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.

Computational Study of Ways by Which exo-Silatranes Might Be Prepared.

exo-Silatranes involve cage structures where the nitrogen lone pair points away from the cage rather than into it. This distinguishes them from the well-established endo-silatranes. exo-Silatranes have not been observed experimentally, consistent with a significant benefit to silicon-nitrogen interactions inside the cages as suggested for endo-silatranes. Identifying examples of exo-silatranes would prove useful in understanding Si-N interactions, as they would represent the "no interaction" extreme of the spectrum. We have found four means by which exo-silatranes might be synthesized: (1) employing smaller cages; (2) employing constrained rings to stiffen the cage backbones; (3) employing steric interactions to enhance preference for the less crowded exo-geometry around nitrogen; (4) modifying the Lewis acidity and basicity of the silicon and nitrogen so significantly as to remove their desire to interact. The preference for exo geometries is established using the parameter Δ, representing the distance between the nitrogen atom and the least-squares plane containing the adjacent carbon atoms. In some cases, Δ values for exo-silatranes are greater than 0.3 Å. In others, they are near zero, indicating a nearly planar nitrogen atom. There are no obvious structural markers besides Δ that distinguish between exo- and endo-silatranes.