Bonding in Copper(II) Chelates: Solvent Effects in Their Visible Absorption Spectra

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Abstract
Absorption spectra, in the visible red and near infrared, are reported for solutions of bis‐acetylacetono‐Cu (II) and its 3‐ethyl variant in various media. The spectra could arise from a set of three Gaussian components whose positions and intensities are shown to depend primarily upon solvent basicity. These component bands are identified with three 3d1—3d1 transitions predicted from crystal field and molecular orbital theories. The behavior of these bands upon alteration of solvent and of chelate ring substituents appears consistent with the postulate that pi‐bonding changes are energetically of less importance than changes in crystal field splitting and in sigma bonding.