The tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl)]methyl ligand, [TismPriBenz], has been employed to form the magnesium carbatrane compound, [TismPriBenz]MgH, which possesses a terminal hydride ligand. Specifically, [TismPriBenz]MgH is obtained via the reaction of [TismPriBenz]MgMe with PhSiH3. The reactivity of [TismPriBenz]MgMe and [TismPriBenz]MgH allows access to a variety of other structurally characterized carbatrane derivatives, including [TismPriBenz]MgX [X = F, Cl, Br, I, SH, N(H)Ph, CH(Me)Ph, O2CMe, S2CMe]. In addition, [TismPriBenz]MgH is a catalyst for (i) hydrosilylation and hydroboration of styrene to afford the Markovnikov products, Ph(Me)C(H)SiH2Ph and Ph(Me)C(H)Bpin, and (ii) hydroboration of carbodiimides and pyridine to form N-boryl formamidines and N-boryl 1,4- and 1,2-dihydropyridines, respectively.

X-ray diffraction studies demonstrate that crystals of the carbodiphosphorane, (Ph3P)2C, obtained from solutions in benzene, exhibit a linear P–C–P interaction. This observation is in contrast to the highly bent structures that have been previously reported for this molecule, thereby providing experimental evidence that the coordination geometry at zerovalent carbon may be very flexible. Density functional theory calculations support the experimental observations by demonstrating that the energy of (Ph3P)2C varies relatively little over the range 130–180°.

A series of bis- and tris(oxobenzimidazolyl)hydroborato compounds, namely, [BoRBenz]Na and [ToRBenz]-Na (R = Me, But, Ad), which feature uncommon sterically demanding LX [O2] and L2X [O3] donor ligands, have been obtained via the reactions of NaBH4 with 1-R-1,3-dihydro-2H-benzimidazol-2-ones. Evidence that the alkyl substituents are suitably located to have a significant impact on the coordination environment is provided by the observation that the methyl derivative [ToMeBenz]Na(κ3-diglyme) exhibits κ3-coordination of the diglyme, whereas the t-butyl and adamantyl derivatives, [ToButBenz]Na(κ2-diglyme) and [ToAdBenz]Na(κ2-diglyme), exhibit κ2-coordination. The [BoRBenz] and [ToRBenz] ligands also allow for isolation of discrete mononuclear thallium compounds, [BoRBenz]Tl and [ToRBenz]Tl, for which the steric demands of the ligands have been quantified in terms of both cone angle and buried volume concepts.

The tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methyl ligand, [TismPriBenz], has been employed to form carbatrane compounds of both the main group metals and transition metals, namely [TismPriBenz]Li, [TismPriBenz]MgMe, [TismPriBenz]Cu and [TismPriBenz]NiBr. In addition to the formation of atranes, a zinc compound that exhibits κ3-coordination, namely [κ3-TismPriBenz]ZnMe, has also been obtained. Furthermore, the [TismPriBenz] ligand may undergo a thermally induced rearrangement to afford a novel tripodal tris(N-heterocyclic carbene) variant, as shown by the conversion of [TismPriBenz]Cu to [κ4-C4-TismPriBenz*]Cu. The transannular M–C bond lengths in the atrane compounds are 0.19–0.32 Å longer than the sum of the respective covalent radii, which is consistent with a bonding description that features a formally zwitterionic component. Interestingly, computational studies demonstrate that the Cu–Catrane interactions in [TismPriBenz]Cu and [κ4-C4-TismPriBenz*]Cu are characterized by an “inverted ligand field”, in which the occupied antibonding orbitals are localized more on carbon than on copper.

The molecular structures of CpMo(PMe3)(CO)2H and CpMo(PMe3)2(CO)H have been determined by X-ray diffraction, thereby revealing four-legged piano-stool structures in which the hydride ligand is trans to CO. However, in view of the different nature of the four basal ligands, the geometries of CpMo(PMe3)(CO)2H and CpMo(PMe3)2(CO)H deviate from that of an idealized four-legged piano stool, such that the two ligands that are orthogonal to the trans H–Mo–CO moiety are displaced towards the hydride ligand. While CpRMo(PMe3)3–x(CO)xH (CpR = Cp, Cp*; x = 1, 2, 3) are catalysts for the release of H2 from formic acid, the carbonyl derivatives, CpRMo(CO)3H, are also observed to form dinuclear formate compounds, namely, [CpRMo(μ-O)(μ-O2CH)]2. The nature of the Mo···Mo interactions in [CpMo(μ-O)(μ-O2CH)]2 and [Cp*Mo(μ-O)(μ-O2CH)]2 have been addressed computationally. In this regard, the two highest occupied molecular orbitals of [CpMo(μ-O)(μ-O2CH)]2 correspond to metal-based δ* (HOMO) and σ (HOMO–1) orbitals. The σ2δ*2 configuration thus corresponds to a formal direct Mo–Mo bond order of zero. The preferential occupation of the δ* orbital rather than the δ orbital is a consequence of the interaction of the latter orbital with p orbitals of the bridging oxo ligands. In essence, lone-pair donation from oxygen increases the electron count so that the molybdenum centers can achieve an 18-electron configuration without the existence of a Mo–Mo bond, whereas a Mo═Mo double bond is required in the absence of lone-pair donation.

Bond lengths between pairs of atoms in covalent molecules are generally predicted well by the sum of their respective covalent radii, such that there are usually only small variations in related compounds. It is, therefore, significant that we have demonstrated that the incorporation of appropriately sized linkers between carbon and a metal center provides a means to modulate the length and nature of a metal–carbon interaction. Specifically, X-ray diffraction studies on a series of tris(1-methylimidazol-2-ylthio)methyl zinc complexes, [TitmMe]ZnX, demonstrate how the Zn–C bond lengths are highly variable (2.17–2.68 Å) and are up to 0.67 Å longer than the average value listed in the Cambridge Structural Database (2.01 Å). Furthermore, density functional theory calculations on [TitmMe]ZnCl demonstrate that the interaction is very flexible, such that either increasing or decreasing the Zn–C length from that in the equilibrium structure is associated with little energy change in comparison to that for other compounds with Zn–C bonds.

The bulky tris(3-tert-butyl-5-pyrazolyl)hydroborato ligand, [TpBut,Me], has been employed to obtain the first structurally characterized example of a molecular magnesium compound that features a terminal fluoride ligand, namely [TpBut,Me]MgF, via the reaction of [TpBut,Me]MgMe with Me3SnF. The chloride, bromide and iodide complexes, [TpBut,Me]MgX (X = Cl, Br, I), can also be obtained by an analogous method using Me3SnX. The molecular structures of the complete series of halide derivatives, [TpBut,Me]MgX (X = F, Cl, Br, I) have been determined by X-ray diffraction. In each case, the Mg–X bond lengths are shorter than the sum of the covalent radii, thereby indicating that there is a significant ionic component to the bonding, in agreement with density functional theory calculations. The fluoride ligand of [TpBut,Me]MgF undergoes halide exchange with Me3SiX (X = Cl, Br, I) to afford [TpBut,Me]MgX and Me3SiF. The other halide derivatives [TpBut,Me]MgX undergo similar exchange reactions, but the thermodynamic driving forces are much smaller than those involving fluoride transfer, a manifestation of the often discussed silaphilicity of fluorine. In accord with the highly polarized Mg–F bond, the fluoride ligand of [TpBut,Me]MgF is capable of serving as a hydrogen bond and halogen bond acceptor, such that it forms adducts with indole and C6F5I. [TpBut,Me]MgF also reacts with Ph3CCl to afford Ph3CF, thereby demonstrating that [TpBut,Me]MgF may be used to form C–F bonds.

The first terminal zinc hydride complex that features a sulfur-rich coordination environment, namely the tris(2-mercapto-1-tert-butylimidazolyl)hydroborato compound, [TmBut]ZnH, has been synthesized via the reaction of [TmBut]ZnOPh with PhSiH3. The Zn–H bond of [TmBut]ZnH is subject to insertion of CO2 and facile protolytic cleavage, of which the latter provides access to heterobimetallic [TmBut]ZnMo(CO)3Cp.

Tris(2-pyridylthio)methane ([Tptm]H) has been employed to synthesize a series of cadmium carbatrane compounds that feature an [N3C] coordination environment. Specifically, [Tptm]H reacts with Cd[N(SiMe3)2]2 to afford [Tptm]CdN(SiMe3)2, which thereby provides access to other derivatives. For example, [Tptm]CdN(SiMe3)2 reacts with (i) CO2 to form {[Tptm]Cd(μ-NCO)}2 and (ii) Me3SiOH and Ph3SiOH to form {[κ3-Tptm]Cd(μ-OSiMe3)}2 and [Tptm]CdOSiPh3, respectively. The siloxide compound {[κ3-Tptm]Cd(μ-OSiMe3)}2 reacts with Me3SiX (X = Cl, Br, O2CMe) to give [Tptm]CdX, while the reaction with PhSiH3 in the presence of CO2 generates the formate complex, [Tptm]CdO2CH, thereby providing evidence for the generation of a proposed cadmium hydride intermediate, {[Tptm]CdH}.

The molecular structure of the nickel formate compound, Ni(py)4(O2CH)2·2py, has been determined by X-ray diffraction, thereby demonstrating that the formate ligand coordinates in a unidentate manner. A similar investigation of the nitrate compound, Ni(py)4(ONO2)2·2py, indicates that the nitrate ligand also coordinates in a unidentate manner; however, the Ni–O–NO2 bond angle is distinctly bent, in contrast to the linear geometry that was previously reported.The molecular structures of the nickel formate and nitrate compounds, Ni(py)4(O2CH)2·2py and Ni(py)4(ONO2)2·2py, have been determined by X-ray diffraction, thereby demonstrating that both ligands coordinate in a unidentate manner. Of note, the Ni–O–NO2 bond angle is distinctly bent, in contrast to the linear geometry that was previously reported.

The phenylselenolate mercury alkyl compounds, PhSeHgMe and PhSeHgEt, have been structurally characterized by X-ray diffraction, thereby demonstrating that both compounds are monomeric with approximately linear coordination geometries; the mercury centers do, nevertheless, exhibit secondary Hg⋯Se intermolecular interactions that serve to increase the coordination number in the solid state. The ethyl derivative, PhSeHgEt, undergoes facile protolytic cleavage of the Hg–C bond to release ethane at room temperature, whereas PhSeHgMe exhibits little reactivity under similar conditions. Interestingly, the cleavage of the Hg–C bond of PhSeHgEt is also more facile than that of the thiolate analog, PhSHgEt, which demonstrates that coordination by selenium promotes protolytic cleavage of the mercury–carbon bond. The phenylselenolate compounds PhSeHgR (R = Me, Et) also undergo degenerate exchange reactions with, for example, PhSHgR and RHgCl. In each case, the alkyl groups preserve coupling to the 199Hg nuclei, thereby indicating that the exchange process involves metathesis of the Hg–SePh/Hg–X groups rather than metathesis of the Hg–R/Hg–R groups.The phenylselenolate mercury alkyl compounds, PhSeHgR (R = Me, Et), have been structurally characterized by X-ray diffraction. Both compounds undergo rapid exchange with RHgX (X = SPh, Cl) on the NMR timescale, while PhSeHgEt also undergoes facile protolytic cleavage of the Hg–C bond by PhSH.

