New titanium carbonyls: Ti2(CO)10, Ti2(CO)11, and Ti2(CO)12

Structural and energetic features of the binuclear titanium carbonyls Ti2(CO)n (n = 12, 11, 10) have been examined using density functional theory. The lowest-energy Ti2(CO)12 structure is a singlet structure consisting of two Ti(CO)6 units linked by Ti═Ti double bonds of lengths 3.0-3.2 Å. A similar slightly higher energy triplet Ti2(CO)12 structure is found with longer Ti-Ti bonds (3.37-3.62 Å), considered to be formal single bonds. The energy required for the dissociation of Ti2(CO)12 into two Ti(CO)6 fragments is 18.5 ± 2 kcal/mol higher than the energy required for the dissociation of V2(CO)12 into two V(CO)6 fragments. For the unsaturated Ti2(CO)11 system, the lowest-energy structures contain a four-electron-donor bridging η(2)-μ-CO group and 10 terminal CO groups with formal Ti═Ti double bonds in the singlet structures and formal Ti-Ti single bonds in the triplet structures. Similarly, the most favored geometries for the more highly unsaturated Ti2(CO)10 contain two four-electron-donor bridging η(2)-μ-CO groups with formal Ti═Ti double bonds for the singlet structures and formal Ti-Ti single bonds for the triplet structures. Higher-energy triplet Ti2(CO)10 structures are found with two or three two-electron-donor semibridging CO groups and formal Ti≡Ti triple bonds of length 2.7-2.8 Å.