STABILITY OF PENTAAMMINEOSMIUM(II) IN AQUEOUS-SOLUTION

The reduction of Os(NH3)5(OTf)3 in D2O or H2O over zinc amalgam gives rise to a species that retains the ability to form (NH3)5Os(II) adducts with the pi-acidic ligands acetone, N,-N-dimethyluracil (DMU), and acetonitrile for periods of hours in the absence of reducing agent. The eta2-DMU complex was shown not to be in equilibrium with free DMU, in contrast to a previous report. Kinetic trapping studies are described that demonstrate the following: (1) an ''active'' source of (NH3)5Os(II) other than the aquo complex is formed and decomposes by a first-order process with a half-life of about 3 h at room temperature in D2O; (2) the rate of the decomposition process increases markedly at lower pH; and (3) the decomposition rate in H2O is about twice that in D2O. No direct spectroscopic or electrochemical evidence of species having osmium-hydrogen bonds was observed at neutral pH.