Polyselenide

In chemistry, a polyselenide usually refers to anions of the formula (Se_n)²⁻, where Se is the atomic symbol for the element selenium. Many main group and transition metals form complexes with polyselenide anions.^[1]

Structures of two polyselenide anions.

Preparation

Conceptually, polyselenides are derived by deprotonation polyselenanes H₂Se_n, but compounds with Se-H bonds are rare or labile. Instead, analogous to the preparation of many Zintl ions, polyselenides can produced by reduction of elemental Se with alkali metals. Such reactions can be conducted by heating a mixture of the solids or by dissolving Se metal in amine solutions of alkali metals. Synthesis can also be conducted in high-boiling, polar, aprotic solvents such as DMF, HMPA, and NMP.^[2] These reactions appear to proceed by initial formation of the alkali metal selenide, followed by the reaction of the latter with additional selenium:

2 Na + Se → Na₂Se

Na₂Se + n Se → Na₂Se_n+1

Once generated, alkali metal polyselenides can be converted to lipophilic salts by treatment cryptand ligands or by ion exchange with quat salts.^[3]

Na₂Se_n + 2 R₄NCl → (R₄N)₂Se_n + 2 NaCl

Sodium polyselenide can also be generated by heating elemental selenium with sodium hydroxide.^[4]

Structures

Salts of polyselenides have often been characterized by X-ray crystallography. Polyselenides salts generally feature open chains, which adopt a zig-zag conformation. In rare cases, cyclic structures are observed as in Li₂Se₅, which features a square-planar Se center. High resolution solid state ⁷⁷Se NMR spectra of [NMe₄]₂Se₆ show three selenium sites. Single-crystal X-ray structure determination of the two salts support the NMR data.^[5]

Reactivity

Structure of (C₅H₅)₂TiSe₅.

Polyselenides are prone to decomposition on exposure to air, in which case they are oxidized back to elemental selenium.

Se²⁻
_n + 2 H⁺ + 1⁄2 O₂ → n Se + H₂O

As ligands in coordination complexs, polyselenides are generally bidentate. Complexes of penta-, tetra-, and triselenide ligands are known. One example is the spirocyclic [Zn(Se₄)₂]²⁻.^[6]

Further reading

• Graf, Christian; Assoud, Abdeljalil; Mayasree, Oottil; Kleinke, Holger (2009). "Solid State Polyselenides and Polytellurides: A Large Variety of Se–Se and Te–Te Interactions". Molecules. 14 (9): 3115–3131. doi:10.3390/molecules14093115. PMC 6255372. PMID 19783911.
• Sheldrick, William S. (2012). "Polychalcogenide Anions: Structural Diversity and Ligand Versatility". Zeitschrift für Anorganische und Allgemeine Chemie. 638 (15): 2401–2424. doi:10.1002/zaac.201200241.

See also

• Polytelluride