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

Computational studies of low spin d cis- and trans-[M(en)X] 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, CH, CF) 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(CF), BCl, SiCl) 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, NO, C≡CH) prefer trans geometries because they combine destabilization of cis geometries with enhanced stabilization of trans geometries resulting from conjugation.