The structure and stability of simple tri-iodides.

In this work the simple tri-iodides are regarded as being those in which the crystal lattice contains only cations, tri-iodide anions and possibly solvate molecules. The alkali metal tri-iodides CsI 3 and KI 3 .H20 exemplify compounds of this type; polyiodides like the penta-iodide (CH3)4NI5 or the hepta-iodide (C2H5) 4I7, in which it is known that the anionic group is made up of I -3 units accompanied by iodine molecules, are not encompassed by this study. In the solid state the simple tri-iodides exhibit a wide range of stability, and the probable causes underlying this range in behaviour are reviewed against existing knowledge concerning the crystalline structure and current models of the anion bonding of these compounds. The presence of solvate molecules in the lattice has a profound effect upon the stability of the simple tri-iodides; also among the unsolvated species there is a range in stability which can be attributed to differences in the electrostatic environment experienced by the anion. This sensitivity to electrostatic environment is reflected in small differences in the geometric configuration of the anion among the unsolvated tri-iodides as revealed by crystal structure analysis.