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→C₂ P₂ ←NHC (1 a,b) (NHC=IPr or SIPr) contain a four-membered C₂ P₂ 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 [M⁰ (CO)₃ ] (M=Cr, Mo), [Co^I (CO)₂ ]⁺ , or [Ni^II Br₂ ], through an η⁴ -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.