The kinetics and thermodynamics of concerted two-electron transfer and metal-metal bond cleavage in the binuclear phosphido-bridged complexes [M 2 (µ-PPh 2 ) 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 1°and 2°a re accompanied by an increase of 1.08 Å in M-M distance and expansion and contraction by 29°of the M-P-M and P-M-P angles, respectively, within an intact M 2 (µ-PPh 2 ) 2 unit. The one-electron electrode potentials of these systems are highly inverted: ∆E°′ ) E 2 °′ -E 1 °′ ) +0.17 V for 1 0/2and +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,1 ) 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 (σ*) 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 RSversus R 2 Pbridged species.

Temperature-dependent measurements of potential, E degrees', and electron-transfer rate constant, k(s,h), are reported for electrochemical reduction (in 0.3 M TBAPF(6)/CH(3)CN) of a series of oxomolybdenum(V) complexes, [(Tp)MoO(X,Y)], where Tp = hydrotris(3,5-dimethyl-1-pyrazolyl)borate and X,Y is a series of bidentate 1,2-disubstituted aliphatic or aromatic ligands in which oxygen donors are replaced sequentially by sulfur. E degrees' values shift in the positive direction, and k(s,h) values increase as O is replaced by S and as the framework of the ligand is changed from aliphatic to aromatic. The electrochemical enthalpy of activation, measured under conditions of zero driving force as DeltaH= -R partial differential[ln(k(s,h))]/ partial differential(1/T) and corrected for an outer-shell component by the mean spherical approximation, is approximately 10 kJ mol(-1) larger for complexes with O versus S donors and with an aliphatic versus aromatic ligand framework. Thus, the rate of Mo(V/IV) electron transfer is modulated primarily by differences in inner-shell reorganization. Following a recent description of electronic structure contributions to electron-transfer reactivity (Kennepohl, P.; Solomon, E. I. Inorg. Chem. 2003, 42, 679 ff), it is concluded that more effective charge distribution over the entire molecular structure, as mediated by electronic relaxation in S versus O and aromatic versus aliphatic systems, is responsible for the influence of ligand structure on the kinetics and thermodynamics of Mo-centered electron transfer. There is no evidence, based on experimentally measured pre-exponential factors, that sulfur donors or an aromatic ligand framework are more effective than their structural counterparts in facilitating electronic coupling between the electrode and the Mo d(xy) redox orbital.

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