Diphosphene

Diphosphene is a compound having the formula (PH)₂. 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 (PR)₂, 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-bis(2,4,6-tri-tert-butylphenyl)diphosphene, but also longer than that of P-P single bond in H₂P−PH₂. Calculation of the dihedral angle of trans-HPPH suggests that it is almost 90 degree, which means the formation of ${\displaystyle \pi }$ and ${\displaystyle \pi ^{*}}$ P-P bonds is forbidden and σ bond is enhanced.