Ligand field torque: a pi-type electronic driving force for determining ligand rotational preferences

Transition metal complexes with triply-degenerate T ground states are formally Jahn-Teller active but do not usually display the significant bond length distortions familiar from their E ground state counterparts like d(9) Cu(II). The electronic 'asymmetry' for T-state systems lies in the d(pi) orbitals, which interact with the ligands relatively weakly compared to the stronger sigma-type interactions for E-state systems. However, in combination with asymmetric M-L pi bonding, T-type systems have an additional mechanism for relieving the electronic strain. Density functional theory, ligand field theory and ligand field molecular mechanics calculations are used to show how rotations around the M-L bonds can affect their pi-pi (d(pi)-L(pi)) interactions and lead to significant energy lowering. For example, d(6) [Fe(OH(2))(6)](2+), which has a (5)T(g) state in cubic T(h) symmetry, 'distorts' to an S(6) structure 4.4 kcal mol(-1) lower in energy (by DFT) but with six equal Fe-O distances via Fe-O rotations of approximately 20 degrees and thus masquerades as an apparently regular geometry. Using model systems, we show that this effect is not restricted to formally Jahn-Teller active complexes. The combination of asymmetric pi bonding and asymmetric d(pi) orbital occupations can generate an M-L 'torque' worth up to 6 kcal mol(-1) per bond which can 'lock' the ligand in a particular orientation relative to the partially-occupied d orbital(s). The effect is particularly marked for imidazole, the donor group of histidine, which, in a model low-spin d(5) Fe(III) system, shows almost no orientational preference in its neutral form but a very strong (approximately 6 kcal mol(-1)) orientational preference in its deprotonated form.