Diphosphene

Diphosphene is a compound having the formula. It exists as two geometric isomers, E and Z. Diphosphene is also the parent member of the entire class of diphosphene compounds with the formula, where R is an organyl group.
Visible radiation induces cis-trans isomerization, although further irradiation can excite the molecule to a triplet diradical state. In triplet trans-HPPH, the P-P bond length is predicted to be 2.291 Å. It is not only longer than the P-P double bond in ground state trans-bisdiphosphene, but also longer than that of P-P single bond in. Calculation of the dihedral angle of trans-HPPH suggests that it is almost 90 degree, which means the formation of and P-P bonds is forbidden and σ bond is enhanced.