Characterisation of the trans-influence, and its inverse

Freeman, Tom and Kerridge, Andrew and Peach, Michael (2020) Characterisation of the trans-influence, and its inverse. Masters thesis, Lancaster University.

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Abstract

The trans-influence (TI), whereby the bond directly opposite a strong σ-donor, in certain d-block complexes, is relatively lengthened. The inverse trans-influence (ITI), whereby the analogous bond in certain f-block complexes, is relatively shortened. The purpose of this work is investigate the origin of the TI and its inverse (ITI) in a variety of d- and f-block species of the [MOX5]− form (M = U, Mo, W, and halide X =F, Cl, Br). Relative magnitudes of the influences as both a function of the metal species and halide ligand are determined computationally. Several model chemistries are tested, spanning eight basis sets and seven DFT exchange–correlation functionals. Characterisation of the complexes in the ground state considers bond length, QTAIM, and natural bond orbital (NBO) analyses. The results demonstrate that the d-block TIs have generally higher magnitudes than the f-block ITIs, and that regardless of metal centre, the magnitudes of the influences are greatest in the F-ligand complexes, and lowest in the Br-ligand complexes. NBO analysis identifies that the trans-bonds, relative to the cis-bonds in the ITI-exhibiting [UOX5]− species, exhibit reduced f- and s-orbital, and enhanced d-orbital character from the U contributions. A novel examination of the influence of electronic excitation (as studied using TDDFT) on the TI and ITI is considered. The geometries of the ground and of pertinent excited states are compared to identify key excitations that significantly alter the influences. Analysis of three excitations proved particularly insightful; two exclusive to the fblock species, and one common to both the d- and f-block species. For the latter excitations yielded a reduction of the TI in the d-block and a reduction (and reversal) of the ITI in the f-block species. The results hint at a possible common electronic origin for the TI and ITI and demonstrate that these influences can be moderated by electronic excitation.