X-RAY STRUCTURE AND BONDING OF 1-PHENYLETHYNYL-2-PHENYL-1,2-DICARBADODECABORANE(12), [1-(PHC=C)-2-PH-1,2-C2B10H10], A MODEL ALKYNE COMPLEX CONTAINING A RICH VARIETY OF CARBON-CARBON BOND TYPES

The crystal and molecular structure of 1-phenylethynyl-2-phenyl-1,2-dicarbadodecaborane(12) (1), prepared by the reaction between 1,4-diphenylbutadi-yne and the decaborane adduct B10H12(NCMe)2 in boiling toluene, have been established by an X-ray crystallographic study. Evidence of some delocalization of pi-electronic charge in the phenylethynyl ligand towards the carborane cage is provided by the carbon-carbon bond distances [1.431(2), 1.194(2) and 1.433(2) angstrom, respectively from benzene ring to carborane isocahedron] and Molecular Orbital Bond Index (MOBI) calculations, which also reveal subtle differences in the bonding of the two skeletal carbon atoms to their boron neighbours in the icosahedron. Consideration of (1) as an adduct in which one alkyne function of PhC=CC=CPh coordinates to an arachno-shaped B10H10 residue reveals the strong electron-withdrawing capacity of the latter, which is compared with other borane and metal cluster residues.