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 Me₂NH₂I

Crystal-structure determinations are reported for Me 2 NH 2 I 3 , (Ph 2 I)I 3 , tropanium-I 3 , [Me 2 HN(CH 2 ) 2 NHMe 2 ](I 3 ) 2 , [Me 2 HN(CH 2 ) 3 NHMe 2 ](I 3 ) 2 , (N-Me 2 -piperazinium) (I 3 ) 2• H 2 O, (N,N′-Me 2 -piperazinium)(I 3 ) 2 , and Me 2 NH 2 I. 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 I 3 − anion; and (3) examine the effect of N—H(N)… I hydrogen bonding on the symmetry of the I 3 − anion. It is found that the d,d* relationship can be represented to a high degree of correlation by the power function d* − d 0 = K(d − d 0 ) −c (d* ≥ d,d 0 = d(I—I) in I 2 (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 I 3 − in two samples, one containing H-bonded I 3 − anions, the other with H-bonding absent, leads to the conclusion that while H-bonding is a factor affecting I 3 − symmetry, it is not a preferential factor. The four 1:2 title triiodides have structures of, or related to, the CdI 2 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 I 4 rings in (Ph 2 I)I 3 and (Ph 2 I)I is examined.