A series of new germylene compounds has been synthesized offering systematic variation in the σ- and π-capabilities of the α-substituent and differing levels of reactivity towards E−H bond activation (E=H, B, C, N, Si, Ge). Chloride metathesis utilizing [(terphenyl)GeCl] proves to be an effective synthetic route to complexes of the type [(terphenyl)Ge(ER_n)] (1–6: ER_n=NHDipp, CH(SiMe₃)₂, P(SiMe₃)₂, Si(SiMe₃)₃ or B(NDippCH)₂; terphenyl=C₆H₃Mes₂-2,6=Ar^Mes or C₆H₃Dipp₂-2,6=Ar^Dipp; Dipp=C₆H₃iPr₂-2,6, Mes=C₆H₂Me₃-2,4,6), while the related complex [{(Me₃Si)₂N}Ge{B(NDippCH)₂}] (8) can be accessed by an amide/boryl exchange route. Metrical parameters have been probed by X-ray crystallography, and are consistent with widening angles at the metal centre as more bulky and/or more electropositive substituents are employed. Thus, the widest germylene units (θ>110°) are found to be associated with strongly σ-donating boryl or silyl ancillary donors. HOMO–LUMO gaps for the new germylene complexes have been appraised by DFT calculations. The aryl(boryl)-germylene system [Ar^MesGe{B(NDippCH)₂}] (6-Mes), which features a wide C-Ge-B angle (110.4(1)°) and (albeit relatively weak) ancillary π-acceptor capabilities, has the smallest HOMO–LUMO gap (119 kJ mol⁻¹). These features result in 6-Mes being remarkably reactive, undergoing facile intramolecular C−H activation involving one of the mesityl ortho-methyl groups. The related aryl(silyl)-germylene system, [Ar^MesGe{Si(SiMe₃)₃}] (5-Mes) has a marginally wider HOMO–LUMO gap (134 kJ mol⁻¹), rendering it less labile towards decomposition, yet reactive enough to oxidatively cleave H₂ and NH₃ to give the corresponding dihydride and (amido)hydride. Mixed aryl/alkyl, aryl/amido and aryl/phosphido complexes are unreactive, but amido/boryl complex 8 is competent for the activation of E−H bonds (E=H, B, Si) to give hydrido, boryl and silyl products. The results of these reactivity studies imply that the use of the very strongly σ-donating boryl or silyl substituents is an effective strategy for rendering metallylene complexes competent for E−H bond activation.

Anion receptors based on a [CpFe(indenyl)] scaffold offer the possibility for the incorporation of adjacent Lewis acidic functions onto a six-membered carbocyclic framework, while at the same time retaining the colorimetric/electrochemical reporter mechanisms available to synthetically simpler ferrocene systems. Thus, [CpFe(indenyl)] systems featuring mutually ortho BMes₂ and PPh₂Me⁺ substituents (with either 4,5 or 5,6 regiochemistry) are accessible which are capable of cooperative fluoride ion fixation. Simultaneous binding at the borane and phosphonium centres can be established by spectroscopic, structural and computational approaches, and is responsible for the favourable thermodynamics associated with F⁻ uptake. Thus, in contrast to simple BMes₂ systems, the binding of fluoride is found to be more favourable than the uptake of cyanide (which interacts only with the borane Lewis acid). Moreover, in the case of a 4-(MePh₂P)-5-(Mes₂B)-7-Me-indenyl derivative, fluoride chelation is signalled not only by a large cathodic shift in the Fe^II/Fe^III potential (>500 mV in THF), but also by a distinct colour change from green (for the free receptor) to maroon for the adduct.

A single-component ambiphilic system capable of the cooperative activation of protic, hydridic and apolar HX bonds across a Group 13 metal/activated β-diketiminato (Nacnac) ligand framework is reported. The hydride complex derived from the activation of H₂ is shown to be a competent catalyst for the highly selective reduction of CO₂ to a methanol derivative. To our knowledge, this process represents the first example of a reduction process of this type catalyzed by a molecular gallium complex.

