# Diphosphenes

Diphosphene is a type of organophosphorus compound that has a phosphorus–phosphorus double bond, denoted by R-P=P-R'. These compounds are not common, but their properties have theoretical importance.
Normally, compounds with the empirical formula RP exist as rings. However, like other multiple bonds between heavy main-group elements, P=P double bonds can be stabilized by large steric hindrance. In general, diphosphenes react like alkenes.

## History

In 1877, Köhler and Michaelis claimed what would have been the first isolated diphosphene (PhP=PPh), The structure of Köhler and Michaelis' product was later revised. and X-ray crystallographic analysis proved that this "diphosphene" only had P-P single bonds and was in fact primarily a four-membered ring of the form (PPh)$_4$. The isolation of phosphorus ylide and phosphaalkenes suggested that compounds with P=P bonds could be made.
Yoshifuji et al's isolated a sterically-hindered diphosphene in 1981. That compound's P-P bond distance is 2.034 Å, which is much shorter than the average bond length in (C$_6$H$_5$P)$_5$ (2.217 Å) and (C$_6$H$_5$P)$_6$ (2.237 Å) and indicates double-bond character.

## Synthesis

Following Maasaka Yoshifuji and his coworkers' 1981 preparation of bis(2,4,6-tri-tert-butylphenyl)diphosphene, most disphosphene syntheses involve dehalogenation of bulkyl aryldichlorophosphine (ArPCl$_2$). Mg is a typical dehalogenation reagent:
    2 ArPCl$_2$ + 2 Mg → ArP=PAr + 2 MgCl$_2$
Such a synthesis works also for trisalkylsilylphosphines, or N-heterocyclic boro-phosphines.

### Ylide-stabilized diphosphenes

Examples of di-vinyl-substituted diphosphenes arise via a ring opening/dimerization process from kinetically unstable 2H-phosphirenes. However, the conjugation caused the compounds to exhibit reactivity closer to a phosphinidene.

## Structure

Cyclic voltammetry and UV/Vis spectra indicate that boryl-substituted diphosphenes have lower LUMO level and larger HOMO-LUMO gap than aryl-substituted diphosphenes.

### Geometry

According to X-ray crystallography, the following parameters describe bis(2,4,6-tri-tert-butylphenyl)diphosphene: P-P = 2.034 (2) Å; P-C = 1.826 (2) Å; $\angle$P-P-C = 102.8 (1)$^o$; $\angle$C-P-P-C = 172.2 (1)$^o$. Compared with the length of a P-P single bond in H$_2$PPH$_2$ (2.238 Å), the P-P bond distance is much shorter, which reveals double bond character. The trans orientation is the thermodynamically preferred isomer.

### Spectroscopic properties

Diphosphene compounds usually exhibit a symmetry-allowed ($\pi \rightarrow \pi ^{*}$) (intense) and symmetry-forbidden ($n\rightarrow \pi ^{*}$) (weak) electronic transitions. In the Raman spectrum, the P=P vibration is enhanced by resonance with allowed the $\pi \rightarrow \pi ^{*}$ transition than with the forbidden $n\rightarrow \pi ^{*}$ transition due to different geometries of excited states and enhancement mechanism. Also the observed strong Raman shifts for (CH(SiMe $_3$) $_2$) $_2$P $_2$and (CH(SiMe$_3$)$_2$P=PC(SiMe$_3$)$_2$) suggest stronger dipnictenes feature of diphosphene compared with P-P single bond.

## Reactivity

Lithium aluminium hydride reduces diphosphene to give diphosphanes.
Carbenes add across the double bond, to give diphosphiranes, which further rearrange to 1,3-diphospha-allenes in strong bases.
Diphosphene is inert to oxygen but cycloadds to ozone to give highly unstable phosphorus-oxygen rings that tend to attack the phosphorus' organyl substituents. The reaction with ozone is much more rapid and indicates a 2:1 (ozone:diphosphene) stoichiometry.
When treated with strongly nucleophilic NHC's, the P=P bond cleaves giving phosphinidene compounds:
    RP=PR + L → 2 RP−L

### Coordination to transition metals

Diphosphines form a variety of [[coordination complexes.. Diphosphenes can bind to transition metal either in a η$^1$ or in a η$^2$ mode.
[{{chem2|Fe(CO)4[P2(CH(SiMe3)2}) is obtained by treating Na$_2$[Fe(CO)$_4$] with dichlorobis(trimethylsilyl)methylphosphine. The related complex [ArP=PAr]Fe(CO)$_4$ (Ar=2,4,6-tri-tert-butylphenyl) arises by treating diphosephene with Fe$_2$(CO)$_9$.
η$^2$-coordination is illustrated by (M(PhP=PPh)L$_2$) (with M=Pt or Pd and L = (PPh$_3$)$_2$ or Ph$_2$P[CH$_2$]$_2$PPh$_2$).