Electrochemical Assessment of the Stability of [Os(NH_3)_5(OH_2)]^(2+) in Aqueous Solutions

The [Os(NH_3)_5(OH_2)]^(2+) complex in aqueous acid has been widely regarded as short-lived on the basis of results reported in an early pioneering electrochemical study.1 More recently, a much longer-lived Os(II) species generated by reduction of [Os(NH)3))5(OH)2)]^(3+) but of uncertain composition has been reported. In the present study the lifetime of [Os(NH_3)_5(OH_2)]^(2+) in aqueous acid was determined by steady-state voltammetry at a carbon microelectrode and by comparing experimental with simulated cyclic voltammograms obtained for solutions of [Os(NH_3)_5(OH_2)]^(3+). Rate constants for the oxidation of [Os(NH_3)_5(OH_2)]^(2+) by H^+, CH_3COOH (HA), and H_2O were evaluated as k_H = 17 M^(-1) s^(-1), k_(HA) = 1.2 M^(-1) s^(-1), and k_(H_2O) = 4.1 × 10^(-4) s^(-1). These values are shown to be compatible with the data in ref 1 and come close to accounting for the long-lived species described in ref 6 which may be [Os(NH_3)_5(OH_2)]^(2+) itself. A revised value of 4.85 was measured for the pKA of [Os(NH_3)_5(OH_2)]^(3+). Adsorption of an Os complex on the surface of mercury, but not glassy carbon, electrodes is shown to enhance greatly the rate with which protons undergo direct electroreduction