Adducts of bis(2,4-pentanediono)-copper (II) with pyridine bases: Stability constants and heats of reaction

Abstract

Co-ordination equilibria involving bis(2,4-pentanediono)-copper (II) and eight pyridine bases have examined spectrophotometrically at 20, 35 and 50°C. The heats of reaction, in benzene solution, for these same systems have also been determined in a solution calorimeter. The complexes formed are relatively weak ones and show no obvious correlation of stability constant with the electron releasing or withdrawing nature of the substituents. The heats of formation decrease in a nearly regular fashion as the electron withdrawing power of the substituents increases but reaches a nearly steady value for the most powerful of these groups. From this evidence and related data in the literature, it is suggested that correlation of ligand base strength and complex stability will not be found in very weak complexes. For these materials, solvation and steric effects will be of considerable relative importance and may overpower weak electronic effects.