title: Phosphorus selenide
text: # 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 $P_2Se_5$ and the polymeric $catena-[P_4Se_4]_x.$ There is also some doubt about the existence of molecular $P_4Se_{10}.$
  
  ## Crystallographically confirmed compounds
  
  ### $P_2Se_5$
  
  Molecular $P_2Se_5$ 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 $(CS_2)$ extraction from a $P_2Se_5$ amorphous phase made from the elements.
  
  ### $P_4Se_3$
  
  $P_4Se_3$ has been characterised crystallographically and has the same structure as the low temperature form of $P_4S_3.$ It can be prepared from the elements. One preparation is to extract and recrystallise using tetralin.
  
  ### $P_4Se_5$
  
  The $P_4Se_5$ molecule has the same structure as $P_4S_5.$ It was prepared by reacting $P_4Se_3$ with bromine in $CS_2.$
  
  ### $catena-[P_4Se_4]_x$
  
  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
  
  ### $P_4Se_4$
  
  $P_4Se_4$ has two crystalline forms $α-P_4Se_4$ with the same molecular structure as $α-P_4S_4$ and $β-P_4Se_4$ with same molecular structure as $β-P_4S_4.$ A fully characterised compound $(CuI)_3P_4Se_4$ contains $P_4Se_4$ with a $β-P_4S_4$ structure.
  
  ### $P_4Se_7$
  
  This has been reported to have the same structure as $P_4S_7.$ One well-known textbook does not mention it at all.
  
  ### $P_4Se_{10}$
  
  Molecular $P_4Se_{10}$ has been reported to share the same structure as $P_4S_{10}$ and $P_4O_{10},$ 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 $P_4Se_{10}.$ The isoelectronic anion $[Ge_4Se_{10}]^{4−}$ which has the adamantane like $P_4O_{10}$ structure is known, an example is the sodium salt $Na_4[Ge_4Se_{10}].$
  
  ## Other compounds
  
  $P_4Se_6,$ $P_{14}Se,$ $P_4Se,$ $P_4Se_2,$ $P_2Se,$ have been reported.
  
  ## Phosphorus - selenium glasses
  
  Phosphorus - selenium glasses have been examined using $^{31}P-NMR$ 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 $P_4Se_3)$ 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 $P_2$ units (P in formal oxidation state +4). There is no evidence for an amorphous phase containing molecular $P_4Se_{10}.$
  
  