Nitrito–nitro linkage isomerisation of the penta-amminecobalt(III) complex in liquid ammonia

Abstract

The nitrito–nitro linkage Isomerisation reaction of the penta-amminecobalt(III) complex in liquid ammonia proceeds entirely via an intramolecular conjugate-base mechanism. The reaction is fully retentive and the penta-ammine(nitro)cobalt(III) complex is the only species that could be detected by ¹H n.m.r. after the reaction was completed. The advantage of liquid ammonia as solvent is demonstrated by the separate determination of the pre-equilibrium constant K^c.b(ΔH= 27 kJ mol^–1; ΔS=–14 J K^–1 mole^–1) and the rate constant of the rate-determining step k₂(ΔH^‡= 78 kJ mol^–1; ΔS^‡= 29 J K^–1 mol^–1). Comparison of the activation parameters of the latter rate constant with the activation parameters reported earlier for the ammoniation reactions of a number of cobalt(III) and rhodium(III) amine complexes in liquid ammonia shows the expected deviation from a limiting dissociative activation.