Borole

Boroles represent a class of molecules known as metalloles, which are heterocyclic 5-membered rings. As such, they can be viewed as structural analogs of cyclopentadiene, pyrrole or furan, with boron replacing a carbon, nitrogen and oxygen atom respectively. They are isoelectronic with the cyclopentadienyl cation C₅H+5 or abbreviated as Cp⁺ and comprise four π electrons. Although Hückel's rule cannot be strictly applied to borole, it is considered to be antiaromatic due to having 4 π electrons. As a result, boroles exhibit unique electronic properties not found in other metalloles.

The parent unsubstituted compound with the chemical formula C₄H₄BH has yet to be isolated outside a coordination sphere of transition metals. Substituted derivatives, which have been synthesized, can have various substituents at the 4 carbons and boron. The high electron deficiency leads to various reactivities such as metal free hydrogen activation and rearrangements upon cycloaddition which are unobserved in other structural analogues like pyrrole or furan.

Once reduced to the dianion, the borolediide complex gains aromaticity and can then participate in similar reactions as the Cp⁻ anion, including forming sandwich complexes.