C-C plasticity in boron chemistry: Modulation of the C-c center dot centerdot center dot C-c distance in mixed pyrrolyl/dicarbollide complexes

The single, double, and triple bond distances are well defined in conventio nal organic compounds. Cluster boron chemistry, in particular the o-carbora nes, provides the possibility to modulate the C-C distance in an almost con tinuous way within the same family of compounds. As an example, mixed pyrro lyl/dicarbollide sandwich cobalt complexes derived from closo-[3-Co(eta (5) -NC4H4)-1,2-C2B9H11] (1) display a continuous range of C-c. . .C-c distance s between 1.640(7) for the parent compound and 1.919(6) for closo- [3-Co(et a (5)-NC4H4)-1,2-(SCH3)(2)-1,2-C2B9H9] (5). Here, C, represents the cluster carbon atom. The C-c. . .C-c modulation was achieved merely by modifying t he substituents on the carbon of the cluster compound. The complexes studie d were synthesized by reaction of the appropriate closo cluster, e.g., 1,2- (SCH3)2-1,2-C2B10H10 with a suspension of K[NC4H4] and anhydrous COCl2. The new compounds are closo- [3-Co(eta (5)-NC4H4)-1,2-(SCH3)(2)-1,2-C2B9H9] (5 ), closo-[3-Co(eta (5)-NC4H4)-1,2-(SCH3)(2)-1,2-C2B9H9] (6), and closo-[3-C o(eta (5)-NC4H4)-1-C6H5-1,2-C2B9H10] (7). Their crystal structures were elu cidated by single-crystal X-ray diffraction and are compared with those of the parent compound closo-[3-Co(eta (5)-NC4H4)-1,2-C2B9H11] (1) and earlier prepared compounds closo-[3-Co(eta (5)-NC4H4)-1-CH3-2-C4H9-1,2-C2B9H9] (2) and closo-[3-Co(eta (5)-NC4H4)-1-C6H5-2-C3H5-1,2-C2B9H9] (3). Purely alkyl substituents alter the C-c. . .C-c distance of the parent compound very li ttle, which implies that steric effects, although relevant, are not the maj or cause of the lengthening. In contrast, substituents with lone pairs alte r the C-c. . .C-c distance substantially. Computational methods suggest tha t the cause of the elongation is the transfer of electron density from the available lone pairs on the substituents to the Psi* orbitals on C-c, produ cing a decrease in the C-c. . .C-c bond order and, thereby, an increase in the C-c. . .C-c distance.