SELENIUM TETRACHLORIDE [SeCl4]

·         CAS Number 10026-03-6

·         Linear Formula SeCl₄

·         Molecular Weight 220.77

·          EC Number 233-053-5

·          MDL number MFCD00011225

·          PubChem Substance ID 24859372

Selenium tetrachloride is the inorganic compound 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, Se₂Cl₂. SeCl₄is used in the synthesis of other selenium compounds.

Detailed description

Synthesis and structure

The compound is prepared by treating selenium with chlorine.^[3] 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 tetrameric 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°.^[4]

Interestingly 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. 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,^[5] 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

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