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 Cp(2)Ti(2)(CO)(n) derivatives (Cp = eta(5)-C(5)H(5); n = 8, 7, 6, 5, 4, 3, 2) have been optimized using density functional theory. Furthermore, the thermodynamics of CO dissociation, disproportionation into Cp(2)Ti(2)(CO)(n+1) + Cp(2)Ti(2)(CO)(n-1), and dissociation into mononuclear fragments of these Cp(2)Ti(2)(CO)(n) derivatives have been studied. An unbridged Cp(2)Ti(2)(CO)(8) structure with a long approximately 3.9 A 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 Cp(2)Ti(2)(CO)(6) structure has two CpTi(CO)(3) units linked by a formal approximately 2.8 A Ti identical withTi triple bond and thus is the next member of the M identical withM triply bonded series Cp(2)V(2)(CO)(5), Cp(2)Cr(2)(CO)(4), Cp(2)Mn(2)(CO)(3), all three of which are stable compounds. The lowest energy structures of Cp(2)Ti(2)(CO)(7), Cp(2)Ti(2)(CO)(5), and Cp(2)Ti(2)(CO)(4) all contain one or two four-electron donor bridging eta(2)-mu-CO groups. However, they are not likely to be stable molecules since their disproportionation energies into Cp(2)Ti(2)(CO)(n+1) + Cp(2)Ti(2)(CO)(n-1) are either nearly thermoneutral (n = 5) or exothermic (n = 7 and 4). The lowest energy structure of Cp(2)Ti(2)(CO)(3), in which all three carbonyl groups are four-electron donor eta(2)-mu-CO groups bridging a approximately 3.05 A 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(2)Ti(2)(CO)(4) + Cp(2)Ti(2)(CO)(2). In the lowest energy Cp(2)Ti(2)(CO)(2) structure both carbonyl groups are four-electron donor eta(2)-mu-CO groups bridging a formal 2.74 A Ti[triple bond]Ti triple bond. These low energy Cp(2)Ti(2)(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.