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]

Preparation

Conceptually, polyselenides are derived by deprotonation polyselenanes H₂Se_n, but such species are rare or unstable. Instead, analogous to the preparation of many Zintl ions, polyselenides are 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

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.^[4]

Reactivity

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-.^[5]

References

^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 763-765. ISBN 978-0-08-037941-8.
^ Thompson, D.; Boudjouk, P. A (1988). "Convenient Synthesis of Alkali Metal Selenides and Diselenides in Tetrahydrofuran and the Reactivity Differences Exhibited By These Salts Toward Organic Bromides". Journal of Organic Chemistry. 53: 2109-2112. doi:10.1021/jo00244a051.
^ Kolis, J. "Coordination Chemistry of Polychalcogen Anions and Transition Metal Carbonyls" Coordination Chemistry Reviews 1990, volume 105, pp. 195-219. doi:10.1016/0010-8545(90)80023-M
^ Barrie, P. J.; Clark, R. J. H.; Selenium Solid-State NMR Spectroscopy and Structures of Tetramethylammonium Pentaselenide and Hexaselenide Complexes. Inorg. Chem, 1995, 34, 4299–4304 DOI: 10.1021/ic00121a006
^ Kanatzidis, Mercouri G. (1990). "Soluble Polychalcogenides of the Late Transition and Main Group Elements". Comments on Inorganic Chemistry. 10 (4–5): 161–195. doi:10.1080/02603599008048650.