Computational studies of cis- and trans-isomer preferences of low-spin d6 [M(DABF)2A2]+ and [M(CO)4A2]+ complexes (M = Co, Rh, Ir; A = anionic ligand): spectator ligand π-backbonding and DFT exchange

Computational studies of low spin d6 cis- and trans-[M(DABF)2A2]+ complexes (M = Co, Rh, Ir; A = anionic ligand) employing multiple model chemistries find that cis geometries are preferred for complexes where the binding atom in A is high and/or to the left in the Periodic Table, while trans geometries are preferred for complexes where the binding atom is heavy or to the right of the Periodic Table. This holds despite the fact that consideration of the trans influence for the pi-acceptor spectator DABF ligand suggest that all such complexes should prefer cis geometries. Energy decomposition analysis ties the phenomenon mostly to the degree of DFT exchange; that this benefits trans geometries more than cis geometries is thought to arise from the greater electronic symmetry of the former. This is supported by predictions for haloborane complexes [M (DABF)2(BX2)2]+. In contrast, [M(CO)4A2]+ complexes, containing better pi-accepting CO spectator ligands (presumably higher in the trans influence series), are predicted to prefer cis geometries except for the halogen complexes [M(CO)4X2]+ (X = F, Cl, I), where no distinct preferences exist.