Mononuclear carbonyl anion complexes of groups IV and V metals

The anionic carbonyl complexes of groups IV and V metals TM(CO)_6,7(TM=Ti, Zr, Hf, V, Nb, Ta) are prepared in the gas phase using a laser vaporation-supersonic expansion ion source. The infrared spectra of TM(CO)_6,7− anion complexes in the carbonyl stretching frequency region are measured by mass-selected infrared photodissociation spectroscopy. The six-coordinated TM(CO)₆− anions are determined to be the coordination saturate complexes for both the group IV and group V metals. The TM(CO)₆− complexes of group IV metals (TM = Ti, Zr, Hf) are 17-electron complexes having a ²A_1g ground state with D_3d symmetry, while the TM(CO)6⁻ complexes of group V metals (TM = V, Nb, Ta) are 18-electron species with a closed-shell singlet ground state possessing O_h symmetry. The energy decomposition analyses indicate that the metal-CO covalent bonding is dominated by TM⁻(d) → (CO)₆ π-backdonation and TM⁻(d) ← (CO)₆ σ-donation interactions.