Polyhalide anions in crystals. Part 2. I3- asymmetry and N-H...I bonding: Triiodides of the Me2NH2+, Ph2I+, tropanium, N,N,N′,N′-Me4-1,2-ethanediammonium, N,N,N′,N′-Me4-1,3-propanediammonium, N-Me-piperazinium(2+), and N,N′-Me2-piperazinium(2+) cations, and

Article abstract of DOI:10.1139/v96-174

Crystal-structure determinations are reported for Me₂NH₂I₃, (Ph₂I)I₃, tropanium·I₃, [Me₂HN(CH₂)₂NHMe₂](I₃)₂, [Me₂HN(CH₂)₃NHMe₂](I₃)₂, (N-Me-piperazinium) (I₃)₂·H₂O, (N,N′-Me₂-piperazinium)(I₃)₂, and Me₂NH₂I. The features of these and relevant literature structures are used to (1) classify triiodide structures by their ion-packing types; (2) analyze the relationship between the two I-I bond lengths d and d* in the I₃^- anion; and (3) examine the effect of N-H(N)...I hydrogen bonding on the symmetry of the I₃^- anion. It is found that the d,d* relationship can be represented to a high degree of correlation by the power function d*-d₀ = K(d-d₀)^-c (d*≥d, d₀ = d(I-I) in I₂(g)) based on the 3c4e model of the anion. An empirical correlation is shown to exist between the H(N)..I and N...I distances both for unbranched and branched N-H(N)...I bonds. Comparison of the degree of asymmetry of I₃^- in two samples, one containing H-bonded I₃^- anions, the other with H-bonding absent, leads to the conclusion that while H-bonding is a factor affecting I₃^- symmetry, it is not a preferential factor. The four 1:2 title triiodides have structures of, or related to, the CdI₂ type, in which the anions form infinite pseudo-hexagonal channels. The positioning of the divalent cations on the axes of these channels gives rise to an interesting `vernier' effect governed by the cation length and H-bonding ability. Bonding in centrosymmetric I₄ rings in (Ph₂I)I₃ and (Ph₂I)I is examined.