The discovery of a diiron organometallic site in nature within the diiron hydrogenase, [FeFe]-H2ase, active site has prompted revisits of the classic organometallic chemistry involving the Fe–Fe bond and bridging ligands, particularly of the (μ-SCH2XCH2S)[Fe(CO)3]2 and (μ-SCH2XCH2S)[Fe(CO)2L]2 (X = CH2, NH; L = PMe3, CN–, and NHC’s (NHC = N-heterocyclic carbene)), derived from CO/L exchange reactions. Through the synergy of synthetic chemistry and density functional theory computations, the regioselectivity of nucleophilic (PMe3 or CN–) and electrophilic (nitrosonium, NO+) ligand substitution on the diiron dithiolate framework of the (μ-pdt)[Fe(CO)2NHC][Fe(CO)3] complex (pdt = propanedithiolate) reveals the electron density shifts in the diiron core of such complexes that mimic the [FeFe]-H2ase active site. While CO substitution by PMe3, followed by reaction with NO+, produces (μ-pdt)(μ-CO)[Fe(NHC)(NO)][Fe(CO)2PMe3]+, the alternate order of reagent addition produces the structural isomer (μ-pdt)[Fe(NHC)(NO)PMe3][Fe(CO)3]+, illustrating how the nucleophile and electrophile choose the electron-poor metal and the electron-rich metal, respectively. Theoretical explorations of simpler analogues, (μ-pdt)[Fe(CO)2CN][Fe(CO)3]−, (μ-pdt)[Fe(CO)3]2, and (μ-pdt)[Fe(CO)2NO][Fe(CO)3]+, provide an explanation for the role that the electron-rich iron moiety plays in inducing the rotation of the electron-poor iron moiety to produce a bridging CO ligand, a key factor in stabilizing the electron-rich iron moiety and for support of the rotated structure as found in the enzyme active site.

The heavy-atom heterocycle Pd[Re2(CO)8(μ-SbPh2)(μ-H)]2 (5) has been synthesized by the palladium-catalyzed ring-opening cyclodimerization of the three-membered heterocycle Re2(CO)8(μ-SbPh2)(μ-H) (3). The Pd atom occupies the center of the ring. The Pd atom in 5 can be removed reversibly to yield the palladium-free heterocycle [Re2(CO)8((μ-SbPh2)(μ-H)]2 (6).

•2+2 cycloaddition to OsSiR is under kinetic control.•Kinetic control is due to sterics.•Product structures confirmed by computed NMR chemical shift.The [2+2] cycloaddition reactivity of an osmium silylyne compound 2, [Cp∗(iPr3P)(H)OsSi(Trip)][HB(C6F5)3], with PhCCPh or PCtBu was studied by density functional theory (DFT) computations. Results indicate that these two reactions are under kinetic control in room temperature and the kinetic products were detected by NMR in experiments. Further tests revealed that the interplay between steric effects and the bonding interactions between two reacting fragments is majorly responsible for the energy differences in different isomers.The [2+2] cycloaddition reactivity of an osmium silylyne compound 2, [Cp∗(iPr3P)(H)OsSi(Trip)][HB(C6F5)3], with PhCCPh or PCtBu was studied by density functional theory (DFT) computations. These two reactions appear to be under kinetic control due to complex interplay between steric effects and the bonding interactions between two reacting fragments.

To explore the effect of delocalization in the Fe(NO)2 unit on possible linkage isomerism of ambidentate ECN– ligands, E = S and O, anionic DNICs, dinitrosyl iron complexes, (SCN)2Fe(NO)2– (1) and (OCN)2Fe(NO)2– (2) were synthesized by the reaction of in situ-generated [Fe(CO)2(NO)2]+ and PPN+ECN–. Other {Fe(NO)2}9 (Enemark–Feltham notation) complexes, (N3)2Fe(NO)2– and (PhS)2Fe(NO)2–, were prepared for comparison. The X-ray diffraction analysis of 1 and 2 yielded the typical tetrahedral structures of DNICs with two slightly bent Fe–N–O oriented toward each other, and linear FeNCE units. The ν(NO) IR values shift to lower values for 1 > 2 > (N3)2Fe(NO)2– > (PhS)2Fe(NO)2–, reflecting the increasing donor ability of the ancillary ligands and consistent with the redox potentials of the complexes, and the small trends in Mössbauer isomer shifts. Computational studies corroborate that the {Fe(NO)2}9 motif prefers N-bound rather than E-bound isomers. The calculated energy differences between the linkage isomers of 1 (Fe-NCS preferred over Fe-SCN by about 6 kcal/mol) are smaller than those of 2 (Fe-NCO preferred over Fe-OCN by about 16 kcal/mol), a difference that is justified by the frontier molecular orbitals of the ligands themselves.

The binding of an alkene by Ni(tfd)2 [tfd = S2C2(CF3)2] is one of the most intriguing ligand-based reactions. In the presence of the anionic, reduced metal complex, the primary product is an interligand adduct, while in the absence of the anion, dihydrodithiins and metal complex decomposition products are preferred. New kinetic (global analysis) and computational (DFT) data explain the crucial role of the anion in suppressing decomposition and catalyzing the formation of the interligand product through a dimetallic complex that appears to catalyze alkene addition across the Ni–S bond, leading to a lower barrier for the interligand adduct.