How to make 8,1,2-closo-MC2B9 metallacarboranes

Three examples of the rare 8,1,2-closo-MC₂B₉ isomeric form of an icosahedral metallacarborane have been isolated as unexpected trace products in reactions. Seeking to understand how these were formed we considered both the nature of the reactions that were being undertaken and the nature of the coproducts. This led us to propose a mechanism for formation of the 8,1,2-closo-MC₂B₉ species. The mechanism was then tested, leading to the first deliberate synthesis of an example of this isomer. Thus, deboronation of 4-(η-C₅H₅)-4,1,8-closo-CoC₂B₁₀H₁₂ selectively removes the B5 vertex to yield the dianion [nido-(η-C₅H₅)CoC₂B₉H₁₁]^2–, oxidative closure of which affords 8-(η-C₅H₅)-8,1,2-closo-CoC₂B₉H₁₁ in moderate yield.