The structure of [7,8-C 2 B 9 H 12 ] – ; correction of a popular misconception

The [7,8-C2B9H12]– ion has been shown, via n.m.r. spectroscopic and X-ray crystallographic study, to contain an endo-H atom bonded to B(10) and not, as generally thought, a B–H–B bridge. Four ion pairs of [H(dmso)2][C2B9H12](dmso = dimethyl sulphoxide) crystallise in a unit cell of dimensions a= 8.2919(14), b= 11.0802(21), c= 18.128(5)Å, β= 100.135(18)°, and space group P21/n. The structure has been refined to R= 0.0436, R′= 0.0698 for 2 355 reflections measured at 185 ± 1 K. The presence of an endo-H atom in [7,8-C2B9H12]– is rationalised by consideration of the form of the highest occupied molecular orbital of [7,8-C2B9H11]2–. Analysis of the results of extended-Hückel molecular orbital calculations confirms that the endo-H atom bonds to the cage via a four-centre two-electron bond, and allows the relative magnitudes of endo and exo B–H coupling constants to be understood as well as the observation of weak coupling between the endo-H atom and B(9) and B(11).