Bonding in Complexes of Bis(pentalene)dititanium, Ti₂(C₈H₆)₂

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. Ti₂Pn₂ with C₂ symmetry has two metal-metal bonds and a low-lying metal-based empty orbital, all three frontier orbitals having a₁ symmetry. 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 an S = ¹/₂ ground-state electronic configuration for the latter, which was confirmed by EPR spectroscopy and SQUID magnetometry. Carbon dioxide binds symmetrically to Ti₂Pn₂, preserving the C₂ symmetry, as does carbon disulfide. The dominant interaction in Ti₂Pn₂CO₂ is σ donation into the LUMO of bent CO₂, and donation from the O atoms to Ti₂Pn₂ is minimal, whereas in Ti₂Pn₂CS₂ there 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. Ti₂Pn₂ is 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)₂ has C symmetry with the Ti₂Pn₂ unit acting as two MCp₂ fragments. Synthetic studies showed that in the presence of excess CO the tricarbonyl complex Ti₂Pn^†₂(CO)₃ is formed, which optimizes to an asymmetric structure with one semibridging and two terminal CO ligands. Low-temperature ¹³C NMR spectroscopy revealed a rapid dynamic exchange between the two bound CO sites and free CO.