The electrochemistry of Os(III) ammines

Electrochemical Assessment of the Stability of [Os(NH3)5(OH2)]2+ in Aqueous Solutions

“…(2−5) × 10 5 M -1 s -1 were obtained from polarographic and cyclic voltammetric data obtained with strongly buffered solutions of [Os(NH 3 ) 5 (OH 2 )] 3+ . Subsequent workers have relied upon this report in analyzing the behavior of solutions of [Os(NH 3 ) 5 (OH 2 )] 2+ . − In one recent report, evidence was presented in support of the proposal that an Os(II) complex produced by the reduction of [Os(NH 3 ) 5 (OTf)] 2+ (OTf - = CF 3 SO 3 - ) with Zn(Hg) in aqueous solutions survived for times much longer than would be compatible with the rate constant found by Gulens and Page . It was concluded that a complex of Os(II) that was not [Os(NH 3 ) 5 (OH 2 )] 2+ must have been formed to account for the observed longevity of the reduced complex …”

“…Our objectives were to employ electrochemical methods to measure the lifetime of the reduced complex by taking advantage of steady-state voltammetry at microelectrodes and by utilizing a commercially available simulation program to analyze cyclic voltammograms . The results reveal that [Os(NH 3 ) 5 (OH 2 )] 2+ reacts with protons at a much lower rate than suggested by the results of Gulens and Page . In addition, we obtained a revised value for the p K A of [Os(NH 3 ) 5 (OH 2 )] 3+ (4.85) that is somewhat lower than the value estimated in ref (5.3) and regularly quoted in the subsequent literature.…”

“…Rate constants for the oxidation of [Os(NH 3 ) 5 (OH 2 )] 2+ by H + , CH 3 COOH (HA), and H 2 O were evaluated as k H = 17 M -1 s -1 , k HA = 1.2 M -1 s -1 , and k H 2 O = 4.1 × 10 -4 s -1 . These values are shown to be compatible with the data in ref and come close to accounting for the long-lived species described in ref which may be [Os(NH 3 ) 5 (OH 2 )] 2+ itself. A revised value of 4.85 was measured for the p K A of [Os(NH 3 ) 5 (OH 2 )] 3+ .…”

“…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 .…”

“…In fact, additional voltammetric data reported in the same study with unbuffered solutions of [Os(NH 3 ) 5 (OH) 2 ] 3+ (but not analyzed to obtain a rate constant) are not compatible with the high values of the rate constant ((2−5) × 10 5 M -1 s -1 ) obtained in acetate buffers. Much smaller rate constants for the oxidation of [Os(NH 3 ) 5 OH 2 ] 2+ by protons result if these data are analyzed by means of digital simulation procedures that were unavailable at the time that Gulens and Page conducted their experiments , we decided to re-examine the electrochemistry of [Os(NH 3 ) 5 (OH 2 )] 3+ and [Os(NH 3 ) 5 (OH 2 )] 2+ in aqueous acid.…”