The Lightest Element Phosphoranylidene: NHC‐Supported Cyclic Borylidene–Phosphorane with Significant B=P Character

A borylidene–phosphorane, the lightest phosphoranylidene, which is stabilized by an N ‐heterocyclic carbene ligand, was synthesized and fully characterized. Experimental electron density analysis and DFT calculations indicate an enhanced ylene character (rather than an ylide character) with an exceptionally strong B→P π‐back bonding related to the less electronegative boron compared to phosphorus.