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 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 solely from electronic differences between chlorine and phosphine donors. Due to inversion of the delta and delta* orbitals, the complexes M2Cl6(PH3)(4) and 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 are 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 phosphine both favor stronger metal-metal bonding in the phosphine-based complexes. The strong linear dependence observed between Delta E-spe and Delta E-ovlp enables the metal-metal bonding in these systems to be predicted simply from single-ion spin-polarization energies. The antiferromagnetic coupling 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 contribution from the pi electrons, particularly for the dimolybdenum complex.