Indium(I,III) iodide, also known as indium diiodide, is an inorganic compound of indium and iodine with the empirical formula InI2. Despite this formula, it does not contain indium in the +2 oxidation state. In the solid state it is a mixed valence compound consisting of In+ cations and tetraiodoindate(III) anions, and is therefore more accurately formulated as In+[InI4]−.
Preparation Indium(I,III) iodide can be prepared by reaction of elemental indium with mercury(II) iodide:
In + HgI2 → InI2 + Hg
It can also be prepared by comproportionation of elemental indium with indium(III) iodide.
A further preparative route uses excess indium metal and elemental iodine in refluxing toluene, from which In[InI4] can be isolated.
Properties Indium(I,III) iodide is a red-brown, crystalline, diamagnetic and hygroscopic solid. Its melt is black.
In water, it disproportionates to elemental indium and indium(III) iodide.
The compound crystallizes in the orthorhombic crystal system in space group Pnna (No. 52). Single-crystal X-ray diffraction gives lattice parameters a = 8.525(5) Å, b = 10.969(9) Å and c = 11.162(8) Å, with four formula units per unit cell.
Its structure consists of discrete tetrahedral [InI4]− anions and In+ cations. The average In(III)–I bond length in the tetraiodoindate anion is about 2.714 Å. The monovalent indium ions are much more weakly coordinated by iodide than the In(III) centres, consistent with the presence of the 5s2 lone pair on In(I).
The compound is therefore better regarded as indium(I) tetraiodoindate(III), In[InI4], rather than as a genuine indium(II) iodide.
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