Laser irradiation of a diphosphene: evidence for the first cis–trans isomerization

Low temperature laser irradiation of trans-diphosphene (1), followed using u.v., and 31P and 1H n.m.r. spectroscopy, gives an equilibrium between (1) and the unstable cis-isomer (2): the activation free energy of the (2)→(1) reaction is 20.3 kcal mol–1(1 kcal = 4.18 kJ).