The synthesis of a phosphorus(III) compound bearing a N,N-bis(3,5-di-tert-butyl-2-phenoxy)amide ligand is reported. This species has been found to react with ammonia and water, activating the EH bonds in both substrates by formal oxidative addition to afford the corresponding phosphorus(V) compounds. In the case of water, both OH bonds can be activated, splitting the molecule into its constituent elements. To our knowledge, this is the first example of a compound based on main group elements that sequentially activates water in this manner.

The synthesis of a phosphorus(III) compound bearing a N,N-bis(3,5-di-tert-butyl-2-phenoxy)amide ligand is reported. This species has been found to react with ammonia and water, activating the EH bonds in both substrates by formal oxidative addition to afford the corresponding phosphorus(V) compounds. In the case of water, both OH bonds can be activated, splitting the molecule into its constituent elements. To our knowledge, this is the first example of a compound based on main group elements that sequentially activates water in this manner.

A single-component ambiphilic system capable of the cooperative activation of protic, hydridic and apolar HX bonds across a Group 13 metal/activated β-diketiminato (Nacnac) ligand framework is reported. The hydride complex derived from the activation of H₂ is shown to be a competent catalyst for the highly selective reduction of CO₂ to a methanol derivative. To our knowledge, this process represents the first example of a reduction process of this type catalyzed by a molecular gallium complex.

We have investigated the fundamental patterns of reactivity for the cationic Ir^III complexes [(6-Mes)(6-Mes′)Ir(H)]⁺ and [(7-Mes)(7-Mes′)Ir(H)]⁺ [6-Mes, 7-Mes = ring-expanded N-heterocyclic carbene (NHC) ligands, ′ indicates that NHC is tethered through an iridium-bound CH₂ group], each of which undergoes degenerate fluxional exchange between an iridium-bound agostic C–H bond and the discrete alkyl/hydride ligands resulting from C–H activation. A comparison between the activation parameters ΔH^‡ and ΔS^‡ determined by variable-temperature NMR (VT-NMR) spectroscopy line-shape analysis and those obtained from DFT calculations on model systems offers a basis to discriminate between different possible exchange mechanisms, and an oxidative addition pathway gives the lowest barrier and also the best agreement between experiment and theory. The addition of neutral Lewis base donors to [(6-Mes)(6-Mes′)Ir(H)]⁺ resulted in the displacement of the weak agostic C–H···Ir interaction and the generation of static (alkyl)iridium(III) hydride complexes such as [(6-Mes)(6-Mes′)Ir(H)(CNtBu)₂]⁺. The analogous carbonyl complex is more labile, with the enhanced π-acceptor properties of the ancillary ligands leading to C–H reductive elimination and to the formation of the Ir^I complex [(6-Mes)₂Ir(CO)₂]⁺, which incorporates unactivated NHC donors and has the ability to reversibly take up H₂.

The modes of interaction of donor-stabilized Group 13 hydrides (E=Al, Ga) were investigated towards 14- and 16-electron transition-metal fragments. More electron-rich N-heterocyclic carbene-stabilized alanes/gallanes of the type NHC⋅EH₃ (E=Al or Ga) exclusively generate κ² complexes of the type [M(CO)₄(κ²-H₃E⋅NHC)] with [M(CO)₄(COD)] (M=Cr, Mo), including the first κ² σ-gallane complexes. β-Diketiminato (′nacnac′)-stabilized systems, {HC(MeCNDipp)₂}EH₂, show more diverse reactivity towards Group 6 carbonyl reagents. For {HC(MeCNDipp)₂}AlH₂, both κ¹ and κ² complexes were isolated, while [Cr(CO)₄(κ²-H₂Ga{(NDippCMe)₂CH})] is the only simple κ² adduct of the nacnac-stabilized gallane which can be trapped, albeit as a co-crystallite with the (dehydrogenated) gallylene system [Cr(CO)₅(Ga{(NDippCMe)₂CH})]. Reaction of [Co₂(CO)₈] with {HC(MeCDippN)₂}AlH₂ generates [(OC)₃Co(μ-H)₂Al{(NdippCme)₂CH}][Co(CO)₄] (12), which while retaining direct AlH interactions, features a hitherto unprecedented degree of bond activation in a σ-alane complex.

