
Tin(IV) chloride

Tin(IV) chloride, also known as tin tetrachloride or stannic chloride, is an inorganic compound with the formula SnCl4. It is a colorless hygroscopic liquid, which fumes on contact with air. It is used as a precursor to other tin compounds. It was first discovered by Andreas Libavius (1550–1616) and was known as "spiritus fumans libavii".
Preparation.
It is prepared from reaction of chlorine gas with tin at .
Structure.
Anhydrous tin(IV) chloride solidifies at −33 °C to give monoclinic crystals with the P21/c space group. It is isostructural with SnBr4. The molecules adopt near-perfect tetrahedral symmetry with average Sn–Cl distances of 227.9(3) pm. 
Hydrates.
Several hydrates of tin tetrachloride are known. The pentahydrate, SnCl4·5H2O was formerly known as "butter of tin". They all consist of [SnCl4(H2O)2] molecules together with varying amounts of water of crystallization. The additional water molecules link together the molecules of [SnCl4(H2O)2] through hydrogen bonds. Although the pentahydrate is the most common hydrate, lower hydrates have also been characterised.
Reactions.
Aside from water, other Lewis bases form adducts with SnCl4. These include ammonia and organophosphines. The complex [SnCl6]2− is formed with hydrochloric acid making hexachlorostannic acid.
Precursor to organotin compounds.
Anhydrous tin(IV) chloride is a major precursor in organotin chemistry. Upon treatment with Grignard reagents, tin(IV) chloride gives tetraalkyltin compounds:
Anhydrous tin(IV) chloride reacts with tetraorganotin compounds in redistribution reactions:
These organotin halides are more useful than the tetraorganotin derivatives.
Applications in high organic synthesis.
Although a specialized application, SnCl4 is used in Friedel-Crafts reactions as a Lewis acid catalyst for alkylation and cyclisation. Stannic chloride is used in chemical reactions with fuming (90%) nitric acid for the selective nitration of activated aromatic rings in the presence of inactivated ones.
Uses.
The main application of SnCl4 is as a precursor to organotin compounds, which are used as catalysts and polymer stabilizers. It can be used in a sol-gel process to prepare SnO2 coatings (for example for toughening glass); nanocrystals of SnO2 can be produced by refinements of this method.
Safety.
Stannic chloride was used as a chemical weapon in World War I, as it formed an irritating (but non-deadly) dense smoke on contact with air: it was substituted for by a mixture of silicon tetrachloride and titanium tetrachloride near the end of the war due to shortages of tin.