Exchange of Ammonia between Metal-Ammine Cations and Solvent in Liquid Ammonia

The rates of exchange of NH3 between solvent liquid ammonia and ligand ammonia in five metal-ammine cations have been measured with N15 labeling. Reactants were separated by distilling off the solvent ammonia. Within the exchange times of 0.5, 3, and 11 minutes, respectively, exchange is complete for Ag(NH3)2+, Cu(NH3)4+2, and Ni(NH3)6+2 at concentrations of ca 0.01 f and in the dark at —33.5°C. Based on a single run, exchange with 0.013 f Cr(NH3)6+3 in the dark at 25.0°C exhibited a good McKay plot with a half-time of 1500 minutes, corresponding to a bimolecular rate constant k=1.3×10—6 l·mole—1·sec—1. Exchange with Co(NH3)6+3 was shown to be slow (half-times of the order of several hours at 0.002–0.005 f in the dark at 25.0°C), but reproducible results were not obtained. All complexes were studied as the nitrates. The observed slow exchange for Cr(NH3)6+3 and Co(NH3)6+3 is consistent with their classification as ``inert inner-orbital'' complexes. Exchange of N15 between NO3— and solvent liquid ammonia, including exchange during the hot NaOH treatment used to distill off the ammonia, is less than 3% at 0.05 f NO3— and 25°C.