Facile Synthesis of a Stable Dihydroboryl {BH₂}⁻ Anion

While the one-electron reduction of (CAACMe )BH₂ Br (CAACMe =1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) yields a hydride-shift isomer of the corresponding tetrahydrodiborane, a further reversible reduction leads to the first stable parent boryl anion, [(CAACMe )BH₂ ]⁻ , which acts as a powerful boron nucleophile.