Nature of the coordination bond in metal complexes of substituted pyridine derivatives. I. The substituent effect on the coordination bond

Abstract

The nature of the coordination bond in some metal complexes of 4-substituted pyridines has been investigated by thermodynamic and infrared spectroscopic methods. The effects of the substituents upon the coordination bond strength in the copper(II) complexes of these ligands have been found by direct measurement of the heats of coordination. These effects have been further described by the substituent effect upon the change in free energy of the coordination center in the ligands. A new parameter, Rπ, measuring the relative π bonding in the complexes of 4-substituted pyridines has been introduced. For this purpose a series of complexes of the types [M(4—R—C5H4N)2Cl2] and [(4—R—C5H4N)•HCl] (R = CH3, CH2OH, H, COCH3, CONH2, COOCH3, CN; M = Cu(II), Zn(II)) has been isolated, and the infrared spectra of these complexes and the free ligands have been examined.