N‐Heterocyclic Carbene Stabilized Dicarbondiphosphides: Strong Neutral Four‐Membered Heterocyclic 6π‐Electron Donors

In contrast to cyclic π‐conjugated hydrocarbons, the coordination chemistry of inorganic heterocycles is less developed. Dicarbondiphosphides stabilized by N‐heterocyclic carbenes (NHCs) NHC→C 2 P 2 ←NHC ( 1 a , b ) (NHC=IPr or SIPr) contain a four‐membered C 2 P 2 ring with an aromatic 6π‐electron configuration. These heterocycles coordinate to a variety of complex fragments with metals from groups 6, 9, and 10, namely [M 0 (CO) 3 ] (M=Cr, Mo), [Co I (CO) 2 ] + , or [Ni II Br 2 ], through an η 4 ‐coordination mode, leading to complexes 2 a , b , 3 a , b , 5 a , b , and 6 a , b , respectively. These complexes were characterized by X‐ray diffraction methods using single crystals, IR spectroscopy, and DFT calculations. In combination these methods indicate that 1 a , b behave as exceptionally strong 6π‐electron donors.