Thermodynamics of metal-ligand bond formation. VI. Trimerization and base addition of nickel(II) β-diketonates

Abstract

Five nickel(II) complexes of β-diketones (LH) have been studied by calorimetric titration with pyridine in benzene solution. Trimerization of the complexes NiL2 is endothermic and entropy-driven, probably as a result of desolvation of the low-spin monomers. Reaction with pyridine occurs in two steps, forming successively Ni2L4(py) and NiL2(py)2. The enthalpy of formation of NiL2(py)2 from the intermediate adduct is about -40 kJ mol-1 and shows no evidence for bulkiness of the ligand leading to steric instability in Ni2L4(py). Variations in the relative stabilities of the monomeric and trimeric forms of NiL2 seem more likely to be due to electronic than steric effects.