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)
Abstract
The geometric and electronic structure of mer-[Ti(NR)Cl2(NH3)3] (R = But, C6H5 or C6H4NO2-4), models for the corresponding crystallographically characterised pyridine complexes [Ti(NR)Cl2(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 But > C6H5 > C6H4NO2-4. This result supplements the crystallographic results, which found no experimentally significant difference in the trans influences in [Ti(NR)Cl2(py)3] (R = But, C6H5 or C6H4NO2-4). The causes of the trans influence have been investigated. Approximately 25% of the trans influence in the fully optimised geometries arises from π orbital driven increases in the RNTi–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 NOs–Cl (cis) angles relax from 90° 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.