ELECTROCHEMICAL ASSESSMENT OF THE STABILITY OF [OS(NH3)(5)(OH2)](2-SOLUTIONS() IN AQUEOUS)

The [Os(NH3)(5)(OH2)](2+) complex in aqueous acid has been widely rega rded as short-lived on the basis of results reported in an early pione ering electrochemical study.(1) More recently, a much longer-lived Os( II) species generated by reduction of [Os(NH3)(5)(OH2)](3+) but of unc ertain composition has been reported.(6) In the present study the life time of [Os(NH3)(5)(OH2)](2+) in aqueous acid was determined by steady -state voltammetry at a carbon microelectrode and by comparing experim ental with simulated cyclic voltammograms obtained for solutions of [O s(NH3)(5)(OH2)](3+). Rate constants for the oxidation of [Os(NH3)(5)(O H2)](2+) by H+, CH3COOH (HA), and H2O were evaluated as k(H) = 17 M-1 s(-1), k(HA) = 1.2 M-1 s(-1), and k(H2O) = 4.1 x 10(-4) S-1. These val ues are shown to be compatible with the data in ref 1 and come close t o accounting for the long-lived species described in ref 6 which may b e [Os(NH3)(5)(OH2)](2+) itself. A revised value of 4.85 was measured f or the pK(A) of [OS(NH3)(5)(OH2)(3+). Adsorption of an Os complex on t he surface of mercury, but not glassy carbon, electrodes is shown to e nhance greatly the rate with which protons undergo direct electroreduc tion.