The kinetics and thermodynamics of concerted two-electron transfer and metal-metal bond cleavage in the binuclear phosphido-bridged complexes [M-2(mu-PPh2)(2)(CO)(8)](0/2-) [M = Mo (1(0/2-)), W(2(0/2-))] have been determined by variable scan-rate cyclic voltammetry in 0.3 M TBAPF(6)/acetone. The reductions of 1degrees and 2degrees are accompanied by an increase of 1.08 Angstrom in M-M distance and expansion and contraction by 29degrees of the M-P-M and P-M-P angles, respectively, within an intact M-2(mu-PPh2)(2) unit. The one-electron electrode potentials of these systems are highly inverted: DeltaE(degrees)' = Edegrees'(2) - E(1)degrees' = +0.17 V for 1(0/2-) and +0.18 V for 2(0/2-), and the rate constant of the second heterogeneous electron-transfer reaction is smaller than the first: k(sh,2)/k(sh,l) = 0- 1 for Mo and 0.018 for W. Results are consistent with progressive cleavage of the metal-metal bond in two one-electron steps, of which the second is rate-limiting, because it is accompanied by a larger part of the structural change. EHMO calculations reveal that the redox-active orbital is a metal-metal antibonding (sigma*) orbital with substantial bridging-ligand character that decreases markedly in energy on passing from the metal-metal bonded M(I)(2) state to nonbonded M(0)(2). Despite this feature, electron-transfer thermodynamics and kinetics are not significantly metal-dependent. Rather, comparisons with structurally similar sulfido-bridged complexes reveal that electron-transfer energetics are influenced more extensively by the bridging ligand, with more positive potentials and larger electron-transfer rates observed for RS- versus R2P- bridged species.