Reaction of N-heterocyclic carbenes with ferroborirene complex [{(η5-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 [(η5-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.
![Graphical abstract: Carbene-induced synthesis of the first borironium cations using the [(η5-C5Me5)Fe(CO)2]− anion as an unlikely leaving group](http://hg.y866.cn/compound/lib/scimg/usr/1/C5CC07503C.jpg)
