Thermodynamics of Metal-Ligand Bond Formation. XVIII Addition of Heterocyclic Bases to Copper(II) and Cobalt(II) Schiff Base Complexes

Abstract

Thermodynamic data are reported for reaction of heterocyclic bases in benzene solution with four-coordinate copper(11) and cobalt(11) Schiff base complexes and also with a five-coordinate nickel(11) complex. The cogper(11) complexes form adducts of low stability with pyridine or 4-methylpyridine, K ≈ 21. mol^-1, ΔHº ≈ -20 kJmol^-l. The cobalt(11) complexes add two molar proportions of base successively 1 < K₁ ≤ K₂ < 101.mol^-1 ΔH₁º ≈ ΔH₂º ≈ -20 kJ mol^-1 They also form adducts with 2,2'-bipyridine 30 < K < 600 ΔHº ≈ -40kJ mol^-1 The nickel(11) complex forms more stable adducts with unidentate bases, K ≈ 1001. mol^-l, ΔHº ≈ 70 kJ mol^-1. Comparison of the three metals indicates that differences between the stabilities of their base adducts can be attributed mainly to entropy factors.