Hafnium(III) iodide

Hafnium iodide is an inorganic compound of hafnium and iodine with the formula Hf I₃. It is a black solid.

Preparation

Like other group 4 trihalides, hafnium iodide can be prepared from hafnium(IV) iodide by high-temperature reduction with hafnium metal, although incomplete reaction and contamination of the product with excess metal often occurs.
Other metals can be used as the reducing agent, for example aluminium. The product is often nonstoichiometric, with the compositions Hf I_3.2–3.3 and Hf I_3.0–3.5 reported.

Hafnium iodide adopts the same crystal structure as zirconium(III) iodide. This is very similar to the β-TiCl₃ structure. The structure is based on hexagonal close packing of iodide ions with one third of the octahedral interstices occupied by Hf³⁺ ions. It consists of parallel chains of face-sharing octahedra.
Hafnium iodide has a lower magnetic moment than is expected for the d¹ metal ion Hf³⁺, indicating non-negligible Hf–Hf bonding. The Hf–Hf separation was originally reported to be 3.295 Å, but a subsequent study of nonstoichiometric hafnium iodide indicated a lower symmetry structure.

Like the chloride and bromide, hafnium iodide is a powerful enough reducing agent to reduce water and therefore does not have any aqueous chemistry.