Possibilities for titanium-titanium multiple bonding in binuclear cyclopentadienyltitanium carbonyls: 16-electron metal configurations and four-electron donor bridging carbonyl groups as alternatives

The structures for the binuclear Cp₂Ti₂(CO) _n derivatives (Cp =η ⁵-C₅H₅; n = 8, 7, 6, 5, 4, 3, 2) have been optimized using density functional theory. Furthermore, the thermodynamics of CO dissociation, disproportionation into Cp₂Ti₂(CO)_n+1 + Cp₂Ti ₂(CO)_n-1, and dissociation into mononuclear fragments of these Cp₂Ti₂(CO)_n derivatives have been studied. An unbridged Cp₂Ti₂(CO)₈ structure with a long̃ 3.9 Å Ti-Ti bond is found. As expected from the long Ti-Ti bond, the predicted dissociation energy of this dimer into CpTi(CO) ₄ fragments is relatively low at 7 ± 3 kcal/mol. The lowest energy Cp₂Ti₂(CO)₆ structure has two CpTi(CO)₃ units linked by a formal ̃ 2.8 Å TitTi triple bond and thus is the next member of the M≡ M triply bonded series Cp ₂V₂(CO)₅, Cp₂Cr₂(CO) ₄, CP₂MN₂(CO)₃, all three of which are stable compounds. The lowest energy structures of Cp₂Ti ₂(CO)₇, Cp₂Ti₂(CO)₅, and Cp₂Ti₂(CO)₄ all contain one or two four-electron donor bridging η 2-μ-CO groups. However, they are not likely to be stable molecules since their disproportionation energies into Cp₂Ti₂(CO)_n+1 + Cp₂Ti ₂(CO)_n-1 are either nearly thermoneutral (n = 5) or exothermic (n = 7 and 4). The lowest energy structure of Cp₂Ti ₂(CO)₃, in which all three carbonyl groups are fourelectron donor η 2-μ-CO groups bridging a ̃ 3.05 Å formal Ti-Ti single bond, is a promising synthetic target since it is thermodynamically stable with respect to both CO dissociation and disproportionation into Cp ₂Ti₂(CO)₄ + Cp₂Ti ₂-(CO)₂. In the lowest energy Cp₂Ti ₂(CO)₂ structure both carbonyl groups are four-electron donor η 2-μ-CO groups bridging a formal 2.74 Å TitTi triple bond. These low energy Cp₂Ti₂(CO)n (n = 3, 2) structures have only a 16-electron titanium configuration rather than the usually favorable 18-electron configuration for metal carbonyl complexes.