Multinuclear (1H, 77Se, and 199Hg) NMR spectroscopy demonstrates that 1-methyl-1,3-dihydro-2H-benzimidazole-2-selone, H(sebenzimMe), a structural analogue of the selenoamino acid, selenoneine, binds rapidly and reversibly to the mercury centers of HgX2 (X = Cl, Br, I), while X-ray diffraction studies provide evidence for the existence of adducts of composition [H(sebenzimMe)]xHgX2 (X = Cl, x = 2, 3, 4; X = I, x = 2) in the solid state. H(sebenzimMe) also reacts with methylmercury halides, but the reaction is accompanied by elimination of methane resulting from protolytic cleavage of the Hg–C bond, an observation that is of relevance to the report that selenoneine demethylates CysHgMe, thereby providing a mechanism for mercury detoxification. Interestingly, the structures of [H(sebenzimMe)]xHgX2 exhibit a variety of different hydrogen bonding patterns resulting from the ability of the N–H groups to form hydrogen bonds with chlorine, iodine, and selenium.

The cyclopentadienyl molybdenum hydride compounds, CpRMo(PMe3)3−x(CO)xH (CpR = Cp, Cp*; x = 0, 1, 2 or 3), are catalysts for the dehydrogenation of formic acid, with the most active catalysts having the composition CpRMo(PMe3)2(CO)H. The mechanism of the catalytic cycle is proposed to involve (i) protonation of the molybdenum hydride complex, (ii) elimination of H2 and coordination of formate, and (iii) decarboxylation of the formate ligand to regenerate the hydride species. NMR spectroscopy indicates that the nature of the resting state depends on the composition of the catalyst. For example, (i) the resting states for the CpMo(CO)3H and CpMo(PMe3)(CO)2H systems are the hydride complexes themselves, (ii) the resting state for the CpMo(PMe3)3H system is the protonated species [CpMo(PMe3)3H2]+, and (iii) the resting state for the CpMo(PMe3)2(CO)H system is the formate complex, CpMo(PMe3)2(CO)(κ1-O2CH), in the presence of a high concentration of formic acid, but CpMo(PMe3)2(CO)H when the concentration of acid is low. While CO2 and H2 are the principal products of the catalytic reaction induced by CpRMo(PMe3)3−x(CO)xH, methanol and methyl formate are also observed. The generation of methanol is a consequence of disproportionation of formic acid, while methyl formate is a product of subsequent esterification. The disproportionation of formic acid is a manifestation of a transfer hydrogenation reaction, which may also be applied to the reduction of aldehydes and ketones. Thus, CpMo(CO)3H also catalyzes the reduction of a variety of ketones and aldehydes to alcohols by formic acid, via a mechanism that involves ionic hydrogenation.

A series of cadmium carboxylate compounds in a sulfur-rich environment provided by the tris(2-tert-butylmercaptoimidazolyl)hydroborato ligand, namely, [TmBut]CdO2CR, has been synthesized via the reactions of the cadmium methyl derivative [TmBut]CdMe with RCO2H. Such compounds mimic aspects of cadmium-substituted zinc enzymes and also the surface atoms of cadmium chalcogenide crystals, and have therefore been employed to model relevant ligand exchange processes. Significantly, both 1H and 19F NMR spectroscopy demonstrate that the exchange of carboxylate groups between [TmBut]Cd(κ2-O2CR) and the carboxylic acid RCO2H is facile on the NMR time scale, even at low temperature. Analysis of the rate of exchange as a function of concentration of RCO2H indicates that reaction occurs via an associative rather than dissociative pathway. In addition to carboxylate compounds, the thiocarboxylate derivative [TmBut]Cd[κ1-SC(O)Ph] has also been synthesized via the reaction of [TmBut]CdMe with thiobenzoic acid. The molecular structure of [TmBut]Cd[κ1-SC(O)Ph] has been determined by X-ray diffraction, and an interesting feature is that, in contrast to the carboxylate derivatives [TmBut]Cd(κ2-O2CR), the thiocarboxylate ligand binds in a κ1 manner via only the sulfur atom.

X-ray diffraction studies demonstrate that oxidative addition of SiH4 to Ir(PPh3)2(CO)Cl yields Ir(PPh3)2(CO)(Cl)(SiH3)H, which features a cis arrangement of the SiH3 and H ligands in which H is located trans to CO, rather than trans to Cl as originally reported. 1H NMR spectroscopic studies indicate that oxidative addition of GeH4 to Ir(PPh3)2(CO)Cl also occurs in a cis manner but results in the formation of two isomers of Ir(PPh3)2(CO)(Cl)(GeH3)H, which are related by H being trans to either CO or Cl.

Tris(2-pyridylthio)methyl zinc hydride, [κ3-Tptm]ZnH, is an effective catalyst for multiple insertions of carbonyl groups into the Si–H bonds of PhxSiH4–x (x = 1, 2). Specifically, [κ3-Tptm]ZnH catalyzes the insertion of a variety of aldehydes and ketones into the Si–H bonds of PhSiH3 and Ph2SiH2 to afford PhSi[OCH(R)R′]3 and Ph2Si[OCH(R)R′]2, respectively. The mechanism for hydrosilylation is proposed to involve insertion of the carbonyl group into the Zn–H bond to afford an alkoxy species, followed by metathesis with the silane to release the alkoxysilane and regenerate the zinc hydride catalyst. Multiple insertion of prochiral ketones results in the formation of diastereomeric mixtures of alkoxysilanes that can be identified by NMR spectroscopy.

The κ1-m-terphenyl complex of tantalum, [ArTol2]Ta(NMe2)3Cl ([ArTol2] = 2,6-di-p-tolylphenyl), has been synthesized by the reaction of [Ta(NMe2)3Cl2]2 with two equivalents of [ArTol2]Li. [ArTol2]Ta(NMe2)3Cl provides access to a variety of monoalkyl compounds, [ArTol2]Ta(NMe2)3R (R = Me, Et, Prn, Bun, and Np; Np = CH2But), via the reactions with the corresponding RLi. In addition, the reaction of [Ta(NMe2)3Cl2]2 with excess [ArTol2]Li affords the bis(terphenyl) complex, [ArTol2]2Ta(NMe2)3, while the reaction of [ArTol2]Ta(NMe2)3Cl with LiBH4 gives the borohydride complex, [ArTol2]Ta(NMe2)3(κ2-BH4). The dichloride compound, [ArTol2]Ta(NMe2)2Cl2, which is obtained via the reaction of [ArTol2]Ta(NMe2)3Cl with Me3SiCl, provides access to a series of dialkyl derivatives, [ArTol2]Ta(NMe2)2R2 (R = Me, Et, Prn, Bun, and Np), via the reactions with the corresponding RLi. The κ1-m-terphenyl ligands in these complexes are susceptible to metalation. Thus, [ArTol2]Ta(NMe2)3R eliminates RH to afford [κ2-ArTol,Tol′]Ta(NMe2)3 (Tol′ = C6H3Me), while [ArTol2]Ta(NMe2)2Np2 eliminates NpH to form [κ2-ArTol,Tol′]Ta(NMe2)2Np.

Mo(PMe3)6 cleaves the Si–H bonds of SiH4, PhSiH3, and Ph2SiH2 to afford a variety of novel silyl, hypervalent silyl, silane, and disilane complexes, as respectively illustrated by Mo(PMe3)4(SiH3)2H2, Mo(PMe3)4(κ2-H2-H2SiPh2H)H, Mo(PMe3)3(σ-HSiHPh2)H4, and Mo(PMe3)3(κ2-H2-H2Si2Ph4)H2. Mo(PMe3)4(κ2-H2-H2SiPh2H)H and Mo(PMe3)3(κ2-H2-H2Si2Ph4)H2 are respectively the first examples of complexes that feature a hypervalent κ2-H2-H2SiPh2H silyl ligand and a chelating disilane ligand, and both compounds convert to the diphenylsilane adduct, Mo(PMe3)3(σ-HSiHPh2)H4, in the presence of H2. Mo(PMe3)4(SiH3)2H2 undergoes isotope exchange with SiD4, and NMR spectroscopic analysis of the SiHxD4–x isotopologues released indicates that the reaction does not occur via initial reductive elimination of SiH4, but rather by a metathesis pathway.

W(PMe3)4(η2-CH2PMe2)H reacts with PhSiH3 to give the first examples of diphenyldisilanyl compounds, W(PMe3)4(SiH2SiHPh2)H3 and W(PMe3)3(SiH2Ph)(SiH2SiHPh2)H4, via a mechanism that is proposed to involve migration of a SiHPh2 group to a silylene ligand. In addition to the formation of the aforementioned mononuclear compounds, the reaction of W(PMe3)4(η2-CH2PMe2)H with PhSiH3 also yields a novel dinuclear compound, [W(PMe3)2(SiHPh2)H2](μ-Si,P-SiHPhCH2PMe2)(μ-SiH2)[W(PMe3)3H2], which features a bridging silylene ligand that participates in 3-center-2-electron interactions with both tungsten centers. The bonding within the [W(μ-H)Si(μ-H)W] core can be described by a variety of resonance structures, some of which possess multiple bond character between tungsten and silicon. In this regard, [W(PMe3)2(SiHPh2)H2](μ-Si,P-SiHPhCH2PMe2)(μ-SiH2)[W(PMe3)3H2] possesses the shortest W–Si bond length reported. The corresponding reaction of W(PMe3)4(η2-CH2PMe2)H with Ph2SiH2 yields the σ-silane compound, W(PMe3)3(σ-HSiHPh2)H4.

The tris(2-pyridylthio)methylzinc bicarbonate complex, [κ4-Tptm]ZnOCO2H, is reduced by PhSiH3 to give the formate derivative, [κ4-Tptm]ZnO2CH. Isotopic labeling studies demonstrate that the generation of the formate moiety occurs via a sequence that involves release of CO2 followed by insertion into a zinc–hydride bond.

The tris(mercaptoimidazolyl)hydroborato complexes, [κ3-S2H-TmBut]Na(THF)3 and [κ3-S2H-TmAd]Na(THF)3, which feature t-butyl and adamantyl substituents, have been synthesized via the reactions of the respective 1-R-1,3-dihydro-2H-imidazole-2-thiones with NaBH4 in THF (R = But, 1-Ad). X-ray diffraction studies indicate that the compounds are monomeric and that the [TmR] ligands coordinate to the metal in a κ3-S2H manner via two of the sulfur donors and the hydrogen attached to boron, a combination that is unprecedented for sodium derivatives. Analysis of the tris(mercaptoimidazolyl)hydroborato compounds that are listed in the Cambridge Structural Database has allowed for the formulation of a set of criteria that enables κx-Sx and κx+1-SxH coordination modes to be identified. Furthermore, the various κx-Sx and κx+1-SxH coordination modes have also been analyzed with respect to the conformations of the [TmR] ligands, which differ by rotation of the imidazolethione moieties about the B–N bond.

The tris(2-mercapto-1-methylbenzimidazolyl)hydroborato cadmium complexes, {[TmMeBenz]Cd(μ-Cl)}2 and [TmMeBenz]CdI, have been synthesized via the reactions of [TmMeBenz]K with CdCl2 and CdI2, respectively. While X-ray diffraction studies demonstrate that the iodide derivative, [TmMeBenz]CdI, is a monomer, the chloride derivative, {[TmMeBenz]Cd(μ-Cl)}2, exists as a dimer, which is unprecedented for Group 12 [TmR]MX (X = Cl, Br, I) compounds. Furthermore, the cadmium centers of {[TmMeBenz]Cd(μ-Cl)}2 are trigonal bipyramidal, which is an uncommon motif for cadmium complexes with a [S3Cl2] coordination sphere.

