Do the discrete dianions C₂B₉H₁₁^2- exist? Characterisation of alkali metal salts of the 11-vertex nido dicarboranes, C₂B₉H₁₁^2-, in solution

Detailed experimental solution-state NMR data are reported for the nine moisture-sensitive salts of M₂C₂B₉H₁₁ (M= Li, Na, K)1-9 generated by deprotonation of 7,8-C₂B₉H₁₃, 7,9-C₂B₉H₁₂^- and 2,9-C₂B₉H₁₃ by butyllithium, sodium hydride and potassium hydride. Indicative of cation-anion interactions, the¹¹B,¹³C and ¹H chemical shifts depend on the identity of the cation and, to a lesser degree, the solvent. Computed NMR shifts generated from MP2-optimised geometries of C₂B₉H₁₁^2-, LiC₂B₉H₁₁^- and NaC₂B₉H₁₁^- suggest that intimate ion-pair cluster anions MC₂B₉H₁₁^- are present in solutions of M₂C₂B₉H₁₁. As a test of the method of comparing experimental structures and chemical shifts with those from optimised geometries, the optimised geometries of the small carborane alkali metal salts M₂C₂B₄H₄(SiR₃) ₂ (M= Li or Na, R= H or Me) were computed and shown to agree well with experimental structures.