Metal-metal bonding trends in mixed-group, face-shared d3d3 bioctahedral dimer systems, M′M"Cl9n-

The results of density functional theory (DFT) calculations on a set of binuclear nonachloride complexes M'M"Cl-9(4-) (M' = V, Nb, Ta; M" = Cr, Mo, W) and M'M"Cl-9(2-) (M' = Cr, Mo, W; M" = Mn, Tc, Re), in which each metal possesses a nominal d(3) valence electronic configuration, are reported. When compared with previous studies on same-group dimers (typified by the M'M"Cl-9(3-) complexes of the chromium triad), the present results display an increased tendency for electron donation from M' to M". Structural trends evident for the M'M"Cl-9(4-) and M'M"Cl-9(2-) series of dimers are remarkably consistent: weak ferromagnetic coupling between M' and M" is the most favorable intermetallic interaction when M" is a first-row transition metal, antiferromagnetic coupling dominates when M' is first-row but M" is second- or third-row, and metal-metal triple bond formation generally yields the lowest-energy structure when neither metal is first-row. These structural trends, and other characteristics of the dimers described here, can be satisfactorily rationalized in terms of the tendency for electron transfer from M' to M", coupled with effects due to spin polarization and ligand field splitting of the valence d orbitals on M' and M". (C) 2002 Elsevier Science Ltd. All rights reserved.