Metal-Metal Bonding in M(2)Cl(6)(H(2)PCH(2)PH(2))(2), M(2)Cl(6)(PH(3))(4), and M(2)Cl(10)(4-) (M = Cr, Mo, W) Edge-Shared Dimer Systems.

Density functional theory is used to determine the electronic structures, geometries, and periodic trends in metal-metal bonding in the homo- and heterobimetallic d(3)d(3) edge-shared systems M(2)Cl(10)(4-), M(2)Cl(6)(PH(3))(4), and M(2)Cl(6)(H(2)PCH(2)PH(2))(2) (M = Cr, Mo, W). The much shorter metal-metal distances in these complexes relative to M(2)Cl(10)(4-) (M = Mo, W) are shown to arise solely from electronic differences between chlorine and phosphine donors. Due to inversion of the delta and delta orbitals, the complexes M(2)Cl(6)(PH(3))(4) and M(2)Cl(6)(H(2)PCH(2)PH(2))(2) (M = Mo, W) are found to possess formal metal-metal double bonds. The periodic trends in metal-metal bonding in these systems are rationalized in terms of the energetic contributions of orbital overlap (DeltaE(ovlp)) and spin polarization (DeltaE(spe)). The reduction in DeltaE(spe) and increase in DeltaE(ovlp) on replacement of axial chlorides with phosphine both favor stronger metal-metal bonding in the phosphine-based complexes. The strong linear dependence observed between DeltaE(spe) and DeltaE(ovlp) enables the metal-metal bonding in these systems to be predicted simply from single-ion spin-polarization energies. The antiferromagnetic coupling in M(2)Cl(6)(H(2)PCH(2)PH(2))(2) (M = Mo, W) and MoWCl(6)(H(2)PCH(2)PH(2))(2) is shown to be mostly due to coupling of the metal delta electrons, with a smaller contribution from the pi electrons, particularly for the dimolybdenum complex