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.