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 of phosphorus selenides are similar to their sulfide analogues, there are some new forms, molecular P2Se5 and the polymeric catena-[P4Se4]_{x}. There is also some doubt about the existence of molecular P4Se10.

P2Se5
Molecular P2Se5 has a norbornane like structure with two phosphorus atoms with oxidation state +3 bridged by two diselenide units (\sSe\sSe\s, analogous to disulfide) and one selenide unit (\sSe\s). 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.

P4Se4
P4Se4 has 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 textbook 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 textbook 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 Na4[Ge4Se10].

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_{x}Se_{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 crystallise. 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.