title: Bonding in Complexes of Bis(pentalene)dititanium, Ti2(C8H6)2.
abstract: Bonding in the bis(pentalene)dititanium "double-sandwich" species Ti2Pn2(Pn = C8H6) and its interaction with other fragments have been investigated by density functional calculations and fragment analysis. Ti2Pn2withC2vsymmetry has two metal-metal bonds and a low-lying metal-based empty orbital, all three frontier orbitals having a1symmetry. The latter may be regarded as being derived by symmetric combinations of the classic three frontier orbitals of two bent bis(cyclopentadienyl) metal fragments. Electrochemical studies on Ti2Pn†2(Pn†= 1,4-{SiiPr3}2C8H4) revealed a one-electron oxidation, and the formally mixed-valence Ti(II)-Ti(III) cationic complex [Ti2Pn†2][B(C6F5)4] has been structurally characterized. Theory indicates anS=1/2ground-state electronic configuration for the latter, which was confirmed by EPR spectroscopy and SQUID magnetometry. Carbon dioxide binds symmetrically to Ti2Pn2, preserving theC2vsymmetry, as does carbon disulfide. The dominant interaction in Ti2Pn2CO2is σ donation into the LUMO of bent CO2, and donation from the O atoms to Ti2Pn2is minimal, whereas in Ti2Pn2CS2there is significant interaction with the S atoms. The bridging O atom in the mono(oxo) species Ti2Pn2O, however, employs all three O 2p orbitals in binding and competes strongly with Pn, leading to weaker binding of the carbocyclic ligand, and the sulfur analogue Ti2Pn2S behaves similarly. Ti2Pn2is also capable of binding one, two, or three molecules of carbon monoxide. The bonding demands of a single CO molecule are incompatible with symmetric binding, and an asymmetric structure is found. The dicarbonyl adduct Ti2Pn2(CO)2hasCssymmetry with the Ti2Pn2unit acting as two MCp2fragments. Synthetic studies showed that in the presence of excess CO the tricarbonyl complex Ti2Pn†2(CO)3is formed, which optimizes to an asymmetric structure with one semibridging and two terminal CO ligands. Low-temperature13C NMR spectroscopy revealed a rapid dynamic exchange between the two bound CO sites and free CO.
authors.affiliation: Department of Chemistry, School of Life Sciences, University of Sussex , Brighton BN1 9QJ, U.K.
authors.forename: Alexander F R
authors.initials: AF
authors.lastname: Kilpatrick
authors.affiliation: Department of Chemistry, University of Oxford , Inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3QR, U.K.
authors.forename: Jennifer C
authors.initials: JC
authors.lastname: Green
authors.affiliation: Department of Chemistry, School of Life Sciences, University of Sussex , Brighton BN1 9QJ, U.K.
authors.forename: F Geoffrey N
authors.initials: FG
authors.lastname: Cloke