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Diphosphane

Names
IUPAC name Diphosphane
Systematic IUPAC name Diphosphane (substitutive) Tetrahydridodiphosphorus(P—P) (additive)
Other names Diphosphine
Identifiers
CAS Number	13445-50-6 N
3D model (JSmol)	Interactive image
ChEBI	CHEBI:35880 N
ChemSpider	122832 N
PubChem CID	139283
CompTox Dashboard (EPA)	DTXSID40924074
InChI InChI=1S/H4P2/c1-2/h1-2H2 N Key: VURFVHCLMJOLKN-UHFFFAOYSA-N N
SMILES PP
Properties
Chemical formula	P2H4
Melting point	−99 °C (−146 °F; 174 K)
Boiling point	63.5 °C (146.3 °F; 336.6 K) (Extrapolated, decomposes)
Related compounds
Other anions	ammonia hydrazine triazane
Other cations	diphosphines
Related Binary Phosphorus halides	diphosphorus tetrafluoride diphosphorus tetrachloride diphosphorus tetrabromide diphosphorus tetraiodide
Related compounds	phosphane triphosphane diphosphene diphosphenes
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Diphosphane, or diphosphine, is an inorganic compound with the chemical formula P₂H₄. This colourless liquid is one of several binary phosphorus hydrides. It is the impurity that typically causes samples of phosphine to ignite in air.

Diphosphane adopts the gauche conformation (like hydrazine, less symmetrical than shown in the image) with a P−P distance of 2.219 angstroms. It is nonbasic, unstable at room temperature, and spontaneously flammable in air. It is only poorly soluble in water but dissolves in organic solvents. Its ¹H NMR spectrum consists of 32 lines resulting from an A₂XX'A'₂ splitting system.^[1]

Diphosphane is produced by the hydrolysis of calcium monophosphide, which can be described as the Ca²⁺ derivative of P4−2. According to an optimized procedure, hydrolysis of 400 g of CaP at −30 °C gives about 20 g of product, slightly contaminated with phosphine.

Reaction of diphosphane with butyllithium affords a variety of condensed polyphosphine compounds.

A variety of organic derivatives of diphosphane are known, but asymmetric diphosphanes are only stable at cryogenic temperatures. Otherwise, the substituents facily redistribute on the phosphorus centers to give a mixture of products. On the other hand, there appears to be a substantial barrier to chiral inversion.^[2]

The central bond is weak, and easily adds substituents.^[3]

The simplest synthesis method heats a phosphorus halide and a phosphane:

Ph₂PCl + HPPh₂ → Ph₂P−PPh₂ + HCl↑

Alkali metals can replace the hydrogen in that reaction (i.e., a dialkylphosphide), and in some rare cases a dialkylamine can replace the halide. Symmetric diphosphanes are easily prepared by reductive coupling, e.g. tetraphenyldiphosphine from chlorodiphenylphosphine:

2 ClPPh₂ + 2 Na → Ph₂P−PPh₂ + 2 NaCl

Ultraviolet radiation decomposes mercury(II) dialkylphosphides to the metal and a dialkylphosphane.^[4]

The methyl compound P₂Me₄ is prepared by the reduction of Me₂P(S)−P(S)Me₂, which is produced by methylation of thiophosphoryl chloride with methylmagnesium bromide.^[5]

Pure phosphine (PH
₃) is not pyrophoric at room temperature; its apparent spontaneous combustion in air was historically clarified in 1844 by Paul Thénard, who isolated liquid diphosphane as the pyrophoric impurity responsible for the ignition.^[6]

Trace formation of diphosphane alongside phosphine and methane during the anaerobic decomposition of organic matter is frequently cited as the chemical ignition mechanism for marsh gas, producing the flickering natural lights historically known as will-o'-the-wisp (ignis fatuus).^[7][8]

• Pnictogen hydride