MOLECULAR-STRUCTURE OF GASEOUS 1,7-DICHLORO-1,7-DICARBA-CLOSO-DODECABORANE(12), 1,7-CL2-1,7-C2B10H10, AS STUDIED BY ELECTRON-DIFFRACTION AND AB-INITIO CALCULATIONS

Gas-phase electron diffraction (GED) data for 1,7-dichloro-1,7-dicarba-closo-dodecaborane(12), 1,7-Cl2-1,7-C2B10H10, were fitted by a structure possessing overall C2v symmetry. Not all of the independent parameters could be refined, and some differences between C-B and B-B bond lengths were fixed at values calculated ab initio. Assumption of local C5v symmetry for the CB5 pentagonal pyramids led to the same GED fit (R(G) = 0.065) as with a model in which the B5 rings adjacent to the carbon atoms were not quite regular, as found by ab initio optimisations [HF/6-31 G* and MP2(fc)/6-31 G*]. Nearest-neighbour C-B and B-B separations do not deviate significantly from normal values, and distortion of the cage from regular icosahedral symmetry is small. The C-Cl bond length, 175.9(9) pm, [MP2/6-31 G* 174.9 pm], is shorter than in C(Sp3)-Cl systems and longer than in C(Sp2)-Cl systems. The geometrical parameters calculated at the MP2(fc)/6-31 G* level, as well as B-11 NMR chemical shifts, calculated by the individual gauge for localised orbitals method, are in satisfactory agreement with the experimental observations.