Tantalum(IV) iodide is an inorganic compound with the chemical formula TaI<sub>4</sub>. It dissolves in water to give a green solution, but the color fades when left in the air and produces a white precipitate.
Tantalum(IV) iodide can be prepared by the reduction reaction of tantalum(V) iodide and tantalum. If pyridine is used as the reducing agent, there is an adduct TaI<sub>4</sub>(py)<sub>2</sub>.
Tantalum(IV) iodide can also be obtained by reacting tantalum(V) iodide with aluminum, magnesium or calcium at 380 ðC. Ta<sub>6</sub>I<sub>14</sub> is also formed. This makes it difficult to produce a very pure crystallized tantalum(IV) iodide.
Tantalum(IV) iodide is a black solid. It has a crystal structure isotypic to that of niobium(IV) iodide. Single-crystalline tantalum(IV) iodide was first obtained in 2008 by Rafal Wiglusz and Gerd Meyer as a chance product of a reaction in a tantalum ampoule that was supposed to lead to the product Rb(Pr<sub>6</sub>C<sub>2</sub>)I<sub>12</sub>. The single crystal has a triclinic crystal structure with space group P1 (space group no. 2) with two formula units per unit cell (a = 707.36 pm, b = 1064.64 pm, c = 1074.99 pm, ñ = 100.440ð, ò = 89.824ð and ó = 104.392ð). The crystal structure differs from that of other transition metal tetraiodides, which usually have a MI<sub>4/2</sub>I<sub>2/1</sub> chain structure, as it consists of TaI<sub>6</sub> octahedra bridged over a common surface to form a dimer. Two such dimers bridge over a common edge to form a tetramer.