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+), andN,N′-Me2-piperazinium(2+) cations, and Me2NH2I

Crystal-structure determinations are reported for Me₂NH₂I₃, (Ph₂I)I₃, tropanium-I₃, [Me₂HN(CH₂)₂NHMe₂](I₃)₂, [Me₂HN(CH₂)₃NHMe₂](I₃)₂, (N-Me₂-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 CdI₂type, 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 I₄rings in (Ph₂I)I₃and (Ph₂I)I is examined. Key words: crystal structures, hydrogen bond, iodine–iodine bonds, polyhalide anions, triiodides.