Hafnium(III) iodide is an inorganic compound of hafnium and iodine with the formula Hf I<sub>3</sub>. It is a black solid.
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.
Other metals can be used as the reducing agent, for example aluminium. The product is often nonstoichiometric, with the compositions Hf I<sub>3.2âÂÂ3.3</sub> and Hf I<sub>3.0âÂÂ3.5</sub> reported.
Hafnium(III) iodide adopts the same crystal structure as zirconium(III) iodide. This is very similar to the ò-TiCl<sub>3</sub> structure. The structure is based on hexagonal close packing of iodide ions with one third of the octahedral interstices occupied by Hf<sup>3+</sup> ions. It consists of parallel chains of face-sharing {HfI<sub>6</sub>} octahedra.
Hafnium(III) iodide has a lower magnetic moment than is expected for the d<sup>1</sup> metal ion Hf<sup>3+</sup>, indicating non-negligible HfâÂÂHf bonding. The HfâÂÂHf separation was originally reported to be 3.295àà, but a subsequent study of nonstoichiometric hafnium(III) iodide indicated a lower symmetry structure.
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.