Octacarbonyl Anion Complexes of Group Three Transition Metals [TM(CO)₈]⁻ (TM=Sc, Y, La) and the 18-Electron Rule

We report the gas-phase synthesis of stable 20-electron carbonyl anion complexes of group 3 transition metals, TM(CO)₈⁻ (TM=Sc, Y, La), which are studied by mass-selected infrared (IR) photodissociation spectroscopy. The experimentally observed species, which are the first octacarbonyl anionic complexes of a TM, are identified by comparison of the measured and calculated IR spectra. Quantum chemical calculations show that the molecules have a cubic (O_h) equilibrium geometry and a singlet (¹A_1g) electronic ground state. The 20-electron systems TM(CO)₈⁻ are energetically stable toward loss of one CO ligand, yielding the 18-electron complexes TM(CO)₇⁻ in the ¹A₁ electronic ground state; these exhibit a capped octahedral structure with C_3v symmetry. Analysis of the electronic structure of TM(CO)₈⁻ reveals that there is one occupied valence molecular orbital with a_2u symmetry, which is formed only by ligand orbitals without a contribution from the metal atomic orbitals. The adducts of TM(CO)₈⁻ fulfill the 18-electron rule when only those valence electrons that occupy metal–ligand bonding orbitals are considered.