Kinetics and thermodynamics of the complex-formation of nickel(II) and cobalt(II) ions with ammonia and pyridine-2-azodimethylaniline in water, at pressures from 1 bar to 2 kbar

Abstract

Thermodynamic and kinetic measurements have been made at a series of pressures from 1 bar to 2 kbar on the reactions of nickel(II) and cobalt(II) ions with ammonia and with the large bidentate ligand PADA (pyridine-2-azodimethylaniline) in aqueous solution. The apparatus consisted of a high-pressure cell and a laser temperature-jump apparatus with spectrophotometric detection. A method is described for determining equilibrium constants at high pressures from the amplitude of the optical-density change on reaction. The values found for ΔV^≠ are all in the region of +5 to +8 cm³ mol^–1. The results are consistent with the interpretation that the transition state involves considerable stretching of the cation-water bond, as postulated in the Eigen-Wilkins mechanism.