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 = Bu^t, 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 Bu^t > 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 = Bu^t, 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.