The 1-arylimidazole-2-thiones, (HmimAr) [Ar = 3,4,5-C6H2(OMe)3, 2,4-C6H3(NO2)(OMe), 2,4,6-C6H2Cl3 and 3,5-C6H3(CF3)2], which feature electronically diverse substituents, may be obtained via acid-catalyzed ring closure of the corresponding N,N′-aryldiethoxyethylthiourea derivatives, ArN(H)C(S)N(H)CH2CH(OEt)2, (H2detuAr), which in turn are obtained via treatment of aminoacetaldehyde diethyl acetal, H2NCH2CH(OEt)2, with the respective arylisothiocyanates (ArNCS). The molecular structures of all of the above N,N′-aryldiethoxyethylthioureas and 1-arylimidazole-2-thiones have been determined by X-ray diffraction, thereby demonstrating that the substituents have a profound effect on the crystal structures. For example, each of the N,N′-aryldiethoxyethylthiourea derivatives adopts a different hydrogen bonding pattern. Specifically, the hydrogen-bonding network in (i) H2detuArCl3 consists of chains of 9-membered rings, with an [R22(9)] motif, that feature one N–H⋯O and one N–H⋯S interaction, (ii) H2detuArOMe,NO2 consists of chains of 6-membered rings, with an [R12(6)] motif, that feature two head-to-tail N–H⋯S interactions, (iii) H2detuAr(CF3)2 consists of a dimer that features two pairs of N–H⋯O interactions, of which each pair is a component of an 8-membered ring with an [R22(8)] motif, and (iv) H2detuAr(OMe)3 consists of a chain of head-to-head dimeric rings with a basic [R22(16)] motif, a notable feature of which is that sulfur does not play a role as a hydrogen bond acceptor. Each of the 1-arylimidazole-2-thiones exists as a “head-to-head” hydrogen-bonded dimer in the solid state, with an [R22(8)] motif. However, while the hydrogen-bonded motifs for HmimArCl3 and HmimAr(OMe)3 are planar, those for HmimAr(CF3)2 and HmimArOMe,NO2 are extremely puckered, with fold angles of 24.2° (mean value) and 45.7°, respectively.

•The molecular structure of W(PMe3)3H6 has been determined by low temperature X-ray diffraction.•W(PMe3)3H6 exists as a classical hydride compound with a tricapped trigonal prismatic geometry.•NMR spectroscopic studies indicate that W(PMe3)3H6 is a classical hydride in solution.The molecular structure of W(PMe3)3H6 has been identified by low temperature (−123 °C) X-ray diffraction studies as a classical hydride compound, with a tricapped trigonal prismatic geometry in which two of the PMe3 ligands adopt eclipsed positions on opposite triangular faces of the prism and one PMe3 ligand caps a rectangular face. The solid state structure is reproduced well by density functional theory geometry optimization calculations on the molecule in the gas phase. NMR spectroscopic studies provide evidence that W(PMe3)3H6 also exists as a classical hydride compound in solution. For example, the T1(min) value for the hydride ligands of W(PMe3)3H6 is 380 ms at 500 MHz and 310 ms at 400 MHz, values that are more in accord with a classical hydride formulation than a nonclassical one. Likewise, the W(PMe3)3H6−xDx isotopologues (obtained by isotope exchange with either deuterobenzene or deuterotoluene) exhibit upfield (+13 ppb) deuterium secondary isotope effects on the hydride chemical shift, which are also consistent with a classical description.Graphical abstractW(PMe3)3H6 has been identified as a classical hydride compound by low temperature X-ray diffraction studies in the solid state and by 1H NMR spectroscopic studies in solution.

The Covalent Bond Classification (CBC) method provides a means to classify covalent molecules according to the number and types of bonds that surround an atom of interest. This approach is based on an elementary molecular orbital analysis of the bonding involving the central atom (M), with the various interactions being classified according to the number of electrons that each neutral ligand contributes to the bonding orbital. Thus, with respect to the atom of interest (M), the ligand can contribute either two (L), one (X), or zero (Z) electrons to a bonding orbital. A normal covalent bond is represented as M–X, whereas dative covalent bonds are represented as either M←L or M→Z, according to whether the ligand is the donor (L) or acceptor (Z). A molecule is classified as [MLlXxZz] according to the number of L, X, and Z ligand functions that surround M. Not only does the [MLlXxZz] designation provide a formal classification of a molecule, but it also indicates the electron configuration, the valence, and the number of nonbonding electrons on M. As such, the classification allows a student to understand relationships between molecules, thereby increasing their ability to conceptualize and learn the chemistry of the elements.Keywords: Analogies/Transfer; Covalent Bonding; First-Year Undergraduate/General; Graduate Education/Research; Inorganic Chemistry; Main-Group Elements; Nonmetals; Organometallics; Second-Year Undergraduate;

A variety of trinuclear, tetranuclear and octanuclear chalcogenido compounds of molybdenum and tungsten that are supported by PMe3 ligands has been synthesized and structurally characterized by X-ray diffraction. For example, the tetranuclear sulfido cluster, Mo4S6(SH)2(PMe3)6, has been obtained by reaction of Mo(PMe3)6 with H2S, while the trinuclear selenido and tellurido compounds, Mo3Se5(PMe3)6 and Mo3Te5(PMe3)6, may be obtained by thermolysis of Mo(PMe3)4Se2 and Mo(PMe3)4Te2, respectively. In contrast to the formation of a trinuclear compound analogous to Mo3E5(PMe3)6 (E = Se, Te), thermolysis of the tungsten complex, W(PMe3)4S2, produces the octanuclear sulfido cluster, W8S16(PMe3)10. Of these compounds, Mo4S6(SH)2(PMe3)6 and W8S16(PMe3)10 possess M:S stoichiometries of 1:2 and so may be regarded as molecular derivatives of the respective disulfides, MS2, which are important components of hydrotreating catalysts.A variety of trinuclear, tetranuclear and octanuclear chalcogenido compounds of molybdenum and tungsten that are supported by PMe3 ligands has been synthesized and structurally characterized by X-ray diffraction.

[Tris(2-pyridylthio)methyl]zinc fluoride, [κ4-Tptm]ZnF, the first example of an organozinc compound that features a terminal fluoride ligand, may be obtained by the reactions of either [Tptm]ZnX (X = H, OSiMe3) with Me3SnF or [κ4-Tptm]ZnI with [Bun4N]F. Not only is the fluoride ligand of [κ4-Tptm]ZnF susceptible to coordination by B(C6F5)3 to give the adduct [κ4-Tptm]ZnFB(C6F5)3, but it is also an effective hydrogen bond and halogen bond acceptor. For example, X-ray diffraction studies demonstrate that [κ4-Tptm]ZnF forms an adduct with water in which hydrogen bonding between the fluoride ligands and water molecules serves to link pairs of [κ4-Tptm]ZnF molecules with a [F···(H-O-H)2···F] motif. Furthermore, 1H and 19F NMR spectroscopic studies provide evidence for hydrogen bonding and halogen bonding interactions with indole and C6F5I, respectively.

A new class of bidentate ligands that feature oxygen donors, namely, the bis(2-oxo-1-tert-butylimidazolyl)hydroborato and bis(2-oxo-1-methylbenzimidazolyl)hydroborato ligands, [BoBut] and [BoMeBenz], have been synthesized via the reactions of MBH4 with 2 equiv of the respective 2-imidazolone. Chelation of [BoBut] and [BoMeBenz] to a metal center results in a flexible eight-membered ring that is capable of adopting a “boatlike” conformation that allows for secondary M···H–B interactions.

The molecular structures of 1-t-butyl-1,3-dihydro-2H-imidazol-2-one [H(oimBut)], 1-methyl-1,3-dihydro-2H-benzimidazol-2-one [H(obenzimMe)], 1-t-butyl-1,3-dihydro-2H-benzimidazol-2-one [H(obenzimBut)], and 1-t-butyl-1,3-dihydro-2H-benzimidazole-2-thione [H(mbenzimBut)] have been determined by single crystal X-ray diffraction. Consideration of the C–O bond lengths in the 2-imidazolones, together with the respective values for 2-thiones and 2-selones, indicates that the C–E bonds in these compounds are intermediate between those of formal C–E single and double bonds, an observation that may be rationalized in terms of a significant contribution of zwitterionic structures that feature single C+–E– dative covalent bonds. In this regard, a natural bond orbital (NBO) analysis of the bonding in H(ximBut) derivatives demonstrates that a doubly bonded C═E resonance structure is most significant for the oxygen derivative, whereas singly bonded C+–E– resonance structures are dominant for the tellurium derivative, despite the fact that oxygen is more electronegative. The C–E bonding in these compounds is, therefore, significantly different from that in chalcogenoformaldehyde derivatives for which the bonding is well represented by a H2C═E double bonded resonance structure. Comparison of the C–E bond lengths of the imidazolechalcogenones with those of C–E single bonds indicates that the C–O bonds are anomalously short. This observation may be rationalized in terms of the oxygen derivative having not only the most significant π–component but also a substantial ionic component. The latter results from the C–O bond being the most polar due to a substantial polarization of the σ-bond in the direction of oxygen, which thereby supplements the π-polarization and increases the negative charge on oxygen. In contrast, the σ-polarization for the heavier chalcogens opposes the zwitterionic C+–E– π-polarization and thereby reduces the negative charge on the chalcogen. As such, the C–E bond becomes less polar as the chalcogen becomes heavier, despite the fact that the zwitterionic C+–E– contribution increases.

The benzannulated bis and tris(mercaptoimidazolyl)borohydride compounds, [BmMeBenz]Na and [TmMeBenz]Na, have been synthesized via the reactions of NaBH4 with two and three equivalents of 1-methyl-1,3-dihydro-2H-benzimidazole-2-thione, respectively. X-ray diffraction studies on the THF adducts, {μ-[BmMeBenz]Na(THF)2}2 and {[TmMeBenz]Na}2(μ-THF)3, indicate that both compounds are dinuclear but differ according to the nature of the bridging ligand. Specifically, {μ-[BmMeBenz]Na(THF)2}2 possesses bridging [BmMeBenz] ligands and terminal THF ligands, while {[TmMeBenz]Na}2(μ-THF)3 possesses terminal [TmMeBenz] ligands and bridging THF ligands. The tris(mercaptoimidazolyl)borohydride ligand of {[TmMeBenz]Na}2(μ-THF)3 coordinates in a κ3-manner, which is in marked contrast to the κ2-, κ1- and κ0-modes that have been reported for various [TmMe]Na derivatives. Density functional theory (DFT) geometry optimization calculations of the anions [TmMeBenz]− and [TmMe]− in the gas phase indicate that the conformation required for κ3-S3 coordination, i.e. one in which the three sulfur donors point away from the B–H group, is relatively more stable for [TmMeBenz]− than for [TmMe]−, and thus provides a rationalization for the observation that benzannulation enables κ3-coordination of tris(mercaptoimidazolyl)borohydride ligand in {[TmMeBenz]Na}2(μ-THF)3. Furthermore, comparison of the molecular structure and IR spectroscopic properties of [TmMeBenz]Re(CO)3 with those of [TmMe]Re(CO)3 indicates that benzannulation reduces the electron donating properties of the ligand, but has little effect on its steric properties. {μ-[BmMeBenz]Na(THF)2}2 and {[TmMeBenz]Na}2(μ-THF)3 react with [Me3PCuCl]4 to give [BmMeBenz]CuPMe3 and [TmMeBenz]CuPMe3, the first pair of structurally related bis and tris(mercaptoimidazolyl)hydroborato copper(I) compounds.

Tris(2-pyridonyl)methanes may be synthesized via the reactions of the respective 2-pyridone with CHX3 (X = Cl, Br) and K2CO3 in the presence of [Bun4N]Br, followed by acid-catalyzed isomerization with camphorsulfonic acid. These compounds provide access to a new class of alkyl ligands that feature oxygen donors and are capable of forming metallacarbatranes and a monovalent thallium alkyl compound.

