Molecular‐orbital calculations on transition metal complexes, charge‐transfer spectra and the sequence of metal and ligand orbitals

An outline is given of a semi‐empirical scheme for molecular‐orbital calculations on transition‐metal complexes according to a revised INDO procedure. To judge the reliability of the results of the calculations, the charge‐transfer transitions of a number of complexes have been calculated and compared with experimental data. Both for the excitation energies and for the oscillator strengths the agreement is very satisfactory. With respect to the sequence of occupied metal and ligand orbitals it was found that for closed‐shell d6 and d8 systems the molecular orbitals which are mainly composed of metal d orbitals have a lower energy than the orbitals built up from ligand p orbitals. Calculations by the extended Hückel method and other similar schemes give the d orbitals as the highest occupied ones as a result of a bad approximation of the diagonal elements of the Fock matrix. The consequences for the interpretation of photo‐electron spectra of transition‐metal complexes are discussed.