Valence Isomerization of a 1,3-Diphosphacyclobutane-2,4-diyl: Photochemical Ring Closure to 2,4-diphosphabicyclo[1.1.0]butane and Its Thermal Ring Opening to gauche-1,4-Diphosphabutadiene

The bond stretch isomer 1,3-diphosphacyclobutane-2,4-diyl 1 was transformed photochemically to give the previously unknown 2,4-diphosphabicyclo1.1.0]butane 2 , which itself can be converted thermally into gauche-1,4-diphosphabutadiene 3 . The crystal structures of these three energy-rich valence isomers of 1,2-diphosphete have been determined. R=SiMe₃; Mes*=2,4,6-tBu₃C₆H₂.