MCSCF study of multiple bonding between Ti and the main-group elements C, Si, N, and P

The nature of the multiple bonding of H2Ti=EH2 (E=C, Si) and H2Ti=EH (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 Ti=E are estimated with the MCSCF + multireference second-order perturbation theory (MRMP2) method. The singlet rotation barriers in Ti=C, Ti=Si, and Ti=P are estimated to be 15.9, 8.6, and 9.3 kcal/mol, respectively. The Ti=C and Ti=Si bond dissociation energies are estimated to be 83.4 and 56.9 kcal/mol, respectively. The Tiequivalent toN triple-bond energy is about 30 kcal/mol larger than that of the carbon species, whereas the energy of the Ti=P bond is about 8 kcal/mol smaller than that of the silicon species.