Synthesis and molecular structure of 1,12-dicarba-closo-dodecaborane(12)-1,12-dithiol, 1,12-(SH)(2)-1,12-C2B10H10, in the gaseous phase, determined by electron diffraction and ab initio calculations; geometrical consequencesof three-dimensional aromaticity in carbaboranes 1,12-X-2-1,12-C2B10H10

The molecular structure of 1,12-dicarba-closo-dodecaborane(12)-1,12-dithiol , 1,12-(SH)(2)-1,12-C2B10H10, prepared by means of an improved synthesis, h as been determined by gas-phase electron diffraction restrained by ab initi o calculations. The carbaborane core, shown by calculations at the MP2(fc)/ 6-31G* level to be very close to D-5d symmetry, gave good agreement between theoretical and experimental B-11 NMR chemical shifts. A model of the enti re molecule in overall C-2 symmetry led to an experimental geometry (R-G=0. 077), in good agreement with the theoretical findings. The substituents do not distort the cage significantly. The well determined parameters, the C-B , B(2)-B(3), and B(2)-B(7) distances, 170.6(4), 177.5(3) and 176.5(9) pm re spectively (r(a)), are consistent with the analogous parameters established experimentally for other 1,12-disubstituted 1,12-dicarbadodecaboranes. Whe reas the C-B and B-B distances are found to be relatively constant in the M P2(fc)/6-31G* geometries of a series of carbaboranes 1,12-X-2-1,12-C2B10H10 (in addition to SH, X=H, Li, BeH, BH2, CH3, SiH3, NH2, OH, F and Cl) the C (1). . .C(12) distances and B(2)-C(1)-C(12) angles are appreciably sensitiv e to the nature of X, in a similar manner to the para-disubstituted benzene derivatives, 2xB(2)-C(1)-C(12) being viewed as an analogue of the ipso ang le in the latter.