Adduct formation of nickel(II) chelates of thio-β-diketones and the related amides and esters with heterocyclic nitrogen bases

Abstract

Thermodynamic constants for the formation of adducts of nickel(II) chelates of thio-β-diketones and the related amides and esters with o-phenanthroline and pyridine bases were determined by spectrophotometric method in chloroform solution. In the adducts formed with pyridine bases, the enthalpy ( ΔH) decreases in the order, 4-cyanopyridine > pyridine > β-picoline > γ-picoline. This trend follows inversely the order of equilibrium constants of adduct formation and the order of basicities of pyridine bases. The ΔH values for the o-phenanthroline adduct formations are not significantly different from those for the pyridine bases adduct formations while the ΔG values of the former are smaller than those of the latter. These results suggest that the effect of the ΔS on the ΔG is important. Both the rearrangements of the mother chelate rings in o-phenanthroline adduct formations and its chelate effect make the values of ΔS more positive than those of pyridine adducts. The hydrogen bond between the secondary amide proton and pyridine is suggested from the extrathermodynamic analysis of the enthalpy and entropy changes for the pyridine adduct formation and infrared studies. The results of electronic spectral and magnetic measurements indicate that mother chelates are square-planar and their adducts are octahedral.