A Study of Gas Phase Heterobimetallic and Organometallic Complexes By Anion Photoelectron Spectroscopy

Abstract

Anion photoelectron spectroscopy was used to study extreme examples of multiple metal-metal bonds, with formal bond orders as high as six, in order to obtain fundamental information about the chemical bonding properties of both neutral and negatively charged transition metal dimers. Anion photoelectron spectroscopy was also used to study small organometallic complexes which could serve as models for understanding elementary processes, such as carbon-hydrogen bond activation and dehydrogenation, that occur in reactions catalyzed by transition metals. Density functional theory (DFT) calculations were used to help assign electronic states as well as determine the ground and excited state energies of heterobimetallic and organometallic complexes observed in anion photoelectron spectroscopy experiments. Heterobimetallic vibrationally-resolved spectra studied include those of NbCr¯ and NbMo¯. These results demonstrated that both anions have a closed-shell 1Σ+ ground electronic state with a dσ2dπ4dδ4sσ2 electron configuration, corresponding to a formal bond order of six. Organometallic anions whose vibrationally-resolved spectra were studied include NbC4H4¯, NbC6H4¯, and NbC6H6¯ products produced via flow tube reactions of Nb with C4H6. The most abundant product anion in this experiment was NbC4H4¯, which indicates a loss of H2 from C4H6 upon reaction with a single Nb atom. A relatively small yield of NbC6H4¯ and NbC6H6¯ product anions indicates carbon-carbon bond activation, carbon-carbon bond formation, and dehydrogenation. Isomeric structures of the organometallic products observed in these experiments were determined by DFT calculations to be a Cs five-membered ring for NbC4H4¯, a planar C2v η2 Nb-benzyne complex for NbC6H4¯, and a C6v η6 Nb-benzene π complex for NbC6H6¯.