Thermodynamics of metal-ligand bond formation. VII. Pyridine adducts of nickel(II) salicylideneiminato complexes

Abstract

Thermodynamic data have been obtained by calorimetric titration in benzene solution at 30°C for the reaction: NiL₂+2py = NiL₂(py)₂ for 16 different nickel(II) complexes of Schiff bases of salicylaldehyde, 5- chlorosalicylaldehyde, and 5-chloro-2-hydroxybenzophenone. It has also been possible to obtain data for the trimerization of bis(N-phenyl-5- chlorosalicylideneiminato)nickel(II), and for two of the complexes estimates have been obtained for the addition of base in two successive steps.     Pyridine adducts, NiL₂(py)₂, of N-aryl complexes have enthalpies of formation about -40 kJ mol^-1, those of N-alkyl complexes about -60 kJ mol^-1 and a small inductive effect can be observed due to chloro substitution, but variations in stabilities of the adducts arise mainly from entropy effects. In the two systems studied most of the enthalpy of adduct formation is associated with addition of the second molecule of base.