The Pentacyano Complexes of Cobalt(III). IV. Kinetics and Mechanism of the Substitution of Water in Co(CN)₅OH₂₂-by Pyridine, Ammonia, Hydrazine, and Hydrazinium Ion

A study has been made of the displacement of H₂O in Co(CN)₅OH₂^2- by pyridine, hydrazine, hydrazinium ion, and ammonia. At low concentration of Co(CN)₅OH₂^2- and 0.05-1.0 M concentration of a given ligand X, the rate law for each reaction is pseudo first order in Co(CN)₅OH₂^2- and may be described by a rate constant k. For each ligand the quantity k/[X] is a constant independent of the X concentration. The numerical values of k/[X] are 52 × 10^-5, 10.0 × 10^-5, 14.7 × 10^-5, and 20.6 × 10^-5 M^-1 sec^-1 for py, N₂H₄, N₂H₅⁺, and NH₃, respectively. The acidity quotient of Co(CN)₅N₂H₅^- has been measured.