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 _\rm1g ground state with D_\rm3d 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_\rmh symmetry. The energy decomposition analyses indicate that the metal-CO covalent bonding is dominated by TM ^- (d) \rightarrow (CO) _6 \pi -backdonation and TM ^- (d) \leftarrow (CO) _6 \sigma -donation interactions.