On the non-existence of a square-planar pentaiodide coordination complex I(I)₄

The properties of two conformers of the pentaiodide ion, a V-shaped and regularly observed I₅^- ion, and a so far undetected square-planar coordination complex of II₄^- composition, have been investigated by computational methods. The latter compound is indicated by the analogy to the coordination chemistry of gold with halide ligands, as well as isoelectronic main-group compounds. Static and dynamic simulations at density-functional and semi-empirical level including effects of solvent and counter ions indicate that the square-planar II₄^- indeed represents a well-defined local minimum on the pentaiodide potential energy surface, albeit less stable than the typically observed V-shaped I₅^-. No simple pathway of transformation between the two forms of the pentaiodide ion can be identified. Molecular dynamics simulations indicate that the presence of cations, unavoidable during the synthesis of polyiodide compounds, may trigger decomposition of the II₄^- coordination complex into smaller polyiodide building blocks and thus constitute the main reason why this conformer so far has not been identified in solid polyiodide compounds. However, its intrinsic stability indicates that the square-planar form should be possible to isolate in solid compounds given the right conditions of synthesis.