Excitation energies and oscillator strengths for Ni, Fe, Cr 2p and Pd 3d excitations in carbonyl, nitrosyl and bis-allyl complexes have been calculated at 1h-1p CI level, providing for an accurate choice of the basis set and an adequate treatment of relaxation. The spectra are found to be strongly sensitive to the nature of the ligands and the character of the metal-ligand bonding, and less dependent on the molecular symmetry and the d count of the metal atoms. An interpretation of the spectral pattern in terms of appropriate MO energy level diagrams is proposed that allows to correlate spectral data to the extent of the backbonding in p-greco ligand complexes
