Energetics of Concerted Two-Electron Transfer and Metal−Metal Bond Cleavage in Phosphido-Bridged Molybdenum and Tungsten Carbonyl Complexes

The kinetics and thermodynamics of concerted two-electron transfer and metal−metal bond cleavage in the binuclear phosphido-bridged complexes [M<INF>2</INF>(μ-PPh<INF>2</INF>)<INF>2</INF>(CO)<INF>8</INF>]^0/2- [M = Mo (<BO>1</BO>^0/2-), W(<BO>2</BO>^0/2-)] have been determined by variable scan-rate cyclic voltammetry in 0.3 M TBAPF<INF>6</INF>/acetone. The reductions of <BO>1</BO>° and <BO>2</BO>° are 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<INF>2</INF>(μ-PPh<INF>2</INF>)<INF>2</INF> unit. The one-electron electrode potentials of these systems are highly inverted:  ΔE°‘ = <EQUATION><EQNDATA>%@mt;sys@%%@ital@%E%@rsf@%%@sb@%2%@sbx@%%@mh;-2.5q@%°‘%@mx@%</EQNDATA> </EQUATION> − <EQUATION><EQNDATA>%@mt;sys@%%@ital@%E%@rsf@%%@sb@%1%@sbx@%%@mh;-2.5q@%°‘%@mx@%</EQNDATA> </EQUATION> = +0.17 V for <BO>1</BO>^0/2- and +0.18 V for <BO>2</BO>^0/2-, and the rate constant of the second heterogeneous electron-transfer reaction is smaller than the first:  k<INF>sh,2</INF>/k<INF>sh,1</INF> = 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)<INF>2</INF> state to nonbonded M(0)<INF>2</INF>. 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 R<INF>2</INF>P^- bridged species.