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(NH_3)_3)(R=Bu~t, C_6H_5 or C_6H_4NO_2-4)

摘要

The geometric and electronic structure of mer-[Ti(NR)Cl_2(NH_3)_3](R=Bu~t, C_6H_5 or C_6H_4NO_2-4), 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-NH_3 (trans) distance than Ti-NH_3 (cis), this trans influence decreasing in the order Bu~t>C_6H_5>C_6H_4NO_2-4. 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, C_6H_5 or C_6H_4NO_2-4). The causes of the trans influence have been investigated. Approximately 25% of the trans influence in the fully optimised geometries arises from #pi# orbital driven increases in the RN ident to Ti-Cl angle, which lead to inversed steric repulsion between the cis Cl atoms and the trans NH_3 group. This contrasts sharply with the situation for [OsNCl_5]~(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 ident to Os-Cl (cis) angles relax from 90 deg 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-NH_3 (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-NH_3 (cis) antibonding.