Thermodynamics of metal-ligand bond formation. XXIX. Nickel(II) dialkylmonothiocarbamate complexes

Abstract

In benzene solution at concentrations below 10^-2 M, measurements of molecular weights, absorption spectra and the enthalpies of reaction with bases are all consistent with Ni(SOCNR₂)₂ (R = Pr or Bu) existing as an equilibrium between low-spin monomeric and high-spin dimeric, probably five-coordinate, species. Enthalpies of dimerization are about -55 kJ (g-atom Ni)^-1 and dimerization constants increase from 250 l. mol^-1 at about 5°C to 1000 l. mol^-1 at 30°C, which shows that the entropy of dimerization is not temperature-invariant. Addition of bidentate bases to the monomer is quantitative with ΔH° about -100 kJ mol^-1. Addition of pyridine, 4-methylpyridine or piperidine occurs in two steps, giving high-spin adducts, first five-coordinate NiL₂B then six-coordinate NiL₂B₂ with K₁ ≈ K₂, ΔH₁° ≈ -100 and ΔH₂° near zero. Comparisons are made with other low-spin nickel(II) complexes which form high-spin polymers or adducts.