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 [M2(μ-PPh2)2(CO)8]0/2- [M = Mo (10/2-), W(20/2-)] have been determined by variable scan-rate cyclic voltammetry in 0.3 M TBAPF6/acetone. The reductions of 1° and 2° are accompanied by an increase of 1.08 A in M−M distance and expansion and contraction by 29° of the M−P−M and P−M−P angles, respectively, within an intact M2(μ-PPh2)2 unit. The one-electron electrode potentials of these systems are highly inverted: ΔE°‘ = − = +0.17 V for 10/2- and +0.18 V for 20/2-, and the rate constant of the second heterogeneous electron-transfer reaction is smaller than the first: ksh,2/ksh,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 i...