Carbene-induced synthesis of the first borironium cations using the [(η(5)-C5Me5)Fe(CO)2](-) anion as an unlikely leaving group.

Reaction of N-heterocyclic carbenes with ferroborirene complex [{(η(5)-C5Me5)Fe(CO)2}{BC2(SiMe3)2}] results in heterolytic Fe-B bond cleavage, yielding borironium ions, a new class of boron-containing heterocycles. The reaction rests on the surprising ability of the reactive [(η(5)-C5Me5)Fe(CO)2](-) anion to act as a leaving group. The properties of these species were investigated by multinuclear NMR spectroscopy, as well as single-crystal X-ray diffraction.