ESR.‐Spektren von CuN4‐Chromophoren: Komplexe des zweiwertigen Kupfers mit Pyridin und Pyridinderivaten

The ESR. spectra of four copper complexes, containing pyridine, 4‐picoline, 4‐dimethylaminopyridine, and 4‐cyanopyridine as ligands, have been measured in the solid state. Single crystals of the corresponding PtII oder CdII compounds were used as host lattices and magnetic diluents. The spectra of the CuN4 chromophores, showing resolved hyperfine structure due to copper and nitrogen nuclear magnetic momenta, could be analysed within a Spin‐Hamiltonian corresponding to effective symmetry D4h. They have been interpreted by means of molecular orbital theory, and the delocalization parameters have been deduced on the assumption of near degeneracy of the excited levels within the widths of the observed visible absorption band. It is shown that the delocalization parameters apparently do not display any influence of the widely different substituents in the 4 position of the ligands. The parameter describing the π‐antibonding character of the excited state, Eg, differs from the expected value. This is probably due to some fundamental deficiency in the MO scheme so far generally applied to chromophores CuO4, CuO2N2 and CuN4.