Polyiodide ions

Iodine is only slightly soluble in water and no hydrates form upon dissolution. The solubiHty increases with temperature, as shown in Table 2 (36). Iodine is soluble in aqueous iodide solutions owing to the formation of polyiodide ions. For example, an equiHbrium solution of soHd iodine and KI H2O at 25°C is highly concentrated and contains 67.8% iodine, 25.6% potassium iodide, and 6.6% water. However, if large cations such as cesium, substituted ammonium, and iodonium are present, the increased solubiHty may be limited, owing to precipitation of sparingly soluble polyiodides. Iodine is also more... 

Amylose-iodine complexes have a deep blue color, which is a result of an electron relay on the polyiodide ions.196 The helix of amylose provides a tunnel for iodine molecules to align. Stability of the amylose-iodine complex has been studied.197 Iodine has been widely used for quantification of amylose contents despite the fact that the blue color development is affected by many factors, including temperature, pH and mechanical mixing. Several improved methods have been reported (see Section 6.III.1). 

A sample prepared in the presence of boric acid shows all three ions I-, I3 and I5. When this sample is stretched, the L disappears, and the ratio of I to I3 increases dramatically. It was also shown that there is considerable alignment of the linear polyiodide ions along the direction of stretching. 

Figure 13-4 Some higher polyiodide ions (distances in A). Distances > 3.0 and < 3.0 A are indicated by — and —, respectively, other anions include I12,1 4, I15, Ifc, and I (see M. Schroder et al., Angew. Chem. Int. Ed. EngL 1995,34, 2374).

Many other polyiodide ions have been characterized in general, these may be viewed as aggregates of I2 and 1 (sometimes 13 ). Examples are shown in Figure 8-30. 

FIGURE 8-30 Polyiodide Ions. (Bond angles and distances, in pm, are from A. F. Wells, Structural Inorganic Chemistry, 5th ed., Oxford University Press, New York, 1984, pp. 396-399.)... 

Cramer S in his studies of the iodine inclusion compounds considers that in some cases the iodine chains can be represented as a one-dimensional electron gas. From the position of the light absorption maxim un (6200 A.) together with one-dimensional Fourier analyses of the iodine gas, Cramer concludes that there are perhaps 14 or more iodine atoms, spaced with an I—I distance of 3.06 A. in a polyiodide ion. 

Fig. 17.8 Structures of polyiodide ions (a) The pentaiodide ion, 17 (b) the octaiodide ion, Ik (c) the heptaiodide ion as found in Ph4Pl7. Solid lines rcpre.scnt csscniinlly normal covalent bonds, dashed lines represent weakened or partial bond.s, and dolled lines represent very long and weak interactions. Distances in pm.

For reasons which will be apparent shortly it is convenient to discuss separately the more complex polyiodide ions. 

Crystals of hepta(tetrathiafulvalene) pentaiodide show a high electrical conductivity at room temperature. The structure of the (hfcO) projection has now been solved, and it has been shown that it contains infinite columns of tetra-thiafulvalene groups and infinite rows of iodide, not polyiodide, ions. ... 

The structure of (theobromine)2H2l8 has revealed that this is a polyiodide salt containing protonated theobromine.The anion It6 is the largest discrete polyiodide ion that has been characterized until now. A projection of this centrosymmetric anion is shown in Figure 1 it is evident that the ion is nearly planar and is best described as being I3——1 3 —13—12—13. 

It has been suggested that the three orders of magnitude differences in conductivity between some highly oriented polyacetylene types (e.g. Tsukamoto polyacetylene versus Durham polyacetylene) are related to differences in the tilt angle of the polyiodide ions (Pouget et al. [137]). Dopants may have a crucial bridging function, providing the necessary interchain conduction pathways [138],... 

I3 and I5" anions are linear with symmetrical charge population. There arises a question of the orientation of polyiodide ions with respect to polyacetylene chains. Mossbauer studies carried out on stretched polyacetylene showed a dependence of the spectra on the angle between the incident y-rays and the stretching direction revealing the alignment of the polyiodide anions in the stretched film [111]. 

A typical rechargeable battery based on this idea has been constructed. It uses the [Ni(TTL)]a, polymer as the anode, poly-2-vinyl-pyridine-iodine (P2VP (x/2)l2) (68) complex as the cathode, and aqueous KI solution as the electrolyte solution (Equation 13). In the discharging process, electrons flow from the anode [Ni(TTL)]a to the cathode P2VP (x/2)l2 through the load circuit the iodide (or polyiodide) ions formed at the cathode then enter the electrolyte while an equivalent amount of iodide ions from the electrolyte solution intercalate into the oxidized metal tetrathiolene polymer (anode). The electrolyte concentration is therefore conserved. Upon recharging with an opposite... 

FIGURE 8.38 Polyiodide Ions. (Data from N. N. Greenwood and A. Earnshaw, Chemistry of the Elements, Pergamon Press, Elmsford, NY, 1984, pp. 821-854.)... 

The ratio d of the diffusion coefficients of the inhibitor and the activator d = cio / r value very close to one. Typically a value ofd = 1.07 is chosen in simulations, which appears to represent a situation rather unfavorable for the formation of Turing patterns. This view is too simple, however it overlooks the role of starch in the reaction. As discussed in Sect. 1.4.9, the activator 1 reacts with iodine, I2, to form triiodide ion 13 , which gets trapped by the starch molecules embedded in the gel or by iodide-binding sites of the gel. (Polyacrylamide binds polyiodide ions even in the absence of starch as does poly(vinyl alcohol), used as a... 

Higher polyiodide ions have a more complicated structure than I3 . For example, I5 anion is an angular species, which can be regarded as formed by the coordination of two iodine molecules by iodide ion. The heptaiodide ion can be described as [(Is ) 2I2], [(I3 ) 3I2], [(I5 ) I2], with a more or less distorted pyramidal geometry or Z-shape. 

---