# Zirconium(IV) chloride

Zirconium(IV) chloride, also known as zirconium tetrachloride, (ZrCl$_4$) is an inorganic compound frequently used as a precursor to other compounds of zirconium. This white high-melting solid hydrolyzes rapidly in humid air.

## Structure

Unlike molecular TiCl$_4$, solid ZrCl$_4$ adopts a polymeric structure wherein each Zr is octahedrally coordinated. This difference in structures is responsible for the disparity in their properties: TiCl $_4$ is distillable, but ZrCl $_4$ is a solid. In the solid state, ZrCl$_4$ adopts a tape-like linear polymeric structure—the same structure adopted by HfCl$_4$. This polymer degrades readily upon treatment with Lewis bases, which cleave the Zr-Cl-Zr linkages.

## Synthesis

This conversion entails treatment of zirconium oxide with carbon in the presence of chlorine at high temperature:
    ZrO$_2$ + 2 C + 2 Cl$_2$ → ZrCl$_4$ + 2 CO
A laboratory scale process uses carbon tetrachloride in place of carbon and chlorine:
    ZrO$_2$ + 2 CCl$_4$ → ZrCl$_4$ + 2 COCl$_2$

## Applications
### Precursor to zirconium metal

ZrCl$_4$ is an intermediate in the conversion of zirconium minerals to metallic zirconium by the Kroll process. In nature, zirconium minerals usually exist as oxides (reflected also by the tendency of all zirconium chlorides to hydrolyze). For their conversion to bulk metal, these refractory oxides are first converted to the tetrachloride, which can be distilled at high temperatures. The purified ZrCl$_4$ can be reduced with Zr metal to produce zirconium(III) chloride.

### Other uses

ZrCl$_4$ is the most common precursor for chemical vapor deposition of zirconium dioxide and zirconium diboride.
In organic synthesis zirconium tetrachloride is used as a weak Lewis acid for the Friedel-Crafts reaction, the Diels-Alder reaction and intramolecular cyclisation reactions. It is also used to make water-repellent treatment of textiles and other fibrous materials.

## Properties and reactions

Hydrolysis of ZrCl$_4$ gives the hydrated hydroxy chloride cluster called zirconyl chloride. This reaction is rapid and virtually irreversible, consistent with the high oxophilicity of zirconium(IV). For this reason, manipulations of ZrCl$_4$ typically require air-free techniques.
ZrCl$_4$ is the principal starting compound for the synthesis of many organometallic complexes of zirconium. Because of its polymeric structure, ZrCl$_4$ is usually converted to a molecular complex before use. It forms a 1:2 complex with tetrahydrofuran: CAS [21959-01-3], mp 175–177 °C. Sodium cyclopentadienide (NaC$_5$H$_5$) reacts with ZrCl$_4$(THF)$_2$ to give zirconocene dichloride, ZrCl$_2$(C$_5$H$_5$)$_2$, a versatile organozirconium complex. One of the most curious properties of ZrCl$_4$ is its high solubility in the presence of methylated benzenes, such as durene. This solubilization arises through the formation of π-complexes.
The log (base 10) of the vapor pressure of zirconium tetrachloride (from 480 to 689 K) is given by the equation: log$_{10}$(P) = −5400/T + 11.766, where the pressure is measured in torrs and temperature in kelvins. The log (base 10) of the vapor pressure of solid zirconium tetrachloride (from 710 to 741 K) is given by the equation log$_{10}$(P) = −3427/T + 9.088. The pressure at the melting point is 14,500 torrs.