Titanium tetraiodide

Titanium tetraiodide is an inorganic compound with the formula TiI₄. It is a black volatile solid, first reported by Rudolph Weber in 1863. It is an intermediate in the van Arkel–de Boer process for the purification of titanium.

TiI₄ is a rare molecular binary metal iodide, consisting of isolated molecules of tetrahedral Ti(IV) centers. The Ti-I distances are 261 pm. Reflecting its molecular character, TiI₄ can be distilled without decomposition at one atmosphere; this property is the basis of its use in the van Arkel–de Boer process. The difference in melting point between TiCl₄ (m.p. -24 °C) and TiI₄ (m.p. 150 °C) is comparable to the difference between the melting points of CCl₄ (m.p. -23 °C) and CI₄ (m.p. 168 °C), reflecting the stronger intermolecular van der Waals bonding in the iodides.

Two polymorphs of TiI₄ exist, one of which is highly soluble in organic solvents. In the less soluble cubic form, the Ti-I distances are 261 pm.

Three methods are well known: 1) From the elements, typically using a tube furnace at 425 °C:

This reaction can be reversed to produce highly pure films of Ti metal.

2) Exchange reaction from titanium tetrachloride and HI.

3) Oxide-iodide exchange from aluminium iodide.

Like TiCl₄ and TiBr₄, TiI₄ forms adducts with Lewis bases, and it can also be reduced. When the reduction is conducted in the presence of Ti metal, one obtains polymeric Ti(III) and Ti(II) derivatives such as CsTi₂I₇ and the chain CsTiI₃, respectively.