Computational Studies of Relative Stabilities of Low-Spin d6 cis- and trans-[M(en)2X2]+ Complexes (M = Co, Rh, Ir): Steric and Electronic Effects in the Context of the Structural Trans Influence.

Computational studies of low spin d6 cis- and trans-[M(en)2X2]+ complexes (M = Co, Rh, Ir) employing multiple model chemistries find that isomer preferences fall into three categories. Complexes where X is largely a σ-donor (H-, CH3-, CF3-) prefer cis geometries, in keeping with predictions associated with the trans influence series. Complexes where this donor characteristic is augmented by π acceptor behavior (B(CF3)2-, BCl2-, SiCl3-) evince even greater preference for cis geometries. QTAIM charge data suggest this is marked by lower positive charge on the metal in cis complexes. In contrast, complexes where X is a π donor and low in the trans influence series (X = OH-, F-, Cl-, I-) prefer trans geometries to varying degrees. QTAIM calculations indicate that this arises because the cis complexes are destabilized by distortions of the electron density in the M-X bonds. This can be viewed conceptually as resulting from repulsions between lone pair electrons on the ligands. Complexes where the X ligands are moderately trans-influencing and can interact conjugatively (CN-, NC-, NO2-, C≡CH-) prefer trans geometries because they combine destabilization of cis geometries with enhanced stabilization of trans geometries resulting from conjugation.