The molecular structures of a series of 1,3-propanedithiols that contain carboxylic acid groups, namely rac- and meso-2,4-dimercaptoglutaric acid (H4DMGA) and 2-carboxy-1,3-propanedithiol (H3DMCP), have been determined by X-ray diffraction. Each compound exhibits two centrosymmetric intermolecular hydrogen bonding interactions between pairs of carboxylic acid groups, which result in a dimeric structure for H3DMCP and a polymeric tape-like structure for rac- and meso-H4DMGA. Significantly, the hydrogen bonding motifs observed for rac- and meso-H4DMGA are very different to those observed for the 1,2-dithiol, rac-2,3-dimercaptosuccinic acid (rac-H4DMSA), in which the two oxygen atoms of each carboxylic acid group hydrogen bond to two different carboxylic acid groups, thereby resulting in a hydrogen bonded sheet-like structure rather than a tape. Density functional theory calculations indicate that 1,3-dithiolate coordination to mercury results in larger S–Hg–S bond angles than does 1,2-dithiolate coordination, but these angles are far from linear. As such, κ2-S2 coordination of these dithiolate ligands is expected to be associated with mercury coordination numbers of greater than two. In vivo studies demonstrate that both rac-H4DMGA and H3DMCP reduce the renal burden of mercury in rats, although the compounds are not as effective as either 2,3-dimercaptopropane-1-sulfonic acid (H3DMPS) or meso-H4DMSA.The molecular structures of a series of 1,3-propanedithiols that contain carboxylic acid groups, namely rac- and meso-2,4-dimercaptoglutaric acid (H4DMGA) and 2-carboxy-1,3-propanedithiol (H3DMCP), have been determined by X-ray diffraction. Each compound exhibits two centrosymmetric intermolecular hydrogen bonding interactions between pairs of carboxylic acid groups, which result in a dimeric structure for H3DMCP and a polymeric tape-like structure for rac- and meso-H4DMGA. In vivo studies demonstrate that both rac-H4DMGA and H3DMCP reduce the renal burden of mercury in rats.

[Tris(2-pyridylthio)methyl]zinc hydride, [κ3-Tptm]ZnH, is a multifunctional catalyst that is capable of achieving (i) rapid release of hydrogen by protolytic cleavage of silanes with either water or methanol and (ii) hydrosilylation of aldehydes, ketones, and carbon dioxide. For example, [κ3-Tptm]ZnH catalyzes the release of 3 equivalents of H2 by methanolysis of phenylsilane, with a turnover number of 105 and a turnover frequency surpassing 106 h–1 for the first 2 equivalents. Furthermore, [κ3-Tptm]ZnH also catalyzes the formation of triethoxysilyl formate by hydrosilylation of carbon dioxide with triethoxysilane. Triethoxysilyl formate may be converted into ethyl formate and N,N-dimethylformamide, thereby providing a means for utilizing carbon dioxide as a C1 feedstock for the synthesis of useful chemicals.

A new class of [CCC] X3-donor pincer ligand for transition metals has been constructed via cyclometalation of a 2,6-di-p-tolylphenyl ([ArTol2]) derivative. Specifically, addition of PMe3 to [ArTol2]TaMe3Cl induces elimination of methane and formation of the pincer complex, [κ3-ArTol′2]Ta(PMe3)2MeCl (Tol′ = C6H3Me), which may also be obtained by treatment of Ta(PMe3)2Me3Cl2 with [ArTol2]Li. Solutions of [κ3-ArTol′2]Ta(PMe3)2MeCl undergo ligand redistribution with the formation of [κ3-ArTol′2]Ta(PMe3)2Me2and [κ3-ArTol′2]Ta(PMe3)2Cl2, which may also be synthesized by the reactions of [κ3-ArTol′2]Ta(PMe3)2MeCl with MeMgBr and ZnCl2, respectively. Reduction of [κ3-ArTol′2]Ta(PMe3)2Cl2 with KC8 in benzene gives the benzene complex [κ3-ArTol′2]Ta(PMe3)2(η6-C6H6) that is better described as a 1,4-cyclohexadienediyl derivative. Deuterium labeling employing Ta(PMe3)2(CD3)3Cl2 demonstrates that the pincer ligand is created by a pair of Ar–H/Ta–Me sigma-bond metathesis transformations, rather than by a mechanism that involves α-H abstraction by a tantalum methyl ligand.

Analysis of a monoclinic modification of Zr(CH2Ph)4 by single-crystal X-ray diffraction reveals that the bond angles Zr–CH2–Ph in this compound span a range of 25.1°, which is much larger than previously observed for the orthorhombic form (12.1°). In accord with this large range, density functional theory calculations demonstrate that little energy is required to perturb the Zr–CH2–Ph bond angles in this compound. Furthermore, density functional theory calculations on Me3ZrCH2Ph indicate that bending of the Zr–CH2–Ph moiety in the monobenzyl compound is also facile, thereby demonstrating that a benzyl ligand attached to zirconium is intrinsically flexible, such that its bending does not require a buffering effect involving another benzyl ligand.

Tris(2-pyridylthio)methane, [Tptm]H, has been employed to synthesize the mononuclear alkyl zinc hydride complex, [κ3-Tptm]ZnH, which has been structurally characterized by X-ray diffraction. [κ3-Tptm]ZnH provides access to a variety of other [Tptm]ZnX derivatives. For example, [κ3-Tptm]ZnH reacts with (i) R3SiOH (R = Me, Ph) to give [κ4-Tptm]ZnOSiR3, (ii) Me3SiX (X = Cl, Br, I) to give [κ4-Tptm]ZnX, and (iii) CO2 to give the formate complex, [κ4-Tptm]ZnO2CH. The bis(trimethylsilyl)amide complex [κ3-Tptm]ZnN(SiMe3)2 also reacts with CO2, but the product obtained is the isocyanate complex, [κ4-Tptm]ZnNCO. The formation of [κ4-Tptm]ZnNCO is proposed to involve initial insertion of CO2 into the Zn–N(SiMe3)2 bond, followed by migration of a trimethylsilyl group from nitrogen to oxygen to generate [κ4-Tptm]ZnOSiMe3 and Me3SiNCO, which subsequently undergo CO2-promoted metathesis to give [κ4-Tptm]ZnNCO and (Me3SiO)2CO.

The reactions of W(PMe3)4(η2-CH2PMe2)H, W(PMe3)5H2, W(PMe3)4H4 and W(PMe3)3H6 towards thiophenes reveal that molecular tungsten compounds are capable of achieving a variety of transformations that are relevant to hydrodesulfurization. For example, sequential treatment of W(PMe3)4(η2-CH2PMe2)H with thiophene and H2 yields the butanethiolate complex, W(PMe3)4(SBun)H3, which eliminates but-1-ene at 100 °C. Likewise, sequential treatment of W(PMe3)4(η2-CH2PMe2)H with benzothiophene and H2 yields W(PMe3)4(SC6H4Et)H3, which releases ethylbenzene at 100 °C. Moreover, W(PMe3)4(η2-CH2PMe2)H desulfurizes dibenzothiophene to form a dibenzometallacyclopentadiene complex, [(κ2-C12H8)W(PMe3)](μ-S)(μ-CH2PMe2)(μ-PMe2)[W(PMe3)3].

W(PMe3)4(η2-CH2PMe2)H reacts with aryl halides to give the alkylidene complex, [W(PMe3)4(η2-CHPMe2)H]+, which reacts with LiAlD4 to give selectively W(PMe3)4(η2-CHDPMe2)H, in which the deuterium resides in the methylene group; subsequent migration of deuterium from the methylene group provides a means to measure the rate constant for the formation of the 16-electron species [W(PMe3)5] from W(PMe3)4(η2-CH2PMe2)H.

A new class of tripodal L2X ligands that feature three oxygen donors, namely the tris(2-oxo-1-tert-butylimidazolyl) and tris(2-oxo-1-methylbenzimidazolyl)hydroborato ligands, [ToBut] and [ToMeBenz], has been synthesized via the reactions of NaBH4 with the respective imidazolone. Structural and spectroscopic studies indicate that both [ToBut] and [ToMeBenz] are significantly more sterically demanding but less electron donating than the related [O3] donor ligand, [CpCo{P(O)(OEt)2}3].

The tris(2-mercapto-1-adamantylimidazolyl)hydroborato ligand, [TmAd], has been synthesized via the reaction of 1-adamantyl-2-mercaptoimidazole with MBH4 (M = Li, K). [TmAd]M has been used to synthesize a variety of compounds of the main-group and transition elements, including [TmAd]ZnI, {[TmAd]GaI}[GaI4], {[TmAd]GaCl}[GaCl4], {[TmAd]GaGa[TmAd]}[GaCl4]2, {[TmAd]2In}[InI4], [TmAd]In(κ2-mimAd)Cl, [TmAd]Ga→B(C6F5)3, [TmAd]In→B(C6F5)3, and [TmAd]Re(CO)3. Structural characterization of [TmAd]Re(CO)3 demonstrates that the [TmAd] ligand is more encapsulating than other [TmR] ligands, including [TmBut], while IR spectroscopic studies indicate that the [TmAd] and [TmBut] ligands have very similar electron-donating properties.

The molybdenum hydride complexes Mo(PMe3)5H2 and Mo(PMe3)4H4 are capable of cleaving the C–S bonds of thiophene, benzothiophene and dibenzothiophene. For example, Mo(PMe3)5H2 reacts with thiophene to give the η5-thiophene and butadiene–thiolate complexes, (η5-C4H4S)Mo(PMe3)3 and (η5-C4H5S)Mo(PMe3)2(η2-CH2PMe2). These complexes are also obtained from the reaction between Mo(PMe3)4H4 and thiophene under photochemical conditions, whereas at elevated temperatures thiophene is desulfurized to liberate but-1-ene. Similarly, Mo(PMe3)4H4 desulfurizes benzothiophene at elevated temperatures to liberate ethylbenzene, while the arylthiolate complex Mo(PMe3)4(SC6H4Et)H3 is obtained photochemically. Furthermore, Mo(PMe3)4H4 cleaves the C–S bond of dibenzothiophene to give [η6,κ1-C6H5C6H4S]Mo(PMe3)2H.Graphical abstractThe molybdenum hydride complexes Mo(PMe3)5H2 and Mo(PMe3)4H4 are capable of cleaving the C–S bonds of thiophene, benzothiophene and dibenzothiophene. For example, Mo(PMe3)5H2 reacts with thiophene to give the η5-thiophene and butadiene–thiolate complexes, (η5-C4H4S)Mo(PMe3)3 and (η5-C4H5S)Mo(PMe3)2(η2-CH2PMe2).Research highlights► Mo(PMe3)5H2 and Mo(PMe3)4H4 are capable of cleaving the C–S bonds of thiophenes. ► Mo(PMe3)5H2 reacts with thiophene to give the η5-thiophene and butadiene–thiolate complexes. ► Mo(PMe3)4H4 desulfurizes thiophene at elevated temperatures to liberate but-1-ene. ► Mo(PMe3)4H4 desulfurizes benzothiophene at elevated temperatures to liberate ethylbenzene. ► Mo(PMe3)4H4 cleaves the C–S bond of dibenzothiophene to give [η6,κ1-C6H5C6H4S]Mo(PMe3)2H.

