Beryllium can coordinate with cyclic alkyl amino carbene (CAAC) ligands and can form beryllium radicals which can be present with beryllium complexes (). A CAAC ligand coordinates a 2 electron -1 charge into the beryllium center. CAAC has an "amino" substituent and an "alkyl" sp carbon atom. CAACs are very good σ donors (higher HOMO) and π acceptors (lower LUMO) compared to NHCs. In addition, the lower heteroatom stability of the carbene center in CAAC compared to NHC results in a lower ΔE.