MOLECULAR-STRUCTURE OF 1,2-DICARBA-CLOSO-DECABORANE(10) AS STUDIED BYTHE CONCERTED USE OF ELECTRON-DIFFRACTION AND AB-INITIO CALCULATIONS

An electron diffraction study of 1,2-dicarba-closo-decaborane (10), 1, 2C(2)B(8)H(10), constrained by ab initio calculations has been underta ken. A satisfactory refinement (R(G) = 0.052) was obtained with a bica pped ''square'' antiprismatic model assuming C-s symmetry. In this str ucture one carbon atom occupies one of the two capping positions, whil e the other is adjacent, in a basal position of a ''square'' pyramid. The experimental molecular dimensions (r(a)) are consistent with the r (e) structure as derived by the MP2(fc)/6-31G optimization. The C-C e dge [r(a) = 153.8(8) pm, r(e) = 153 pm] leads to distortion from the r egular square pyramidal. shape. The compression of the carbon atoms to ward the center of the cluster results in a substantial opening of the B(3)C(2)B(5) bond angle [both electron diffraction and MP2(fc)/6-31G calculations give 95 degrees] from the parent 90 degrees. As a conseq uence, the C(2)B(3) distance is the longest CB nearest-neighbor separa tion so far observed in the gas phase, r(a) = 179.4(4) pm [cf. r(e) = 177.7 pm, MP2(fc)/6-31G)]. The reliability of the experimental struct ure is assessed both by IGLO (individual gauge for localized orbitals) calculations of B-11 chemical shifts and by MP2(fc)/6-31G single-poi nt energy calculations.