Theoretical study of the geometric and electronic structures of pseudo-octahedral d0 imido compounds of titanium:: the trans influence in mer-[Ti(NR)Cl2(NH3)3] (R = But, 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 characterised pyridine complexes [Ti(NR)Cl-2(py)(3)], have been studied computationally using non-local density functional theory. In general, excellent agreement is found between the fully optimised calculated geometries and the experimental structures Each of the molecules is calculated to have a significantly longer Ti-NH3 (trans) distance than Ti-NH3 (cis), this trans influence decreasing in the order Bu-t > C6H5 > C6H4NO2-4. This result supplements the crystallographic results which found no experimentally significant 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 to increased steric repulsion between the cis Cl atoms and the trans NH3 group. This contrasts sharply with the situation for [OsNCl5](2-) (studied previously by other workers and revisited in the present contribution) in which 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 value. The remaining 75% of the trans influence for the title titanium imides is attributed to their intrinsic electronic structures, and in particular to two occupied molecular orbitals which are Ti-NH3 (trans) antibonding and which vary in composition according to the identity of the imido N-substituent. By contrast, none of the molecules has an occupied orbital which is Ti-NH3 (cis) antibonding.