A series of tris(2-mercapto-1-tert-butylimidazolyl)hydroborato gallium compounds have been synthesized. While GaI3 and GaCl3 afford mononuclear {[TmBut]Ga} compounds, namely {[TmBut]GaI}I, {[TmBut]GaCl}[GaCl4], and [κ2-TmBut]2GaI, the reactions of “GaI”, Ga[GaCl4] and (HGaCl2)2 yield compounds that feature Ga–Ga bonds, namely [TmBut]GaGaI3, [TmBut]GaGaCl3, {[TmBut]GaGa[TmBut]}I2, {[TmBut]GaGa[TmBut]}[GaCl4]2, {[TmBut]Ga(GaI2)Ga[TmBut]}I and {[TmBut]GaGa[TmBut]}{[μ-κ1,κ2-TmBut]GaI2GaI2GaI}2. These Ga–Ga bonded compounds may be formally regarded as donor–acceptor adducts between monovalent [TmBut]Ga and various trivalent moieties; for example, [TmBut]GaGaI3 may be described as an adduct of [TmBut]Ga and GaI3, while {[TmBut]Ga(GaI2)Ga[TmBut]}+ is an adduct between two molecules of [TmBut]Ga and [GaI2]+. Comparison of the structure of [TmBut]Ga→B(C6F5)3 with that of [TmBut]In→B(C6F5)3 indicates that [TmBut]Ga is a more effective Lewis base than is [TmBut]In.

The reactions of bis(mercaptoimidazolyl)hydroborato derivatives [BmR]M′ (R = Me, But; M′ = Li, Na, Tl) with MX3 trihalides of aluminium, gallium and indium yield both 1:1 and 2:1 complexes of the types [BmR]MX2 and [BmR]2MX, respectively. Structurally characterized examples of the [BmR]MX2 series include [BmMe]AlCl2, [BmMe]GaI2, [BmMe]InI2, [BmBut]AlCl2 and [BmBut]GaX2 (X = Cl, Br, I), while structurally characterized examples of the [BmR]2MX series include [BmBut]2InX (X = Cl, Br, I). In addition to the halide complexes, the trivalent dimethyl thallium complex [BmBut]TlMe2 has been synthesized via the reaction of [BmBut]Tl with Me2TlCl. The reactions of [BmR]M′ with the monovalent halides, “GaI”, InCl and InI, result in disproportionation. In the case of indium, the mononuclear complexes [BmBut]2InI and [BmBut]InCl(κ2-mimBut) are obtained, whereas for gallium, dinuclear compounds that feature Ga–Ga bonds, namely [BmR](GaI)(GaI)[BmR] (R = Me, But) have been isolated.

The new bis(phosphino)amido ligand, [MePNPPh], that incorporates (i) an ortho-tolylene linker between nitrogen and phosphorus and (ii) phenyl substituents on phosphorus, has been synthesized as its protonated derivative, [MePNPPh]H, via sequential treatment of (2-Br,4-Me-C6H3)2NH with (i) BunLi, (ii) Ph2PCl and (iii) HCl. Deprotonation of [MePNPPh]H with BunLi in THF affords the lithium derivative which has been isolated as both mono and bis THF adducts, [MePNPPh]Li(THF) and [MePNPPh]Li(THF)2. Treatment of [MePNPPh]Li(THF)2 with GaCl3 and InX3 (X = Cl, Br, I) gives a series of [MePNPPh]MX2 complexes in which the [PNP] donor binds in a “T”-shaped manner and the metal has a distorted trigonal bipyramidal geometry. The reaction of [MePNPPh]Li(THF)2 with “GaI” yields the Ga–Ga bonded complex [κ2-MePNPPh](GaI)(GaI)[κ2-MePNPPh] in which the [MePNPPh] ligand binds in a κ2-P,N manner. The bis(phosphino)amine [MePNPPh]H may also serve as a ligand and treatment of [MePNPPh]H with GaBr3 affords [κ2-{[MePNPPh]H}GaBr2][GaBr4], in which the [MePNPPh]H ligand coordinates in a κ2-P,P manner such that the gallium adopts a tetrahedral geometry.The new bis(phosphino)amido ligand, [MePNPPh], has been used to synthesize a series of gallium and indium halide complexes of the types [MePNPPh]MX2, [κ2-MePNPPh](GaI)(GaI)[κ2-MePNPPh], and [κ2-{[MePNPPh]H}GaX2]+, in which the [PNP] donor binds with a variety of coordination modes.

[\( {\text{Tp}}^{{{\text{Me}}_{2} }} \)]M (M = K, Tl) reacts with “GaI” to give a series of compounds that feature Ga–Ga bonds, namely [\( {\text{Tp}}^{{{\text{Me}}_{2} }} \)]Ga→GaI3, [\( {\text{Tp}}^{{{\text{Me}}_{2} }} \)]GaGaI2GaI2(\( {\text{Hpz}}^{{{\text{Me}}_{2} }} \)) and [\( {\text{Tp}}^{{{\text{Me}}_{2} }} \)]Ga(GaI2)2Ga[\( {\text{Tp}}^{{{\text{Me}}_{2} }} \)], in addition to the cationic, mononuclear Ga(III) complex {[\( {\text{Tp}}^{{{\text{Me}}_{2} }} \)]2Ga}+. Likewise, [\( {\text{Tp}}^{{{\text{Me}}_{2} }} \)]M (M = K, Tl) reacts with (HGaCl2)2 and Ga[GaCl4] to give [\( {\text{Tp}}^{{{\text{Me}}_{2} }} \)]Ga→GaCl3, {[\( {\text{Tp}}^{{{\text{Me}}_{2} }} \)]2Ga}[GaCl4], and {[\( {\text{Tp}}^{{{\text{Me}}_{2} }} \)]GaGa[\( {\text{Tp}}^{{{\text{Me}}_{2} }} \)]}[GaCl4]2. The adduct [\( {\text{Tp}}^{{{\text{Me}}_{2} }} \)]Ga→B(C6F5)3 may be obtained via treatment of [\( {\text{Tp}}^{{{\text{Me}}_{2} }} \)]K with “GaI” followed by addition of B(C6F5)3. Comparison of the deviation from planarity of the GaY3 ligands in [\( {\text{Tp}}^{{{\text{Me}}_{2} }} \)]Ga→GaY3 (Y = Cl, I) and [\( {\text{Tm}}^{{{\text{Bu}}^{\text{t}} }} \)]Ga→GaY3, as evaluated by the sum of the Y–Ga–Y bond angles, Σ(Y–Ga–Y), indicates that the [\( {\text{Tm}}^{{{\text{Bu}}^{\text{t}} }} \)]Ga moiety is a marginally better donor than [\( {\text{Tp}}^{{{\text{Me}}_{2} }} \)]Ga. In contrast, the displacement from planarity for the B(C6F5)3 ligand of [\( {\text{Tp}}^{{{\text{Me}}_{2} }} \)]Ga→B(C6F5)3 is greater than that of [\( {\text{Tm}}^{{{\text{Bu}}^{\text{t}} }} \)]Ga→B(C6F5)3, an observation that is interpreted in terms of interligand steric interactions in the former complex compressing the C–B–C bond angles.

The transformation of petroleum-derived feedstocks into useful chemicals often requires controllable cleavage of C–H or C–C bonds. There are many examples of achieving this through the oxidative addition of C–H bonds to metal centres, but analogous transformations of C–C bonds are rare. Here, using a tungsten centre and exploiting the formation of an unusual chelating ligand, a strong C–C bond is cleaved; other metal centres with suitable ancillary ligands could perform the same function.

Mo(PMe3)6 reacts with phenazine (PhzH) to give (η6-C6-PhzH)Mo(PMe3)3, (μ-η6,η6-PhzH)[Mo(PMe3)3]2 and (η4-C4-PhzH)2Mo(PMe3)2, each of which displays previously unknown coordination modes for phenazine. Both mononuclear (η6-C6-PhzH)Mo(PMe3)3 and dinuclear (μ-η6,η6-PhzH)[Mo(PMe3)3]2 react with H2 at room temperature to give the respective dihydride complexes, (η4-C4-PhzH)Mo(PMe3)3H2 and (μ-η6,η4-PhzH)[Mo(PMe3)3][Mo(PMe3)3H2]. A comparison of (η6-C6-PhzH)Mo(PMe3)3 with the anthracene (AnH) and acridine (AcrH) counterparts, (η6-AnH)Mo(PMe3)3 and (η6-C6-AcrH)Mo(PMe3)3, indicates that oxidative addition of H2 is promoted by incorporation of nitrogen substituents into the central ring. Furthermore, comparison of (η6-C6-PhzH)Mo(PMe3)3 with the quinoxaline (QoxH) analogue, (η6-C6-QoxH)Mo(PMe3)3, indicates that ring fusion also promotes oxidative addition of H2. The mononitrogen quinoline (QH) and acridine compounds, (η6-C6-QH)Mo(PMe3)3 and (η6-C6-AcrH)Mo(PMe3)3, which respectively possess two and three fused six-membered rings, exhibit a similar trend, with the former being inert towards H2, while the latter reacts rapidly to yield (η4-C4-AcrH)Mo(PMe3)3H2. Ring fusion also promotes hydrogenation of the heterocyclic ligand, with (η6-C6-AcrH)Mo(PMe3)3 releasing 9,10-dihydroacridine upon treatment with H2 in benzene at 95 °C. Furthermore, catalytic hydrogenation of acridine to a mixture of 9,10-dihydroacridine and 1,2,3,4-tetrahydroacridine may be achieved by treatment of (η6-C6-AcrH)Mo(PMe3)3 with acridine and H2 at 95 °C.

The reaction between a transition metal carbonyl compound, LnMCO, and Li[Me3SiNR] yields the corresponding isocyanide derivative, LnMCNR, thereby providing a new route to transition metal isocyanide compounds that does not require the use of free isocyanides as reagents.

p-tert-Butyltetrathiatetramercaptocalix[4]arene, [S4CalixBut(SH)4], reacts with Mo(PMe3)6, W(PMe3)4(η2-CH2PMe2)H and Ni(PMe3)4 to yield molybdenum, tungsten and nickel compounds in a sulfur-rich coordination environment.

The susceptibility of two-coordinate mercury alkyl compounds of the type X−Hg−R (where X is a monodentate sulfur donor) towards protolytic cleavage has been investigated as part of ongoing efforts to obtain information relevant to understanding the mechanism of action of the organomercurial lyase, MerB. Specifically, the reactivity of the two-coordinate mercury alkyl compounds PhSHgR, [mimBut]HgR and {[HmimBut]HgR}+ (HmimBut = 2-mercapto-1-t-butylimidazole; R = Me, Et) towards PhSH was investigated, thereby demonstrating that the ability to cleave the Hg−C bond is very dependent on the nature of the system. For example, whereas the reaction of PhSHgMe with PhSH requires heating at 145 °C for several weeks to liberate CH4, the analogous reaction of PhSHgEt with PhSH leads to evolution of C2H6 over the course of 2 days at 100 °C. Furthermore, protolytic cleavage of the Hg−C bond by PhSH is promoted by HmimBut. For example, whereas the reaction of {[HmimBut]HgEt}+ with PhSH eliminates C2H6 at elevated temperatures, the protolytic cleavage occurs over a period of 2 days at room temperature in the presence of HmimBut. The ability of HmimBut to promote the protolytic cleavage is interpreted in terms of the formation of a higher coordinate species {[HmimBut]nHgR}+ that is more susceptible to Hg−C bond cleavage than is two-coordinate {[HmimBut]HgR}+. These observations support the notion that access to a species with a coordination number greater than two is essential for efficient activity of MerB.

