Abstract
The [Os(NH3)5(OH2)]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(NH3)5(OH2)]3+ but of uncertain composition has been reported.6 In the present study the lifetime of [Os(NH3)5(OH2)]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(NH3)5(OH2)]3+. Rate constants for the oxidation of [Os(NH3)5(OH2)]2+ by H+, CH3COOH (HA), and H2O were evaluated as kH = 17 M-1 s-1, kHA = 1.2 M-1 s-1, and kH2O = 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(NH3)5(OH2)]2+ itself. A revised value of 4.85 was measured for the pKA of [Os(NH3)5(OH2)]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.
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