Room temperature stable ,-diphosphenes: their isomerization, coordination, and cycloaddition chemistry.

,-isomers display distinct physical properties and chemical reactivities. However, investigations on heavy main group elements remain limited. In this work, we present the isolation and X-ray crystallographic characterization of N-heterocyclic vinyl (NHV) substituted diphosphenes as both - and -isomers (L[double bond, length as m-dash]CH-P[double bond, length as m-dash]P-CH[double bond, length as m-dash]L, ,-2b; L = N-heterocyclic carbene). -2b is thermodynamically more stable and undergoes reversible photo-stimulated isomerization to -2b. The less stable -isomer -2b can be thermally reverted to -2b. Theoretical studies support the view that this ↔ isomerization proceeds P[double bond, length as m-dash]P bond rotation, reminiscent of the isomerization observed in alkenes. Furthermore, both ,-2b coordinate to an AuCl fragment affording the complex [AuCl(η--2b)] with the diphosphene ligand in -conformation, exclusively. In contrast, ,-2b undergo [2 + 4] and [2 + 1] cycloadditions with dienes or diazo compounds, respectively, yielding identical cycloaddition products in which the phosphorus bound NHV groups are in -position to each other. DFT calculations provide insight into the /-isomerisation and stereoselective formation of Au(i) complexes and cycloaddition products.