Computational Studies of cis- and trans-Isomer Preferences of d0 MX4L2 Complexes: Overcoming the trans Influence with Steric Demands

Computational energy comparisons of cis– and trans d0 MX4L2 complexes (M = Zr/Hf; X = F, Cl, I, L = Lewis base ligand lacking π-acceptor capability) show that cis geometries are preferred when the steric demands of the X and L ligands are small, while trans geometries are preferred for larger ligands. This holds because of the removal of the largest factor affecting the thermodynamic trans influence: metal–ligand π backbonding. Although a mathematical equation relating ligand size and isomer preference could not be found, thresholds for preference changes were determined for π-neutral NR3/PR3 and potential π-donor OR2/SR2 ligands using the SUMΔ parameter obtained from cis structures. The model predicts computationally determined preferences well and agrees with experimental findings.