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

Bonding in the bis(pentalene)dititanium "double-sandwich" species Ti(2)Pn(2) (Pn = C8H6) and its interaction with other fragments have been investigated by density functional calculations and fragment analysis. Ti(2)Pn(2) with C-2v symmetry has two metal-metal bonds and a low-lying metal-based empty orbital, all three frontier orbitals having a(1) 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. Electro-chemical studies on Ti(2)Pn(2)(dagger) (Pn(dagger) =1,4-{(SiPr3)-Pr-i}(2)C8H4) revealed a one-electron oxidation, and the formally mixed-valence Ti(II)-Ti(III) cationic complex [Ti(2)Pn(2)(dagger)] [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 Ti(2)Pn(2), preserving the C-2v symmetry, as does carbon disulfide. The dominant interaction in Ti(2)Pn(2)CO(2) is sigma donation into the LUMO of bent CO2, and donation from the O atoms to Ti(2)Pn(2) is minimal, whereas in Ti(2)Pn(2)CS(2) there is significant interaction with the S atoms. The bridging O atom in the mono(oxo) species Ti(2)Pn(2)O, 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 Ti(2)Pn(2)S behaves similarly. Ti(2)Pn(2) 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 Ti(2)Pn(2)(CO)(2) has C-5 symmetry with the Ti2Pn2 unit acting as two MCp2 fragments. Synthetic studies showed that in the presence of excess CO the tricarbonyl complex Ti(2)Pn(2)(dagger)(CO)(3) is formed, which optimizes to an asymmetric structure with one semibridging and two terminal CO ligands. Low-temperature C-13 NMR spectroscopy revealed a rapid dynamic exchange between the two bound CO sites and free CO.