Metal-metal bonding in M2Cl6(H2PCH2PH2)(2), M2Cl6(PH3)(4), and M2Cl104-(M = Cr, Mo, W) edge-shared dimer systems

Density functional theory is used to determine the electronic structures, g eometries, and periodic trends in metal-metal bonding in the homo- and hete robimetallic d(3)d(3) edge-shared systems M2Cl104-, M2Cl6(PH3)(4), and M-2- Cl-6(H2PCH2PH2)(2) (M = Cr, Mo, W). The much shorter metal-metal distances in these complexes relative to M2Cl104- (M = MO, W) are shown to arise sole ly from electronic differences between chlorine and phosphine donors. Due t o inversion of the delta and delta* orbitals, the complexes M2Cl6(PH3)(4) a nd M2Cl6(H2PCH2PH2)(2) (M = Mo, W) are found to possess formal metal-metal double bonds. The periodic trends in metal-metal bonding in these systems a re rationalized in terms of the energetic contributions of orbital overlap (Delta E-ovlp) and spin polarization (Delta E-spe). The reduction in Delta E-spe and increase in Delta E-ovlp On replacement of axial chlorides with p hosphine both favor stronger metal-metal bonding in the phosphine-based com plexes. The strong linear dependence observed between Delta E-spe and Delta E-ovlp enables the metal-metal bonding in these systems to be predicted si mply from single-ion spin-polarization energies. The antiferromagnetic coup ling in M2Cl6(H2PCH2PH2)(2) (M = Mo, Wr) and MoWCl6(H2PCH2PH2)(2) is shown to be mostly due to coupling of the metal d electrons, with a smaller contr ibution from the pi electrons, particularly for the dimolybdenum complex.