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-R2-1,2-C2BnHn (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 A!) of the cage CC distances, from 1.626 to 2.638 A, 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 Cc...Cc separation in neutral species (1.860 A), Cc...Cc distances can be much larger in the corresponding dianions. These range from 1.823 A (R- = e-) to 2.638 A (R- = CH2-) for 1,2-(R-)2-1,2-C2B10H10 and from 1.626 A (R- = SiH2-) to 3.099 A (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.