Electronic spectra of copper(II) β-ketoenolates: intraligand and charge transfer transitions

Abstract

The electronic spectra (in methanol) of the sodium salts and copper(II) complexes of thirteen β-diketones are reported within the energy range 25—48 kK. Spectra of the sodium salts of ligands with alkyl substituents exhibit one band which is assigned to the π 3→π 4 intraligand transition. Where aryl substituents are present, an additional benzenoid band is observed. The spectra of the copper complexes exhibit two bands whether aryl substituents are present or not. The band of lower energy is assigned to the π 3→π 4 transition and the band of higher energy to the σ L→3d xy charge transfer transition. Intensity relationships suggest that, where aryl substituents are present, the benzenoid band is superimposed on the charge transfer band. The observed transition energies exhibit a good level of agreement with those predicted by Huckel MO calculations. Comparison of the π 3→π 4, transition energies of the copper complexes with those available for the β-ketoenolates of other metal ions supports indications from ESR spectra that there is very little π-orbital overlap in the copper—oxygen bonds.