Chemistry:Selenium tetrachloride

Selenium tetrachloride

Names
IUPAC name Selenium tetrachloride
Other names Selenium(IV) chloride, tetrachloro-λ4-selane
Identifiers
CAS Number	10026-03-6 Y
3D model (JSmol)	Interactive image
ChemSpider	59590 Y
EC Number	233-053-5
PubChem CID	66205
RTECS number	VS7875000
UNII	4GB8868P5J Y
InChI InChI=1S/Cl4Se/c1-5(2,3)4 Y Key: LNBXMNQCXXEHFT-UHFFFAOYSA-N Y InChI=1/Cl4Se/c1-5(2,3)4 Key: LNBXMNQCXXEHFT-UHFFFAOYAD
SMILES Cl[Se](Cl)(Cl)Cl
Properties
Chemical formula	SeCl4
Molar mass	220.771 g/mol
Appearance	white to yellow crystals
Density	2.6 g/cm3, solid
Melting point	sublimes at 191.4 °C
Solubility in water	decomposes in water
Structure
Crystal structure	Monoclinic, mS80
Space group	C12/c1, No. 15
Molecular shape	Seesaw (gas phase)
Hazards
GHS pictograms
GHS Signal word	Danger
GHS hazard statements	H301, H331, H373, H410
GHS precautionary statements	P260, P261, P264, P270, P271, P273, P301+310, P304+340, P311, P314, P321, P330, P391, P403+233, P405, P501
NFPA 704 (fire diamond)	0 3 2 W
Flash point	non-flammable
Related compounds
Other anions	Selenium tetrafluoride Selenium tetrabromide Selenium dioxide
Other cations	Dichlorine monoxide Sulfur tetrachloride Tellurium tetrachloride
Related compounds	Selenium dichloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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. SeCl₄ is used in the synthesis of other selenium compounds.

Synthesis and structure

Structure of selenium tetrachloride

The compound is prepared by treating selenium with chlorine. 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°.

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, 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