REDUCTION OF [OS(NH3)5(ETA-1-CH3CN)]2+ TO [OS(NH3)5(ETA-2-CH3CH=NH2)]3+

The reduction of [Os(NH3)5(CH3CN)](O3SCF3)3 or of [Os(NH3)5(O3SCF3)](O3SCF3)2 in acetonitrile by zinc amalgam yields a product of composition corresponding to the formula [Os(NH3)5(eta(2)-CH3CH = NH2)](O3SCF3)3 (4). Proton NMR spectroscopy shows a large splitting between cis and trans ammines and for the heteroligand shows a pattern consistent with the structure proposed. In particular three equivalent protons and one other proton are not exchangeable with deuterium in methanol-d4 while two protons, inequivalent as observed in acetonitrile-d3, are. Although some 4 is formed also in dried CH3CN, the yield is much increased when a small amount of water is added. Compound 4 is quite stable in solution in DME, but when Proton Sponge (1,8-bis(dimethylamine)naphthalene) is added, deprotonation to produce [Os(NH3)5(CH3CH = NH)]2+ takes place, which then disproportionates to yield the acetonitrile complex as one of the products. The value of pK(a) for the corresponding protonation-deprotonation equilibrium quotient, [Os(NH3)5(eta(2)-(CH3)2C = NH2)]3+ = [Os(NH3)5(eta(1)-(CH3)2C = NH)]2+ + H+, for the more stable analogue derived from acetone in water at room temperature was measured as 10.3 +/- 0.2.