Infrared photodissociation spectroscopic investigation of TMO(CO)n+ (TM = Sc, Y, La): testing the 18-electron rule

Abstract

Gaseous TMO(CO)_n⁺ (TM = Sc, Y, La) complex cations prepared via laser vaporization were mass-selected and studied by infrared photodissociation spectroscopy in the C–O stretching frequency region. The structures and vibrational frequencies were calculated by density functional theory to support and interpret the experimental results. The saturated coordination number of CO ligands for ScO(CO)_n⁺, YO(CO)_n⁺ and LaO(CO)_n⁺ was demonstrated to be six, seven and nine, respectively, namely, the nominal 18-, 20- and 24-electron gaseous cation complexes were synthesized. Based on our analysis of the electronic structure, the YO(CO)₇⁺ complex also obeys the 18-electron rule, since one of the occupied valence molecular orbitals is formed only by ligand orbitals. The contribution of 4f orbitals in LaO(CO)₉⁺ accounts for its high coordination number with a 24-electron valence shell.