chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...

, a polyselenide usually refers to anions of the formula (Se_n)^2-, where Se is the atomic symbol for the element

selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...

. Many

main group In chemistry and atomic physics, the main group is the group of elements (sometimes called the representative elements) whose lightest members are represented by helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, and fluorine as arra ...

and

transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...

s form complexes with polyselenide anions.

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 ion Nanoclusters are atomically precise, crystalline materials most often existing on the 0-2 nanometer scale. They are often considered kinetically stable intermediates that form during the synthesis of comparatively larger materials such as semico ...

s, 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. These reactions appear to proceed by initial formatiion 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 In chemistry, cryptands are a family of synthetic, bicyclic and polycyclic, multidentate ligands for a variety of cations. The Nobel Prize for Chemistry in 1987 was given to Donald J. Cram, Jean-Marie Lehn, and Charles J. Pedersen for their e ...

ligands or by ion exchange with

quat salts In chemistry, quaternary ammonium cations, also known as quats, are positively charged polyatomic ions of the structure , R being an alkyl group or an aryl group. Unlike the ammonium ion () and the primary, secondary, or tertiary ammonium cations ...

.Kolis, J. "Coordination Chemistry of Polychalcogen Anions and Transition Metal Carbonyls" Coordination Chemistry Reviews 1990, volume 105, pp. 195-219. : Na₂Se_''n'' + 2 R₄NCl → (R₄N)₂Se_''n'' + 2 NaCl

Structures

Salts of polyselenides have often been characterized by

X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

. 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 Me₄sub>2Se₆ show three selenium sites. Single-crystal X-ray structure determination of the two salts support the NMR data.

Reactivity

Polyselenides are prone to decomposition on exposure to air, in which case they are oxidized back to elemental selenium. : + 2 H⁺ + O₂ → ''n'' Se + H₂O As ligands in

coordination complex A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...

s, polyselenides are generally bidentate. Complexes of penta-, tetra-, and triselenide ligands are known. One example is the spirocyclic n(Se₄)₂sup>2-.

References

{{Selenides Selenides

Preparation

Structures

Reactivity

Further reading

See also

References