MCSCF Study of Multiple Bonding between Ti and the Main-Group Elements C, Si, N, and P

The nature of the multiple bonding of H2TiEH2 (E = C, Si) and H2TiEH (E = N, P) molecules has been investigated with the multiconfiguration SCF (MCSCF) method. It is shown that the Ti−C and Ti−P bond lengths decrease with bond rotation, whereas the Ti−Si bond length increases. MCSCF geometry optimization shows that Ti−N has triple-bond character with a linear Ti−N−H bond angle that results from strong back-bonding from the N lone pair into the empty Ti d orbitals. The rotation barrier and bond dissociation energy for TiE are estimated with the MCSCF + multireference second-order perturbation theory (MRMP2) method. The singlet rotation barriers in TiC, TiSi, and TiP are estimated to be 15.9, 8.6, and 9.3 kcal/mol, respectively. The TiC and TiSi bond dissociation energies are estimated to be 83.4 and 56.9 kcal/mol, respectively. The Ti⋮N triple-bond energy is about 30 kcal/mol larger than that of the carbon species, whereas the energy of the TiP bond is about 8 kcal/mol smaller than that of the silicon species.