KINETICS AND MECHANISM OF OXIDATIVE CLEAVAGE OF THE METAL METAL BOND IN [M2(CO)10]2-, M=CR, MO AND W

Electrochemically-induced metal-metal bond cleavage in [M2(CO)10]2- (M = Cr, Mo, W) has been studied using cyclic voltammetry, bulk electrolysis and double potential step chronocoulometry. Metal-metal bond homolysis, which occurs on oxidation in THF, can be modeled with an EC second-order disproportionation mechanism. The key step involves reaction of 2[M2(CO)10].- to form [M2(CO)10]2- and M(CO)5(THF). Electron transfer rate constants for the homogeneous disproportionation step, k(d), were determined by double potential step chronocoulometry: 2.5 +/- 0.1 x 10(3) m-1 s-1 for [W2(CO)10].- and 5.9 +/-0.3 x 10(2) M-1 s-1 for [Mo2(CO)10].- in THF at 20.0-degrees-C. The [Cr2(CO)10].- radical underwent disproportionation too slowly to observe at room temperature, but a value of k(d) less-than-or-equal-to 1.7 x 10(2) M-1 s-1 was estimated to be the upper limit of the rate constant at 35.0-degrees-C. Activation parameters for k(d) were determined. For [Mo2(CO)10].-, DELTA-H(double dagger) = 6.9 +/- 0.5 kcal mol-1 and DELTA-S(double dagger) = - 22.4 +/- 1.9 cal K-1 mol-1. For [W2(CO)10].-, DELTA-H(double dagger) = 3.3 +/- 0.5 kcal mol-1 and DELTA-S(double dagger) = -31.7 +/- 0.5 cal K-1 mol-1. The relative reactivities of the radicals W > Mo much greater than Cr parallel the driving force for electron transfer, as measured by the potential difference for the oxidation Of [M2(CO)10]2- and [M2(CO)10].-. The k(d) step is proposed to involve outer-sphere electron transfer.