The carboxylate oxygen of thimerosal, [(ArCO2)SHgEt]Na, is subject to facile electrophilic attack by H+ and [HgEt]+ to give (ArCO2H)SHgEt and [(ArCO2HgEt)SHgEt]2, respectively. X-Ray diffraction demonstrates that (ArCO2H)SHgEt exists as a hydrogen bonded dimer in the solid state whereas [(ArCO2HgEt)SHgEt]2 is tetranuclear, with the mercury centers being connected by bridging carboxylate groups. 1H NMR spectroscopic studies indicate that the form of the 199Hg satellites of the ethyl group of (ArCO2H)SHgEt are dependent on the magnetic field, such that the inner pair of CH2 and CH3 satellites appear as a singlet at 400 MHz, as a consequence of 2JHg–H and 3JHg–H having opposite signs and the difference in chemical shifts of the central CH2 and CH3groups being equal to ½{|2JHg–H−3JHg–H|}.

Nickel and palladium paddlewheel complexes that feature 2-mercapto-1-t  -butylimidazolyl (mimButmimBut) bridging ligands, namely Ni2[mimButmimBut]4 and Pd2[mimButmimBut]4, have been synthesized and structurally characterized by X-ray diffraction. Since the mimButmimBut ligand bridges in an asymmetric manner via a sulfur and nitrogen donor, paddlewheel compounds of the type M2[mimButmimBut]4 may exist as isomers that are distinguished by the relative orientations of the ligands. In this regard, the (4,0)-Ni2[mimButmimBut]4 and trans-(2,2)-Ni2[mimButmimBut]4 isomers have been isolated for the nickel system, while the (4,0)-Pd2[mimButmimBut]4 and (3,1)-Pd2[mimButmimBut]4 isomers have been isolated for the palladium system.Nickel and palladium paddlewheel complexes that feature 2-mercapto-1-t-butylimidazolyl bridging ligands, namely Ni2[mimButmimBut]4 and Pd2[mimButmimBut]4, have been synthesized and structurally characterized by X-ray diffraction.

The nickel boratrane complexes [κ4-B(mimBut)3]Ni(κ1-OAc), [κ4-B(mimBut)3]NiNCS and [κ4-B(mimBut)3]NiN3 are obtained viametathesis of the chloride ligand of [κ4-B(mimBut)3]NiCl with TlOAc, KSCN and NaN3, respectively; the Ni→B bond in these complexes is a site of reactivity, thereby providing a means of synthesizing nickel complexes that feature B-functionalized tris(mercaptoimidazolyl)borate derivatives, [YTmBut]NiZ.

[TmBut]In, the first structurally-characterized monovalent indium compound that features a sulfur-rich coordination environment, has been synthesized via treatment of InCl with [TmBut]K; in contrast to the thallium counterpart, the lone pair of [TmBut]In is a site of reactivity, thereby allowing formation of [TmBut]In→B(C6F5)3 and [TmBut]In(κ2-S4) upon treatment with B(C6F5)3 and S8, respectively.

The molecular structure of sodium ethylmercury thiosalicylate (also known as thimerosal and Merthiolate) and related arylthiolate mercury alkyl compounds, namely PhSHgMe and PhSHgEt, have been determined by single crystal X-ray diffraction. 1H NMR spectroscopic studies indicate that the appearance of the 199Hg mercury satellites of the ethyl group of thimerosal is highly dependent on the magnetic field and the viscosity of the solvent as a consequence of relaxation due to chemical shift anisotropy.

One of the paradigms of Zn2+ metallobiochemistry is that coordination of water to Zn2+ provides a mechanism of activation that involves lowering the pKa by approximately 7 pH units. This idea has become central to the development of mechanisms of action for zinc metalloproteins. However, the direct measurement of the pKa of water bound to Zn2+ in a metalloprotein has yet to be accomplished. Developing models for Zn2+−OH2 species has been a significant challenge, but we have utilized solid-state 67Zn NMR spectroscopy as a means to characterize one of the few examples of water bound to mononuclear tetrahedral Zn2+: {[TpBut,Me]Zn(OH2)}[HOB(C6F5)3]. The measured quadrupole coupling (Cq) constant is 4.3 MHz with an asymmetry parameter of ηq of 0.6. Likewise, due to the small value of Cq, anisotropic shielding also contributed to the observed 67Zn NMR lineshape. As expected, the computed values of the magnetic resonance parameters depend critically on the nature of the anion. The predicted value of Cq for {[TpBut,Me]Zn(OH2)}[HOB(C6F5)3] is –4.88 MHz. We discuss the results of these calculations in terms of the nature of the anion, the local electrostatics, and its subsequent hydrogen bonding to [TpBut,Me]Zn(OH2)+.

The reactivity of Mo(PMe3)6 towards 6-membered heterocyclic aromatic nitrogen compounds, namely pyridine, pyrazine, pyrimidine and triazine, has been investigated as part of an effort to define the coordination chemistry of molybdenum relevant to hydrodenitrogenation. For example, Mo(PMe3)6 reacts with pyridine to yield initially (η2-N,C-pyridyl)Mo(PMe3)4H, an uncommon example of an η2-pyridyl–hydride complex. The formation of (η2-N,C-pyridyl)Mo(PMe3)4H is reversible and treatment with PMe3 regenerates Mo(PMe3)6 and pyridine. At elevated temperatures, (η2-N,C-pyridyl)Mo(PMe3)4H dissociates PMe3 and converts to the η6-pyridine complex (η6-pyridine)Mo(PMe3)3. Pyrazine, pyrimidine and 1,3,5-triazine likewise react with Mo(PMe3)6 to yield (η2-N,C-pyrazinyl)Mo(PMe3)4H, (η2-N,C-pyrimidinyl)Mo(PMe3)4H and (η2-N,C-triazinyl)Mo(PMe3)4H, respectively. At elevated temperatures (η2-N,C-pyrazinyl)Mo(PMe3)4H and (η2-N,C-pyrimidinyl)Mo(PMe3)4H dissociate PMe3 and convert to (η6-pyrazine)Mo(PMe3)3 and (η6-pyrimidine)Mo(PMe3)3 in which the heterocycle coordinates to molybdenum in an unprecedented η6-manner.Mo(PMe3)6 reacts with 6-membered heterocyclic aromatic nitrogen compounds (NHetH = pyridine, pyrazine, pyrimidine, and triazine) to yield (η2-NHet)Mo(PMe3)4H resulting from cleavage of a C–H bond adjacent to the nitrogen atom. The C–H bond cleavage is reversible and (η2-NHet)Mo(PMe3)4H converts to (η6-NHetH)Mo(PMe3)3 upon heating for NHetH = pyridine, pyrazine, and pyrimidine.

[Me2Si(CpMe2)2]W(H)Cl is obtained via reaction of WCl6 with a mixture of [Me2Si(CpMe2)2]Li2 and NaBH4, from which the dichloride [Me2Si(CpMe2)2]WCl2 is obtained via treatment with CHCl3. [Me2Si(CpMe2)2]WCl2 provides a means to access other ansa tungstenocene compounds, such as [Me2Si(CpMe2)2]WH2, [Me2Si(CpMe2)2]WMe2, and [Me2Si(CpMe2)2]WCO. Of most interest, the reactions of [Me2Si(CpMe2)2]W(H)Cl with organolithium reagents do not yield simple ansa tungstenocene derivatives. Specifically, the reactions of [Me2Si(CpMe2)2]W(H)Cl with MeLi, BunLi, or PhLi result in the formation of mixed-ring tungstenocene compounds resulting from C–Si cleavage and functionalization of the ansa bridge, namely (CpMe2)(η5,κ1–C5H2Me2SiMe2CH2)WH, (CpMe2)[η5,κ1–C5H2Me2Si(Me)(Bun)CH2]WH, and (CpMe2)[η5,κ1–C5H2Me2SiMe2(C6H4)]WH, respectively. In contrast to the C–Si cleavage achieved by MeLi, BunLi, and PhLi, the ansa bridge of [Me2Si(CpMe2)2]W(H)Cl is inert to ButLi and the product obtained is the fulvene (“tuck-in”) complex [Me2Si(CpMe2)(η6–C5MeH2CH2)]WH derived from dehydrohalogenation.

Linear nickel nitrosyl compounds supported by tridentate nitrogen and selenium ligands, namely the tris(3,5-dimethylpyrazolyl)hydroborato and tris(2-seleno-1-mesitylimidazolyl)hydroborato complexes, [TpMe2]NiNO and [TseMes]NiNO, have been synthesized and structurally characterized by X-ray diffraction. Computational studies demonstrate that the linear nitrosyl ligand behaves as a trivalent 3 ligand such that the Ni–N interaction has multiple bond character.

The impact of agostic interactions (i.e., 3-center–2-electron MHC bonds) on the structures and reactivity of organotransition metal compounds is reviewed.

Abstract

The extreme toxicity of organomercury compounds that are found in the environment has focused attention on the mechanisms of action of bacterial remediating enzymes. We describe facile room-temperature protolytic cleavage by a thiol of the Hg-C bond in mercury-alkyl compounds that emulate the structure and function of the organomercurial lyase MerB. Specifically, the tris(2-mercapto-1-t-butylimidazolyl)hydroborato ligand [Tm^But], which features three sulfur donors, has been used to synthesize [Tm^But]HgR alkyl compounds (R = methyl or ethyl) that react with phenylthiol (PhSH) to yield [Tm^But]HgSPh and RH. Although [Tm^But]HgR compounds exist as linear two-coordinate complexes in the solid state, ¹H nuclear magnetic resonance spectroscopy indicates that the complexes exist in rapid equilibrium with their higher-coordinate [κ²-Tm^But]HgR and [κ³-Tm^But]HgR isomers in solution. Facile access to a higher-coordinate species is proposed to account for the exceptional reactivity of [Tm^But]HgR relative to that of other two-coordinate mercury-alkyl compounds.

Oxidative addition of H2 and D2 to the anthracene complex (η6-AnH)Mo(PMe3)3 giving (η4-AnH)Mo(PMe3)3X2 (X = H, D) is characterized by a normal equilibrium isotope effect (KH/KD > 1) at temperatures close to ambient; calculations on (η4-AnH)Mo(PH3)3H2 indicate that this is a consequence of relatively low energy Mo–H vibrational modes.

A new tripodal ligand that features three selenium donors, namely the tris(2-seleno-1-mesitylimidazolyl)hydroborato ligand, [TseMes], has been constructed via the reaction of KBH4 with 1-mesitylimidazole-2-selone; comparison of the IR spectroscopic data of [TseMes]Re(CO)3 with those of a variety of related LRe(CO)3 complexes demonstrates that the [TseMes] ligand is more strongly electron donating than Cp, Cp*, [Tp], [TpMe2] and [TmMes] ligands.

The dinuclear complex {[μ-κ1,κ3-B(mimBut)3]Pd}2, which features a Pd→B dative bond, may be obtained by the reaction of [TmBut]K with Pd(OAc)2; treatment of {[μ-κ1,κ3-B(mimBut)3]Pd}2 with PMe3 affords the mononuclear boratrane derivative [κ4-B(mimBut)3]Pd(PMe3), for which a molecular orbital analysis indicates that the palladium center possesses a d8 configuration.

A series of arylchalcogenolate complexes of cadmium supported by tris(2-mercapto-1-tert-butylimidazolyl)hydroborato ligation, namely [TmBut]CdEAr (EAr = OC6H3Ph2, SPh, SePh, TePh), has been synthesized from [TmBut]CdMe; structural characterization by X-ray diffraction indicates that the variation in Cd–EAr bond lengths is similar to that of Zn–EAr and correlates closely with the covalent radius of the chalcogen, in marked contrast to the large variation in M–OAr and M–SAr bond lengths observed for other metals (Zr and Sm).