Strategies for the synthesis of highly electrophilic Au^I complexes from either hydride- or chloride-containing precursors have been investigated by employing sterically encumbered Dipp-substituted expanded-ring NHCs (Dipp=2,6-iPr₂C₆H₃). Thus, complexes of the type (NHC)AuH have been synthesised (for NHC=6-Dipp or 7-Dipp) and shown to feature significantly more electron-rich hydrides than those based on ancillary imidazolylidene donors. This finding is consistent with the stronger σ-donor character of these NHCs, and allows for protonation of the hydride ligand. Such chemistry leads to the loss of dihydrogen and to the trapping of the [(NHC)Au]⁺ fragment within a dinuclear gold cation containing a bridging hydride. Activation of the hydride ligand in (NHC)AuH by B(C₆F₅)₃, by contrast, generates a species (at low temperatures) featuring a [HB(C₆F₅)₃]⁻ fragment with spectroscopic signatures similar to the “free” borate anion. Subsequent rearrangement involves BC bond cleavage and aryl transfer to the carbophilic metal centre. Under halide abstraction conditions utilizing Na[BAr^f₄] (Ar^f=C₆H₃(CF₃)₂-3,5), systems of the type [(NHC)AuCl] (NHC=6-Dipp or 7-Dipp) generate dinuclear complexes [{(NHC)Au}₂(μ-Cl)]⁺ that are still electrophilic enough at gold to induce aryl abstraction from the [BAr^f₄]⁻ counterion.

The consequences of extremely high steric loading have been probed for late transition metal complexes featuring the expanded ring N-heterocyclic carbene 6-Dipp. The reluctance of this ligand to form 2:1 complexes with d-block metals (rationalised on the basis of its percentage buried volume, % V_bur, of 50.8 %) leads to CH and CN bond activation processes driven by attack at the backbone β-CH₂ unit. In the presence of Ir^I (or indeed H⁺) the net result is the formation of an allyl formamidine fragment, while Au^I brings about an additional ring (re-)closure step via nucleophilic attack at the coordinated alkene. The net transformation of 6-Dipp in the presence of [(6-Dipp)Au]⁺ represents to our knowledge the first example of backbone CH activation of a saturated N-heterocyclic carbene, proceeding in this case via a mechanism which involves free carbene in addition to the Au^I centre.

The ability of substituted carbazol-9-yl systems to ligate in σ fashion through the amido N-donor, or to adopt alternative coordination modes through the π system of the central five-membered ring, can be tuned by systematic variation in the steric demands of substituents in the 1- and 8-positions. The differing affinities of the two modes of coordination for hard and soft metal centres can be shown to influence not only cation selectivity, but also the redox properties of the metal centre. Thus, the highly sterically sterically demanding 1,3,6,8-tetra-tert-butylcarbazolyl ligand can be used to generate the structurally characterised amido-indium(I) complex, [{(tBu₄carb)In}_n], (together with its isostructural thallium counterpart) in which the metal centre interacts with the central pyrrolyl ring in η³ fashion [d(InN)=2.679(3) Å; d(InC)=2.819(3), 2.899(3) Å]. By contrast, the smaller 3,6-di-tert-butylcarbazolyl system is less able to restrict the metal centre from binding at the anionic nitrogen donor in the plane of the carbazolyl ligand (i.e. in σ fashion). Analogous chemistry with In^I precursors therefore leads to disproportionation to the much harder In^II [and In⁰], and the formation of the mixed-valence product, [In₂{In₂(tBu₂carb)₆}], a homoleptic molecular [In₄(NR₂)₆] system. This chemistry reveals a flexibility of ligation for carbazolyl systems that contrasts markedly with that of the similarly sterically encumbered terphenyl ligand family.