Modulation of the CC distance in disubstituted l,2-R2-o-carboranes. Crystal structure of closo l,2-(SPh)2-l,2-C2B10H10

1,2-Cc-substituted o-carboranes, where Cc represents the cluster carbon atoms, display a range of experimental/computed Cc⋯Cc distances from 1.629/1.624 Å for the unsubstituted parent compound 1,2-C2B10H12 (1) to 1.798/1.818 Å for l,2-(SPh)2-l,2-C2B10H10 (5) 1 or 1.858(5)/1.826(5) for l,2-μ-SCH2(CH2OCH2)2CH2S-1,2-C2B10H10 (7). Different Cc⋯Cc distances can be achieved by modifying the substituents on the carbon of the cluster compound. The crystal structure of a new disubstituted o-carborane 1,2-(SPh)2-l,2-C2B10H10 (5) was elucidated by single crystal X-ray diffraction. Purely alkyl substituents alter the Cc⋯Cc distance of the parent compound very little, which implies that steric effects, although relevant, are not the major cause of the lengthening. In contrast, substituents with lone pairs alter the Cc⋯Cc distance substantially. Our calculations suggest that the cause of the elongation is the transfer of electron density from the available lone pairs on the substituents to the Ψ* low-lying virtual orbitals mainly distributed around Cc, producing a decrease in the Cc⋯Cc bond order and, thereby, an increase in the Cc⋯Cc distance. A Bader analysis shows that the electron density at the bond-critical point, which is found at the mid-point of the Cc⋯Cc distance, decreases considerably with the presence of lone pairs of the sulfur atoms bound to the Cc atoms of the carborane cage.