title: Metal-Metal Quintuple and Sextuple Bonding in Bent Dimetallocenes of the Third Row Transition Metals.
abstract: Theoretical studies on the dimetallocenes Cp2M2 (M = Os, Re, W, Ta) predict bent structures with short metal-metal distances suggesting high-order metal-metal multiple bonds. Analysis of the frontier bonding molecular orbitals indicates a formal Os-Os quintuple bond (σ + 2π + 2δ) in singlet Cp2Os2 and a formal Re-Re sextuple bond (2σ + 2π + 2δ) in singlet Cp2Re2, thereby giving the metals in both molecules the favored 18-electron metal configurations. Predicted low-energy triplet structures for Cp2M2 (M = Os, Re) have formal quintuple bonds but with only two δ one-electron "half" bonds (M = Os) or a single δ two-electron bond (M = Re) and a second σ component derived from overlap of the d(z(2)) orbitals. A quintuple bond similar to that found in triplet Cp2Re2 is found in singlet Cp2W2, giving both tungsten atoms a 16-electron configuration. The formal Ta-Ta quadruple bond in the lowest energy singlet Cp2Ta2 structure is different from that in the original Re2Cl8(2-) in that it is a 2σ + 2π bond with no δ components but only σ and π components.
authors.affiliation: Beijing University of Posts and Telecommunication, Beijing 100876, China, Institute of Chemical Physics, Beijing Institute of Technology, Beijing 100081, China, Center for Computational Quantum Chemistry, South China Normal University, Guangzhou, 510631 China, and Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602.
authors.forename: Bing
authors.initials: B
authors.lastname: Xu
authors.affiliation: Beijing University of Posts and Telecommunication, Beijing 100876, China, Institute of Chemical Physics, Beijing Institute of Technology, Beijing 100081, China, Center for Computational Quantum Chemistry, South China Normal University, Guangzhou, 510631 China, and Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602.
authors.forename: Qian-Shu
authors.initials: QS
authors.lastname: Li
authors.affiliation: Beijing University of Posts and Telecommunication, Beijing 100876, China, Institute of Chemical Physics, Beijing Institute of Technology, Beijing 100081, China, Center for Computational Quantum Chemistry, South China Normal University, Guangzhou, 510631 China, and Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602.
authors.forename: Yaoming
authors.initials: Y
authors.lastname: Xie
authors.affiliation: Beijing University of Posts and Telecommunication, Beijing 100876, China, Institute of Chemical Physics, Beijing Institute of Technology, Beijing 100081, China, Center for Computational Quantum Chemistry, South China Normal University, Guangzhou, 510631 China, and Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602.
authors.forename: R Bruce
authors.initials: RB
authors.lastname: King
authors.affiliation: Beijing University of Posts and Telecommunication, Beijing 100876, China, Institute of Chemical Physics, Beijing Institute of Technology, Beijing 100081, China, Center for Computational Quantum Chemistry, South China Normal University, Guangzhou, 510631 China, and Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602.
authors.forename: Henry F
authors.initials: HF
authors.lastname: Schaefer