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 (2)A(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) pi-backdonation and TM (d) <- (CO)(6) sigma-donation interactions.