Borinium ions, that is, two-coordinate boron cations, are the most electron-deficient isolable boron compounds. As borinium ions have only four formal valence electrons on boron, they should show a strong tendency to accept electron pairs on the boron atom to fill its valence shell. Thus chemical reactions of borinium ions are expected to give products in which the coordination number of boron is increased from two to three or four. However, contrary to this expectation, we found that the dimesitylborinium ion (Mes2B+) undergoes twofold 1,2-carboboration reactions with two equivalents of diphenylacetylene to yield an unprecedented borinium ion (1+) with two substituted vinyl groups on the boron center. NMR spectroscopy and X-ray diffraction analysis of 1+, together with electronic-structure calculations, revealed that the positive charge is delocalized over the entire π-conjugated system. The fact that the chemical transformation of a borinium ion gives rise to a different borinium ion without a change in the coordination number is remarkable and should provide new insight into the chemistry of the Group 13 elements.
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