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-C₂B_nH_n (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 Å!) of the cage CC distances, from 1.626 to 2.638 Å, 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 C_c···C_c separation in neutral species (1.860 Å), C_c···C_c distances can be much larger in the corresponding dianions. These range from 1.823 Å (R^- = e^-) to 2.638 Å (R^- = CH₂^-) for 1,2-(R^-)₂-1,2-C₂B₁₀H₁₀ and from 1.626 Å (R^- = SiH₂^-) to 3.099 Å (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 C_c···C_c bond indices, and Bader AIM method. Cluster carboranes, and possibly other heteroboranes, thus appear to offer unique opportunities for modulating C_c···C_c distances.