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

Abstract

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.

Files

Additional details