The structure of [7,8-C₂B₉H₁₂] ^-; 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-C₂B₉H ₁₂]^- is rationalised by consideration of the form of the highest occupied molecular orbital of [7,8-C₂B₉H ₁₁]^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).