Hafnium(III) iodide

Hafnium(III) iodide is an inorganic compound of hafnium and iodine with the formula Hf&thinsp;I3. It is a black solid.

Preparation
Like other group 4 trihalides, hafnium(III) 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.


 * 3 Hf&thinsp;I4 + Hf → 4 Hf&thinsp;I3

Other metals can be used as the reducing agent, for example aluminium. The product is often nonstoichiometric, with the compositions Hf&thinsp;I3.2–3.3 and Hf&thinsp;I3.0–3.5 reported.

Structure and bonding
Hafnium(III) 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 {HfI6} octahedra.

Hafnium(III) 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(III) iodide indicated a lower symmetry structure.

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