Selenium tetrachloride is the

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemi ...

composed with the formula SeCl₄. This compound exists as yellow to white volatile solid. It is one of two commonly available selenium chlorides, the other example being

selenium monochloride Selenium monochloride is an inorganic compound with the formula Se2Cl2. Although it is called selenium monochloride, a more descriptive name might be diselenium dichloride. It is a reddish-brown, oily liquid that hydrolyses slowly. It exists in c ...

, Se₂Cl₂. SeCl₄ is used in the synthesis of other

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 telluriu ...

compounds.

Synthesis and structure

The compound is prepared by treating

with

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is ...

. When the reacting selenium is heated, the product

sublimes Sublimation is the transition of a substance directly from the solid to the gas state, without passing through the liquid state. Sublimation is an endothermic process that occurs at temperatures and pressures below a substance's triple point i ...

from the reaction flask. The volatility of selenium tetrachloride can be exploited to purification of selenium. Solid SeCl₄ is actually a

tetramer A tetramer () (''tetra-'', "four" + '' -mer'', "parts") is an oligomer formed from four monomers or subunits. The associated property is called ''tetramery''. An example from inorganic chemistry is titanium methoxide with the empirical formula T ...

cubane-type cluster A cubane-type cluster is an arrangement of atoms in a molecular structure that forms a cube. In the idealized case, the eight vertices are symmetry equivalent and the species has Oh symmetry. Such a structure is illustrated by the hydrocarbon ...

, for which the Se atom of an SeCl₆ octahedron sits on four corners of the cube and the bridging Cl atoms sit on the other four corners. The bridging Se-Cl distances are longer than the terminal Se-Cl distances, but all Cl-Se-Cl angles are approximately 90°. SeCl₄ has often been used as an example for teaching VSEPR rules of hypervalent molecules. As such, one would predict four bonds but five electron groups giving rise to a seesaw geometry. This clearly is not the case in the

crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns t ...

. Others have suggested that the crystal structure can be represented as SeCl₃⁺ and Cl⁻. This formulation would predict a pyramidal geometry for the SeCl₃⁺ cation with a Cl-Se-Cl bond angle of approximately 109°. However, this molecule is an excellent example of a situation where maximal bonding cannot be achieved with the simplest molecular formula. The formation of the tetramer (SeCl₄)₄,Wells, ''Structural Inorganic Chemistry'', fifth ed, Oxford, p. 709, with delocalized sigma bonding of the bridging chloride is clearly preferred over a "hypervalent" small molecule. Gaseous SeCl₄ contains SeCl₂ and chlorine, which recombine upon condensation.

Reactions

Selenium tetrachloride can be reduced in situ to the dichloride using triphenylstibine: :SeCl₄ + SbPh₃ → SeCl₂ + Cl₂SbPh₃ Selenium tetrachloride reacts with water to give selenous and hydrochloric acids: :SeCl₄ + 3 H₂O → H₂SeO₃ + 4 HCl Upon treatment with selenium dioxide, it gives selenium oxychloride: :SeCl₄ + SeO₂ → 2SeOCl₂

References

{{Prostanoidergics Selenium(IV) compounds Chlorides Nonmetal halides Chalcohalides