Paramagnetism in Metallacarboranes: The Polyhedral Chromadicarbaborane Systems

The chromadicarbaboranes CpCrC2Bn–3Hn–1 (8 ≤ n ≤ 12) are of
interest in providing stable paramagnetic deltahedral metallaboranes
among which the 12-vertex CpCrC2B9H11 has been synthesized by Hawthorne and co-workers. Density functional
theory shows that the lowest-energy such structures are quartet spin-state
Cr­(III) structures in which the central CrC2Bn–3 units exhibit most spherical closo deltahedral geometries similar to those found in the borane dianions
BnHn2–. Higher-energy doublet CpCrC2Bn‑3Hn–1 (8 ≤ n ≤ 11) structures are found exhibiting central CrC2Bn–3 isocloso deltahedral geometries, thereby providing a degree 6 vertex for
the chromium atom. The lowest-energy CpCrC2Bn‑3Hn–1 (8
≤ n ≤ 11) structures all have both
carbon atoms at degree 4 vertices. However, the lowest-energy CpCrC2B9H11 structures all have central CrC2B9 icosahedra and thus lack degree 4 vertices for
the carbon atoms. For all of the CpCrC2Bn–3Hn–1 (8 ≤ n ≤ 12) systems the lowest-energy isomers are those
with the maximum number of Cr–C edges in contrast to the related
CpCoC2Bn–3Hn–1 systems.