Modulation of the C-C distance in disubstituted 1,2-R2-o-carboranes.: Crystal structure of closo 1,2-(SPh)2-1,2-C2B10H10

1,2-C-c-substituted o-carboranes, where C-c represents the cluster carbon atoms, display a range of experimental/computed C-c...C-c distances from 1.629/1.624 Angstrom for the unsubstituted parent compound 1,2-C2B10H12 (1) to 1.798/1.818 Angstrom for 1,2-(SPh)(2)-1,2-C2B10H10 (5)1 or 1.858(5)/1.826(5) for 1,2-mu-SCH2(CH2OCH2)(2)CH2S-1,2-C2B2H10 (7). Different C-c...C-c 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)2-1,2-C2B10H10 (5) was elucidated by single crystal X-ray diffraction. Purely alkyl substituents alter the C-c...C-c 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 tone pairs alter the C-c...C-c 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 Psi* low-lying virtual orbitals mainly distributed around C-c, producing a decrease in the C-c...C-c bond order and, thereby, an increase in the C-c...C-c distance. A Bader analysis shows that the electron density at the bond-critical point, which is found at the mid-point of the C-c...C-c distance, decreases considerably with the presence of lone pairs of the sulfur atoms bound to the Cc atoms of the carborane cage. (C) 2002 Elsevier Science B.V. All rights reserved.