Calculation of the molecular-orbital parameters for some CuN4 chromophores

Abstract

Molecular-orbital parameters (a₁, b₁, and c₁) have been calculated for the square-coplanar CuN₄ chromophore in [Cu(NH₃)₄][PtCl₄], β-phthalocyaninatocopper(II), and tetraphenylporphinatocopper(II) using a more complete model than has been used previously involving metal–ligand and ligand–ligand overlap integrals and charge-transfer states. In order to obtain a reasonable correlation of the magnetic g and A values and the electronic transition energies, it is necessary to use a Cu⁺ wavefunction to represent the radially expanded Cu²⁺ wavefunction in these complexes. The experimental g and A values are reproduced with reasonable σ- and π-bonding coefficients with σ bonding > out-of-plane π bonding > in-plane π bonding. The σ-bonding coefficients are essentially independent of the π-bonding coefficients but very dependent on the electronic energies involved. In the non-centrosymmetric tetraphenylporphinatocopper(II), d–p mixing is important.