Novel cyclen-polyiodide complexes: a reappraisal of I-I covalent and secondary bond limits

Supramolecular stabilization of polyiodides and iodine-dense phases is of high interest: this study explores the possibilities offered in this sense by diprotonated cyclen, affording two novel crystal structures. One of them contains at least one peculiar I⋯I interatomic distance (3.305(1) Å), falling well below the region commonly described by secondary bonding (3.4-3.7 Å) and essentially equal to the accepted limit for covalent bonding (3.30 Å): in other words, according to threshold distance values, we are relatively free to regard this interaction either as a bond or as contact. Lest the flip of a coin decides if we should or should not draw a bond in a polyiodide, statistical insights based on CSD surveys were used to put in perspective literature material and work out a meaningful assignment (as I₈^2-). In doing so, we address how currently accepted threshold distance values came to be in the first place, their significance, soundness, and shortcomings in describing I₈^2- and its formal fragments (I₂, I₃^-, I₅^-). Discussion of the chemical meaning of the line representing bonding in I-I fragments in similar fringe cases, relating CSD data herein presented with the previous literature, is provided. Available information coincides quite well in supporting the necessity of a revision of broadly accepted threshold distance values.