Strikingly long C•••C distances in 1,2-disubstituted ortho-carboranes and their dianions

Neutral and especially dianionic 6- and 12-vertex closo ortho-carboranes (o-carboranes) 1,2-R-2-1,2-C2B,H, (R = H, CH3, NH2, OH, F, SiH3, PH2, SH, Cl, as well as e(-), CH2-, NH-, O-, SiH2-, PH-, and S-) exhibit extremely large variations (over 1 angstrom!) of the cage CC distances, from 1.626 to 2.638 angstrom, at the B3LYP/6-31G*//B3LYP/6-31G* DFT level. These CC bond lengths, among the longest ever reported, generally are greater in the icosahedral than those in the corresponding octahedral systems and depend strongly on the substituents. While 1,2-(NH2)(2)-1,2-C2B10H10 has the longest (CcCc)-C-... separation in neutral species (1.860 angstrom), Cc(...)Cc distances can be much larger in the corresponding dianions. These range from 1.823 angstrom (R- = e(-)) to 2.638 angstrom (R- = CH2-) for 1,2(-)(R-)(2)-1,2-C2B10H10 and from 1.626 angstrom (R- = SiH2-) to 3.099 angstrom (R- NH-) for 1,2-(R-)(2)-1,2-C2B4H4. Remarkably, there is no abrupt discontinuity over the entire range of CC lengths. Consequently, the relationship between the gradual changes in the distances and the nature of the bonding was analyzed by means of the form of the Kohn-Sham orbitals, the Wiberg Cc(...)Cc bond indices, and Bader AIM method. Cluster carboranes, and possibly other heteroboranes, thus appear to offer unique opportunities for modulating Cc(...)Cc distances.