Selenium tetrachloride is the

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

composed with the

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...

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, Se₂Cl₂. SeCl₄ is used in the synthesis of other

selenium Selenium is a chemical element; it has symbol (chemistry), symbol Se and atomic number 34. It has various physical appearances, including a brick-red powder, a vitreous black solid, and a grey metallic-looking form. It seldom occurs in this elem ...

compounds.

Synthesis and structure

The compound is prepared by treating

with

chlorine Chlorine is a chemical element; it has Symbol (chemistry), 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 ...

. When the reacting selenium is heated, the product sublimes 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 ...

ic cubane-type cluster, 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 Valence shell electron pair repulsion (VSEPR) theory ( , ) is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. It is also named the Gillespie-Nyholm theor ...

rules of

hypervalent molecule In chemistry, a hypervalent molecule (the phenomenon is sometimes colloquially known as expanded octet) is a molecule that contains one or more main group elements apparently bearing more than eight electrons in their valence shells. Phosphorus ...

s. 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 ordered arrangement of atoms, ions, or molecules in a crystalline material. Ordered structures occur from intrinsic nature of constituent particles to form symmetric patterns that repeat ...

. 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

acid An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...

s: :SeCl₄ + 3 H₂O → H₂SeO₃ + 4 HCl Upon treatment with

selenium dioxide Selenium dioxide is the chemical compound with the formula SeO2. This colorless solid is one of the most frequently encountered compounds of selenium. It is used in making specialized glasses as well as a reagent in organic chemistry. Properties ...

, it gives selenium oxychloride: :SeCl₄ + SeO₂ → 2SeOCl₂

References

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