Cp2∗MoCl2 (Cp* = C5Me5) is obtained via reaction of MoCl5 with a mixture of Cp*K and NaBH4 followed by treatment with CHCl3. Cp2∗MoCl2 provides access to a large variety of other permethylmolybdenocene complexes which include Cp2∗MoH2,Cp2∗MoMe2,Cp2∗MoCO,Cp2∗MoO,Cp2∗Mo(Me)Cl,Cp2∗Mo(H)I,Cp2∗Mo(EPh)H (E = S, Se, Te), Cp2∗Mo(η2-E2) (E = S, Se, Te), Cp2∗Mo(η2-E4) (E = S, Se), Cp2∗Mo(OSiMe3)CN,Cp2∗Mo(NCS)2, and Cp2∗Mo(N3)2.Cp2∗MoCl2 (Cp* = C5Me5) is obtained via reaction of MoCl5 with a mixture of Cp*K and NaBH4 followed by treatment with CHCl3. Cp2∗MoCl2 provides access to a large variety of other permethylmolybdenocene complexes.

The tris  (pyrazolyl)hydroaluminate derivatives, [HAlTpBut,Me]Li[HAlTpBut,Me]Li and {[HAlTpMe2]Li}2{[HAlTpMe2]Li}2, have been synthesized via the reactions of LiAlH4 with 3-tert-butyl-5-methylpyrazole and 3,5-dimethylpyrazole, respectively, while the tris  (pyrazolyl)hydrogallate counterparts, [HGaTpBut,Me]Li[HGaTpBut,Me]Li and {[HGaTpMe2]Li}2{[HGaTpMe2]Li}2, are obtained from the corresponding reactions with LiGaH4. X-ray diffraction studies demonstrate that the 3-tert  -butyl-5-methylpyrazolyl derivatives, [HAlTpBut,Me]Li[HAlTpBut,Me]Li and [HGaTpBut,Me]Li[HGaTpBut,Me]Li, are monomeric with lithium in a trigonal pyramidal coordination environment, whereas the less sterically demanding 3,5-dimethylpyrazolyl derivatives, {[HAlTpMe2]Li}2{[HAlTpMe2]Li}2 and {[HGaTpMe2]Li}2{[HGaTpMe2]Li}2, are dimeric. In addition to the tris  (pyrazolyl)hydroaluminate derivative, [HAlTpBut,Me]Li[HAlTpBut,Me]Li, the tetrakis   derivative, [(pzBut,Me)AlTpBut,Me]Li[(pzBut,Me)AlTpBut,Me]Li, has also been isolated. X-ray diffraction studies demonstrate that [(pzBut,Me)AlTpBut,Me]Li[(pzBut,Me)AlTpBut,Me]Li, as obtained from solutions in benzene, exists as a monomer in the solid state with tridentate coordination, whereas crystals obtained from THF exhibit a bidentate coordination mode, which is supplemented by coordination of THF, i.e.  [(pzBut,Me)2AlBpBut,Me]Li(THF)[(pzBut,Me)2AlBpBut,Me]Li(THF). NMR spectroscopic studies, however, indicate that [(pzBut,Me)AlTpBut,Me]Li[(pzBut,Me)AlTpBut,Me]Li is fluxional in both benzene and THF solutions.Tris  (pyrazolyl)hydroaluminate derivatives, [HAlTpBut,Me]Li[HAlTpBut,Me]Li and {[HAlTpMe2]Li}2{[HAlTpMe2]Li}2, have been synthesized via the reactions of LiAlH4 with 3-tert-butyl-5-methylpyrazole and 3,5-dimethylpyrazole, respectively, while the tris  (pyrazolyl)hydrogallate counterparts, [HGaTpBut,Me]Li[HGaTpBut,Me]Li and {[HGaTpMe2]Li}2{[HGaTpMe2]Li}2, are obtained from the corresponding reactions with LiGaH4.

Ni(PMe3)4 serves as a catalyst for the release of H2 and CO2 from formic acid. The capacity of Ni(PMe3)4 to achieve this transformation is linked to the ability of the PMe3 ligand to induce decarboxylation, as illustrated by the observation that both Ni(py)4(O2CH)2 and Ni(O2CH)2·2H2O react with PMe3 to afford Ni(PMe3)4; the latter transformation also provides a convenient method for the synthesis of a zerovalent nickel compound.

The benzannulated tris(mercaptoimidazolyl)borohydride sodium complex, [TmButBenz]Na, has been synthesized via the reaction of NaBH4 with 1-tert-butyl-1,3-dihydro-2H-benzimidazole-2-thione, while [TmMeBenz]K has been synthesized via the reaction of KBH4 with 1-methyl-1,3-dihydro-2H-benzimidazole-2-thione. The molecular structures of the solvated adducts, {[TmButBenz]Na(THF)}2(μ-THF)2 and [TmMeBenz]K(OCMe2)3, have been determined by X-ray diffraction, which demonstrates that the [TmR] ligands in these complexes adopt different coordination modes to that in {[TmMeBenz]Na}2(μ-THF)3. Specifically, while the [TmMeBenz] ligand of the sodium complex {[TmMeBenz]Na}2(μ-THF)3 adopts a κ3-S3 coordination mode, the potassium complex [TmMeBenz]K(OCMe2)3 adopts a most uncommon inverted κ4-S3H coordination mode in which the potassium binds to all three sulfur donors and the hydrogen of the B–H group in a linear K⋯H–B manner. Furthermore, the [TmButBenz] ligand of {[TmButBenz]Na(THF)}2(μ-THF)2 adopts a κ3-S2H coordination mode, thereby demonstrating the flexibility of this ligand system. The monovalent thallium compounds, [TmMeBenz]Tl and [TmButBenz]Tl, have been obtained via the corresponding reactions of [TmMeBenz]Na and [TmButBenz]Na with TlOAc. X-ray diffraction demonstrates that the three sulfur donors of the [TmRBenz] ligands of both [TmMeBenz]Tl and [TmButBenz]Tl chelate to thallium. This coordination mode is in marked contrast to that in other [TmR]Tl compounds, which exist as dinuclear molecules wherein two of the sulfur donors coordinate to different thallium centers. As such, this observation provides further evidence that benzannulation promotes κ3-S3 coordination in this system.

This article provides a means to classify and represent compounds that feature 3-center 4-electron (3c–4e) interactions in terms of the number of electrons that each atom contributes to the interaction. Specifically, Class I 3c–4e interactions are classified as those in which two atoms provide one electron each and the third atom provides a pair of electrons (i.e. LX2), while Class II 3c–4e interactions are classified as those in which two atoms each provide a pair of electrons and the third atom contributes none (i.e. L2Z). These classes can be subcategorized according to the nature of the central atom. Thus, Class I interactions can be categorized according to whether the central atom provides one (i.e. μ–X) or two (i.e. μ–L) electrons, while Class II interactions can be categorized according to whether the central atom provides none (i.e. μ–Z) or two (i.e. μ–L) electrons. The use of appropriate structure-bonding representations for these various interactions provides a means to determine the covalent bond classification of the element of interest.

The tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methyl ligand, [TismPriBenz], has been employed to form carbatrane compounds of both the main group metals and transition metals, namely [TismPriBenz]Li, [TismPriBenz]MgMe, [TismPriBenz]Cu and [TismPriBenz]NiBr. In addition to the formation of atranes, a zinc compound that exhibits κ3-coordination, namely [κ3-TismPriBenz]ZnMe, has also been obtained. Furthermore, the [TismPriBenz] ligand may undergo a thermally induced rearrangement to afford a novel tripodal tris(N-heterocyclic carbene) variant, as shown by the conversion of [TismPriBenz]Cu to [κ4-C4-TismPriBenz*]Cu. The transannular M–C bond lengths in the atrane compounds are 0.19–0.32 Å longer than the sum of the respective covalent radii, which is consistent with a bonding description that features a formally zwitterionic component. Interestingly, computational studies demonstrate that the Cu–Catrane interactions in [TismPriBenz]Cu and [κ4-C4-TismPriBenz*]Cu are characterized by an “inverted ligand field”, in which the occupied antibonding orbitals are localized more on carbon than on copper.

The tris(2-pyridylthio)methylzinc bicarbonate complex, [κ4-Tptm]ZnOCO2H, is reduced by PhSiH3 to give the formate derivative, [κ4-Tptm]ZnO2CH. Isotopic labeling studies demonstrate that the generation of the formate moiety occurs via a sequence that involves release of CO2 followed by insertion into a zinc–hydride bond.

The first terminal zinc hydride complex that features a sulfur-rich coordination environment, namely the tris(2-mercapto-1-tert-butylimidazolyl)hydroborato compound, [TmBut]ZnH, has been synthesized via the reaction of [TmBut]ZnOPh with PhSiH3. The Zn–H bond of [TmBut]ZnH is subject to insertion of CO2 and facile protolytic cleavage, of which the latter provides access to heterobimetallic [TmBut]ZnMo(CO)3Cp.

A new class of tripodal L2X ligands that feature three oxygen donors, namely the tris(2-oxo-1-tert-butylimidazolyl) and tris(2-oxo-1-methylbenzimidazolyl)hydroborato ligands, [ToBut] and [ToMeBenz], has been synthesized via the reactions of NaBH4 with the respective imidazolone. Structural and spectroscopic studies indicate that both [ToBut] and [ToMeBenz] are significantly more sterically demanding but less electron donating than the related [O3] donor ligand, [CpCo{P(O)(OEt)2}3].

The reaction between a transition metal carbonyl compound, LnMCO, and Li[Me3SiNR] yields the corresponding isocyanide derivative, LnMCNR, thereby providing a new route to transition metal isocyanide compounds that does not require the use of free isocyanides as reagents.

W(PMe3)4(η2-CH2PMe2)H reacts with aryl halides to give the alkylidene complex, [W(PMe3)4(η2-CHPMe2)H]+, which reacts with LiAlD4 to give selectively W(PMe3)4(η2-CHDPMe2)H, in which the deuterium resides in the methylene group; subsequent migration of deuterium from the methylene group provides a means to measure the rate constant for the formation of the 16-electron species [W(PMe3)5] from W(PMe3)4(η2-CH2PMe2)H.

p-tert-Butyltetrathiatetramercaptocalix[4]arene, [S4CalixBut(SH)4], reacts with Mo(PMe3)6, W(PMe3)4(η2-CH2PMe2)H and Ni(PMe3)4 to yield molybdenum, tungsten and nickel compounds in a sulfur-rich coordination environment.

The cyclopentadienyl molybdenum hydride compounds, CpRMo(PMe3)3−x(CO)xH (CpR = Cp, Cp*; x = 0, 1, 2 or 3), are catalysts for the dehydrogenation of formic acid, with the most active catalysts having the composition CpRMo(PMe3)2(CO)H. The mechanism of the catalytic cycle is proposed to involve (i) protonation of the molybdenum hydride complex, (ii) elimination of H2 and coordination of formate, and (iii) decarboxylation of the formate ligand to regenerate the hydride species. NMR spectroscopy indicates that the nature of the resting state depends on the composition of the catalyst. For example, (i) the resting states for the CpMo(CO)3H and CpMo(PMe3)(CO)2H systems are the hydride complexes themselves, (ii) the resting state for the CpMo(PMe3)3H system is the protonated species [CpMo(PMe3)3H2]+, and (iii) the resting state for the CpMo(PMe3)2(CO)H system is the formate complex, CpMo(PMe3)2(CO)(κ1-O2CH), in the presence of a high concentration of formic acid, but CpMo(PMe3)2(CO)H when the concentration of acid is low. While CO2 and H2 are the principal products of the catalytic reaction induced by CpRMo(PMe3)3−x(CO)xH, methanol and methyl formate are also observed. The generation of methanol is a consequence of disproportionation of formic acid, while methyl formate is a product of subsequent esterification. The disproportionation of formic acid is a manifestation of a transfer hydrogenation reaction, which may also be applied to the reduction of aldehydes and ketones. Thus, CpMo(CO)3H also catalyzes the reduction of a variety of ketones and aldehydes to alcohols by formic acid, via a mechanism that involves ionic hydrogenation.

