Phosphorus selenide

Phosphorus selenides are a relatively obscure group of compounds. There have been some studies of the phosphorus - selenium phase diagram and the glassy amorphous phases are reported. The compounds that have been reported are shown below. While some are similar to their phosphorus analogues, the phosphorus sulfides, there are some new forms, molecular P₂Se₅ and the polymeric catena-[P₄Se₄]_x. There is also some doubt about the existence of molecular P₄Se₁₀.

Crystallographically confirmed compounds

P₂Se₅

Molecular P₂Se₅ has a norbornane like structure with two phosphorus atoms with oxidation state +3 bridged by two diselenide units, (-Se-Se-), (analogous to disulfide) and one selenide unit(-Se-). It was isolated by solvent (CS₂) extraction from a P₂Se₅ amorphous phase made from the elements.

P₄Se₃

P₄Se₃ has been characterised crystallographically and has the same structure as the low temperature form of P₄S₃. It can be prepared from the elements. One preparation is to extract and recrystallise using tetralin.

P₄Se₅

The P₄Se₅ molecule has the same structure as P₄S₅. It was prepared by reacting P₄Se₃ with bromine in CS₂.

catena-[P₄Se₄]_x

This compound consists of polymeric chains of norbornane-like units joined by Se atoms. As each P atom in the repeat unit is bonded to another P atom and to two Se atoms, each P atom has a formal oxidation state of +2.

Compounds confirmed spectroscopically

P₄Se₄

P₄Se₄has two crystalline forms α-P₄Se₄ with the same molecular structure as α-P₄S₄ and β-P₄Se₄ with same molecular structure as β-P₄S₄. A fully characterised compound (CuI)₃P₄Se₄ contains P₄Se₄ with a β-P₄S₄ structure.

P₄Se₇

This has been reported to have the same structure as P₄S₇. One well known text book does not mention it at all.

P₄Se₁₀

Molecular P₄Se₁₀ has been reported to share the same structure as P₄S₁₀ and P₄O₁₀, but one well known text book does not mention it at all. A review (2001) examining P-Se amorphous phases did not confirm the presence of molecular P₄Se₁₀. The isoelectronic anion [Ge₄Se₁₀]⁴⁻ which has the adamantane like P₄O₁₀ structure is known, an example is the sodium salt Na₄Ge₄Se₁₀.

Other compounds

P₄Se₆, P₁₄Se, P₄Se, P₄Se₂, P₂Se, have been reported.

Phosphorus - Selenium glasses

Phosphorus -selenium glasses have been examined using ³¹P NMR and Raman spectroscopy. Glasses are formed in P_xSe_1-x over the range of compositions 0 < x < 0.8 with a small window around 0.52-0.60 centred on 0.57 (corresponding to the compound P₄Se₃) where there is a tendency to crytallise. For x < 0.47 the glasses contain Se_n chain fragments, pyramidal P units (P oxidation state +3), quasi-tetrahedral P units (P oxidation state +5, with P=Se double bond) and P₂ units (P in formal oxidation state +4). There is no evidence for an amorphous phase containing molecular P₄Se₁₀.