Nature of the coordination bond in metal complexes of substituted pyridine derivatives. II. The far infrared spectra and metal–ligand force constants of copper complexes of 4-substituted pyridines

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Abstract
Infrared spectra, including the lower frequency region, of copper (II) complexes of the type [CuL2Cl2], where L = 4-methylpyridine, 4-carbinylpyridine, pyridine, 4-acetylpyridine, 4-pyridinecarboxamide, 4-carbomethoxypyridine, and 4-cyanopyridine have been measured. The substituent effect upon the copper–nitrogen (ligand) and copper–chlorine stretching frequencies has also been examined. It was found that the substituents on the pyridine ring not only affected the nature of copper–nitrogen bond, but also the copper–chlorine bond. It appears that an entirely delocalized system exists, involving the chlorine atoms and the substituents in the complexes. Force constants associated with the coordination bond between copper and the nitrogen of the ligands have been calculated by means of an IBM 1620 computer. By comparison of the copper–nitrogen stretching frequencies with the corresponding force constants, and the heats of coordination, it may be concluded that the magnitude of the metal–ligand stretching frequency is directly related to the metal–ligand coordination bond strength for the systems under study.