Theoretical study of the geometric and electronic structures of pseudo-octahedral d(0) imido compounds of titanium: the trans influence in mer-[Ti(NR)Cl-2(NH3)(3)] (R = Bu-t, C6H5 or C6H4NO2-4)

The geometric and electronic structure of mer-[Ti(NR)Cl-2(NH3)(3)] (R = Bu- t, C6H5 or C6H4NO2-4), models for the corresponding crystallographically ch aracterised pyridine complexes [Ti(NR)Cl-2(py)(3)], have been studied compu tationally using non-local density functional theory. In general, excellent agreement is found between the fully optimised calculated geometries and t he experimental structures Each of the molecules is calculated to have a si gnificantly longer Ti-NH3 (trans) distance than Ti-NH3 (cis), this trans in fluence decreasing in the order Bu-t > C6H5 > C6H4NO2-4. This result supple ments the crystallographic results which found no experimentally significan t difference in the trans influences in [Ti(NR)Cl-2(py)(3)] (R = Bu-t, C6H5 or C6H4NO2-4). The causes of the traits influence have been investigated. Approximately 25% of the trans influence in the fully optimised geometries arises from pi orbital driven increases in the RN=Ti-Cl angle, which lead t o increased steric repulsion between the cis Cl atoms and the trans NH3 gro up. This contrasts sharply with the situation for [OsNCl5](2-) (studied pre viously by other workers and revisited in the present contribution) in whic h most of the trans influence depends on cis-trans-Cl ligand repulsions as the N=Os-Cl (cis) angles relax from 90 degrees to their fully optimised val ue. The remaining 75% of the trans influence for the title titanium imides is attributed to their intrinsic electronic structures, and in particular t o two occupied molecular orbitals which are Ti-NH3 (trans) antibonding and which vary in composition according to the identity of the imido N-substitu ent. By contrast, none of the molecules has an occupied orbital which is Ti -NH3 (cis) antibonding.