Diphosphenes

However, the heavier pnictogens were known to form oligomers in oxidation state I, and by 1958, chemists had begun to reconsider the structure of Köhler and Michaelis' product.

During the subsequent decade (the 1960s), molecular weight determination[4] and X-ray crystallographic analysis[5] proved that this "diphosphene" only had P-P single bonds and was in fact primarily a four-membered ring of the form (PPh)4.

Nevertheless, the contemporary discoveries of the first diphosphorus ylide and first phosphaalkene suggested that compounds with multiply-bonded phosphorus could be made.

[7] Following Maasaka Yoshifuji and his coworkers' 1981 isolation of bis(2,4,6-tri-tert-butylphenyl)diphosphene,[7] most disphosphene syntheses dehalogenate a bulkyl alkyldichlorophosphine with an active metal.

[9] Examples of di-vinyl-substituted diphosphenes arise via a ring opening/dimerization process from kinetically unstable 2H-phosphirenes.

[13] Raman spectroscopy observes significant enhancement of P=P stretch in the resonance with allowed electron

[15][failed verification] Lithium aluminium hydride reduces diphosphene to give diphosphanes:[16] Visible radiation induces cis-trans isomerization,[12] 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.

Calculation of the dihedral angle of trans-HPPH suggests that it is almost 90 degree, which means the formation of

[17] Diphosphene is inert to oxygen but cycloadds to ozone to give highly unstable phosphorus-oxygen rings that tend to attack the phosphorus' organyl substituents.

[19] Carbenes add across the double bond, to give diphosphiranes, which further rearrange to 1,3-diphospha-allenes in strong bases.

[20] Unlike with olefins, the ylides traditionally called persistent carbenes instead tend to cleave the central bond, forming two phosphaalkene/phosphinidene compounds.

[21] The compounds form a wide variety of transition metal alkene complexes (see § Coordination to transition metals), as well as the traditional complexation to the phosphorus lone pair, or to any aryl moieties present.

Diphosphenes can bind to transition metal either in a η1 mode by donating a lone pair on phosphorus, or in a η2 behavior via a

[22] ArP=PArFe(CO)4 (Ar=2,4,6-tri-tert-butylphenyl) arises simply by treating diphosephene with Fe2(CO)9.Cowley, A. H.; Kilduff, J. E.; Lasch, J. G.; Norman, N. C.; Pakulski, M.; Ando, F.; Wright, T. C. (1983-12-01).