σ-Donor and π-acceptor properties of ten valence electron isoelectronic diatomic molecules being potential ligands in transition metal complexes

Abstract

σ-donor and π-acceptor properties of 14-electron (10 valence electron) isoelectronic diatomic ligands with empty π-orbitals (CO, N 2, CN −, NO +, C 2− 2,O + 2) were investigated by the CNDO/2 method. The LUMO energies, the total energies of the free ligands and of the ligands bonded in Fe 2+ complexes, the changes in local anisotropies, reactivities and bond lengths of the ligands, calculated from electronic data, are correlated with the π-acceptor abilities of the ligands. The σ-donor properties are interpreted by the sum of atomic orbital populations 2s and 2pσ, the σ-proportion electrons on the bonding atom, the degree of bonding between the atoms of the ligands, and the eccentricity of the polarization ellipsoid weighted with σ charge densities. Other factors influencing the electron rearranging effects on coordination are also discussed. To simulate the back donation effect the number of electrons of the ligands were increased by the charge −e and the electronic structures of 11 valence electron species so obtained (CO −, N − 2, CN 2−, NO, C 3− 2 and O + 2) were also calculated. The differences in electronic data, as compared to those of the 10 valence electron species, were used to explain the π-acceptor properties of the ligands.