Ab initio calculation of the dipole moment of cyanamide

The He(I) photoelectron spectra of the isoelectronic series Fe(CO)₂(NO)₂, Co(CO)₃NO and Ni(CO)₄ are reported and interpreted by means of ab initio SCF-MO calculations. For the nitrosyl complexes it is found that ionization potentials calculated assuming Koopmans' theorem are seriously in error due to the considerably greater orbital relaxation accompanying ionization from metal than from NO valence orbitals. When such allowance is made for orbital relaxation by performing restricted Hartree-Fock (RHF) calculations on the ionic states, the experimental spectra are accurately reproduced and the observed similarity of the spectra of all three molecules is explained.