Electronic structure and metal-metal bonding in nominal d³d³ M^IIM^IVCL₉^3- (M^II = V, Nb, Ta; M^IV = Mn, Tc, Re) face-shared binuclear complexes

Density functional theory (DFT) calculations have been employed to study structural and electronic configuration trends in the series of mixed-group, face-shared, bimetallic complexes M′M″Cl₉^3- (M′ = V, Nb, Ta; M″ = Mn, Tc, Re), in which each metal possesses a nominal d³ valence electronic configuration. While the tendency of complexes to exhibit either multiple metal-metal bonded structures (with short intermetallic separations) or weakly-coupled systems (characterized by large intermetallic distances) is broadly consistent with that seen in our earlier studies on same-group dimers (e.g. Cr₂Cl₉^3-), there are also several novel structural and electronic effects which are directly attributable to electron transfer from M^II to M^IV. The general tendency towards intermetallic electron transfer is well modelled by a simple expression involving the spin polarization energy of each metal, and the ligand-field splitting of t_2g and e_g orbitals, in the corresponding d³-valence hexachloro octahedral complex. The effects of this electron transfer include, in some instances, a preference towards ferromagnetic coupling between metal atoms; diminished barriers to complex dissociation; and formation of edge-shared dimers with one five-coordinate metal atom. The heavier congeners, i.e. those lacking V and Mn, are predicted to have strong multiple metal-metal bonds with significant barriers to dissociation.