Kinetics and mechanism of oxidative cleavage of the metal-metal bond in [M 2(CO) 10] 2−, MCr, Mo and W

Abstract

Electrochemically-induced metal-metal bond cleavage in [M 2(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[M 2(CO) 10] ·− to form [M 2(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 × 10 3 M −1 s −1 for [W 2(CO) 10] ·− and 5.9 ± 0.3 × 10 2 M −1 s −1 for [Mo 2(CO) 10] ·− in THF at 20.0 °C. The [Cr 2(CO) 10] ·− radical underwent disproportionation too slowly to observe at room temperature, but a value of k d ⩽ 1.7 × 10 2 M −1 s −1 was estimated to be the upper limit of the rate constant at 35.0 °C. Activation parameters for k d were determined. For [Mo 2(CO) 10] ·−, Δ H ‡=6.9 ± 0.5 kcal mol −1 and Δ S ‡=−22.4 ± 1.9 cal K −1 mol −1. For [W 2(CO) 10] ·−, Δ H ‡=3.3 ± 0.5 kcal mol −1 and Δ S ‡=−31.7 ± 0.5 cal K −1 mol −1. The relative reactivities of the radicals W > Mo ⪢ Cr parallel the driving force for electron transfer, as measured by the potential difference for the oxidation of [M 2(CO) 10] 2− and [M 2(CO) 10] ·−. The k d step is proposed to involve outer-sphere electron transfer.