The nickel boratrane complexes [κ4-B(mimBut)3]Ni(κ1-OAc), [κ4-B(mimBut)3]NiNCS and [κ4-B(mimBut)3]NiN3 are obtained viametathesis of the chloride ligand of [κ4-B(mimBut)3]NiCl with TlOAc, KSCN and NaN3, respectively; the Ni→B bond in these complexes is a site of reactivity, thereby providing a means of synthesizing nickel complexes that feature B-functionalized tris(mercaptoimidazolyl)borate derivatives, [YTmBut]NiZ.

[TmBut]In, the first structurally-characterized monovalent indium compound that features a sulfur-rich coordination environment, has been synthesized via treatment of InCl with [TmBut]K; in contrast to the thallium counterpart, the lone pair of [TmBut]In is a site of reactivity, thereby allowing formation of [TmBut]In→B(C6F5)3 and [TmBut]In(κ2-S4) upon treatment with B(C6F5)3 and S8, respectively.

The bulky tris(3-tert-butyl-5-pyrazolyl)hydroborato ligand, [TpBut,Me], has been employed to obtain the first structurally characterized example of a molecular magnesium compound that features a terminal fluoride ligand, namely [TpBut,Me]MgF, via the reaction of [TpBut,Me]MgMe with Me3SnF. The chloride, bromide and iodide complexes, [TpBut,Me]MgX (X = Cl, Br, I), can also be obtained by an analogous method using Me3SnX. The molecular structures of the complete series of halide derivatives, [TpBut,Me]MgX (X = F, Cl, Br, I) have been determined by X-ray diffraction. In each case, the Mg–X bond lengths are shorter than the sum of the covalent radii, thereby indicating that there is a significant ionic component to the bonding, in agreement with density functional theory calculations. The fluoride ligand of [TpBut,Me]MgF undergoes halide exchange with Me3SiX (X = Cl, Br, I) to afford [TpBut,Me]MgX and Me3SiF. The other halide derivatives [TpBut,Me]MgX undergo similar exchange reactions, but the thermodynamic driving forces are much smaller than those involving fluoride transfer, a manifestation of the often discussed silaphilicity of fluorine. In accord with the highly polarized Mg–F bond, the fluoride ligand of [TpBut,Me]MgF is capable of serving as a hydrogen bond and halogen bond acceptor, such that it forms adducts with indole and C6F5I. [TpBut,Me]MgF also reacts with Ph3CCl to afford Ph3CF, thereby demonstrating that [TpBut,Me]MgF may be used to form C–F bonds.

A series of tris(2-mercapto-1-tert-butylimidazolyl)hydroborato gallium compounds have been synthesized. While GaI3 and GaCl3 afford mononuclear {[TmBut]Ga} compounds, namely {[TmBut]GaI}I, {[TmBut]GaCl}[GaCl4], and [κ2-TmBut]2GaI, the reactions of “GaI”, Ga[GaCl4] and (HGaCl2)2 yield compounds that feature Ga–Ga bonds, namely [TmBut]GaGaI3, [TmBut]GaGaCl3, {[TmBut]GaGa[TmBut]}I2, {[TmBut]GaGa[TmBut]}[GaCl4]2, {[TmBut]Ga(GaI2)Ga[TmBut]}I and {[TmBut]GaGa[TmBut]}{[μ-κ1,κ2-TmBut]GaI2GaI2GaI}2. These Ga–Ga bonded compounds may be formally regarded as donor–acceptor adducts between monovalent [TmBut]Ga and various trivalent moieties; for example, [TmBut]GaGaI3 may be described as an adduct of [TmBut]Ga and GaI3, while {[TmBut]Ga(GaI2)Ga[TmBut]}+ is an adduct between two molecules of [TmBut]Ga and [GaI2]+. Comparison of the structure of [TmBut]Ga→B(C6F5)3 with that of [TmBut]In→B(C6F5)3 indicates that [TmBut]Ga is a more effective Lewis base than is [TmBut]In.

X-ray diffraction studies demonstrate that oxidative addition of SiH4 to Ir(PPh3)2(CO)Cl yields Ir(PPh3)2(CO)(Cl)(SiH3)H, which features a cis arrangement of the SiH3 and H ligands in which H is located trans to CO, rather than trans to Cl as originally reported. 1H NMR spectroscopic studies indicate that oxidative addition of GeH4 to Ir(PPh3)2(CO)Cl also occurs in a cis manner but results in the formation of two isomers of Ir(PPh3)2(CO)(Cl)(GeH3)H, which are related by H being trans to either CO or Cl.

Tris(2-pyridonyl)methanes may be synthesized via the reactions of the respective 2-pyridone with CHX3 (X = Cl, Br) and K2CO3 in the presence of [Bun4N]Br, followed by acid-catalyzed isomerization with camphorsulfonic acid. These compounds provide access to a new class of alkyl ligands that feature oxygen donors and are capable of forming metallacarbatranes and a monovalent thallium alkyl compound.

The tris(mercaptoimidazolyl)hydroborato complexes, [κ3-S2H-TmBut]Na(THF)3 and [κ3-S2H-TmAd]Na(THF)3, which feature t-butyl and adamantyl substituents, have been synthesized via the reactions of the respective 1-R-1,3-dihydro-2H-imidazole-2-thiones with NaBH4 in THF (R = But, 1-Ad). X-ray diffraction studies indicate that the compounds are monomeric and that the [TmR] ligands coordinate to the metal in a κ3-S2H manner via two of the sulfur donors and the hydrogen attached to boron, a combination that is unprecedented for sodium derivatives. Analysis of the tris(mercaptoimidazolyl)hydroborato compounds that are listed in the Cambridge Structural Database has allowed for the formulation of a set of criteria that enables κx-Sx and κx+1-SxH coordination modes to be identified. Furthermore, the various κx-Sx and κx+1-SxH coordination modes have also been analyzed with respect to the conformations of the [TmR] ligands, which differ by rotation of the imidazolethione moieties about the B–N bond.

The tris(2-mercapto-1-methylbenzimidazolyl)hydroborato cadmium complexes, {[TmMeBenz]Cd(μ-Cl)}2 and [TmMeBenz]CdI, have been synthesized via the reactions of [TmMeBenz]K with CdCl2 and CdI2, respectively. While X-ray diffraction studies demonstrate that the iodide derivative, [TmMeBenz]CdI, is a monomer, the chloride derivative, {[TmMeBenz]Cd(μ-Cl)}2, exists as a dimer, which is unprecedented for Group 12 [TmR]MX (X = Cl, Br, I) compounds. Furthermore, the cadmium centers of {[TmMeBenz]Cd(μ-Cl)}2 are trigonal bipyramidal, which is an uncommon motif for cadmium complexes with a [S3Cl2] coordination sphere.

The benzannulated bis and tris(mercaptoimidazolyl)borohydride compounds, [BmMeBenz]Na and [TmMeBenz]Na, have been synthesized via the reactions of NaBH4 with two and three equivalents of 1-methyl-1,3-dihydro-2H-benzimidazole-2-thione, respectively. X-ray diffraction studies on the THF adducts, {μ-[BmMeBenz]Na(THF)2}2 and {[TmMeBenz]Na}2(μ-THF)3, indicate that both compounds are dinuclear but differ according to the nature of the bridging ligand. Specifically, {μ-[BmMeBenz]Na(THF)2}2 possesses bridging [BmMeBenz] ligands and terminal THF ligands, while {[TmMeBenz]Na}2(μ-THF)3 possesses terminal [TmMeBenz] ligands and bridging THF ligands. The tris(mercaptoimidazolyl)borohydride ligand of {[TmMeBenz]Na}2(μ-THF)3 coordinates in a κ3-manner, which is in marked contrast to the κ2-, κ1- and κ0-modes that have been reported for various [TmMe]Na derivatives. Density functional theory (DFT) geometry optimization calculations of the anions [TmMeBenz]− and [TmMe]− in the gas phase indicate that the conformation required for κ3-S3 coordination, i.e. one in which the three sulfur donors point away from the B–H group, is relatively more stable for [TmMeBenz]− than for [TmMe]−, and thus provides a rationalization for the observation that benzannulation enables κ3-coordination of tris(mercaptoimidazolyl)borohydride ligand in {[TmMeBenz]Na}2(μ-THF)3. Furthermore, comparison of the molecular structure and IR spectroscopic properties of [TmMeBenz]Re(CO)3 with those of [TmMe]Re(CO)3 indicates that benzannulation reduces the electron donating properties of the ligand, but has little effect on its steric properties. {μ-[BmMeBenz]Na(THF)2}2 and {[TmMeBenz]Na}2(μ-THF)3 react with [Me3PCuCl]4 to give [BmMeBenz]CuPMe3 and [TmMeBenz]CuPMe3, the first pair of structurally related bis and tris(mercaptoimidazolyl)hydroborato copper(I) compounds.

The carboxylate oxygen of thimerosal, [(ArCO2)SHgEt]Na, is subject to facile electrophilic attack by H+ and [HgEt]+ to give (ArCO2H)SHgEt and [(ArCO2HgEt)SHgEt]2, respectively. X-Ray diffraction demonstrates that (ArCO2H)SHgEt exists as a hydrogen bonded dimer in the solid state whereas [(ArCO2HgEt)SHgEt]2 is tetranuclear, with the mercury centers being connected by bridging carboxylate groups. 1H NMR spectroscopic studies indicate that the form of the 199Hg satellites of the ethyl group of (ArCO2H)SHgEt are dependent on the magnetic field, such that the inner pair of CH2 and CH3 satellites appear as a singlet at 400 MHz, as a consequence of 2JHg–H and 3JHg–H having opposite signs and the difference in chemical shifts of the central CH2 and CH3groups being equal to ½{|2JHg–H−3JHg–H|}.

The reactions of bis(mercaptoimidazolyl)hydroborato derivatives [BmR]M′ (R = Me, But; M′ = Li, Na, Tl) with MX3 trihalides of aluminium, gallium and indium yield both 1:1 and 2:1 complexes of the types [BmR]MX2 and [BmR]2MX, respectively. Structurally characterized examples of the [BmR]MX2 series include [BmMe]AlCl2, [BmMe]GaI2, [BmMe]InI2, [BmBut]AlCl2 and [BmBut]GaX2 (X = Cl, Br, I), while structurally characterized examples of the [BmR]2MX series include [BmBut]2InX (X = Cl, Br, I). In addition to the halide complexes, the trivalent dimethyl thallium complex [BmBut]TlMe2 has been synthesized via the reaction of [BmBut]Tl with Me2TlCl. The reactions of [BmR]M′ with the monovalent halides, “GaI”, InCl and InI, result in disproportionation. In the case of indium, the mononuclear complexes [BmBut]2InI and [BmBut]InCl(κ2-mimBut) are obtained, whereas for gallium, dinuclear compounds that feature Ga–Ga bonds, namely [BmR](GaI)(GaI)[BmR] (R = Me, But) have been isolated.

Linear nickel nitrosyl compounds supported by tridentate nitrogen and selenium ligands, namely the tris(3,5-dimethylpyrazolyl)hydroborato and tris(2-seleno-1-mesitylimidazolyl)hydroborato complexes, [TpMe2]NiNO and [TseMes]NiNO, have been synthesized and structurally characterized by X-ray diffraction. Computational studies demonstrate that the linear nitrosyl ligand behaves as a trivalent 3 ligand such that the Ni–N interaction has multiple bond character.