Triiodide anion

C09-0023. The fourth molecular shape arising from a steric number of 5 is represented by the triiodide anion I3. Determine the molecular geometry and draw a three-dimensional picture of the triiodide ion. 

Molecular iodine dissolves in a solution containing iodide anions to form triiodide anions ... 

Initially, the forward reaction to form triiodide anions is the only process that occurs. As the concentration of the product increases, however, the reverse reaction also takes place ... 

Write the equilibrium constant expression for the decomposition equilibrium for the triiodide anion,... 

According to the definition given above, the term XB comprehensively covers a vast class of non-covalent interactions, from the weak N- Cl XB [35] to the very strong I" I2 interaction, which forms the triiodide anion 10 and 200 kj mol-1 can be assumed as the energy extremes for these interactions, respectively. 

All forms of iodine including the elemental iodine, hypoiodous acid (HOI), hypoiodite anion (OI ), free iodide anion (I-), and triiodide anion (I3 ) in water also may be measured by the Leuco crystal violet method. The sample is treated with potassium peroxymonosulfate to oxidize all iodide species in the sample. It then is treated with leukocrystal violet reagent for color development. Interference from free chlorine may be eliminated by addition of an ammonium salt. 

A short synthesis of imidazoles 47 was discovered on the basis of the reaction of 3,5-diaryl-l,2,4-dithiazolium triiodides 46 (X = I3) with glycinates or aminoacetonitrile. Intermediate (thiocarbonyl)amidines 48 were oxidized in situ to 1,2,4-thiadiazolium salts 49 by the triiodide anion resulting in imidazoles 47. The same reaction with 3,5-diaryl-l,2,4-dithiazolium perchlorates 46 (X = C104) stops at the formation of amidines 48 due to the lack of an oxidizing counterion (Scheme 3) <1997JOC3480>. 

The solid-state structures of the mixed valence salts containing 1,2,3,5-diselenadiazole [50]i[l(l have been determined by X-ray crystallography <1994CM508>. The molecular (asymmetric) unit consists of a trimeric cation [50]3+ and an associated triiodide anion. Within the cation the mean interannular Se-Se contact is 3.377 A. The trimer units form dovetailed stacks in which consecutive layers are oriented in a transantipodal fashion. This arrangement introduces close interannular contacts between the blocks along the stack, as well as close lateral contacts. 

I] and a triiodide anion, and the individual [PPh3l] units are further linked into a polymer by weak interactions between the triiodide anions. Further, the compound PPr 314 contains two [PP/3I] cations weakly linked independently to one of the two terminal iodine atoms of the same triiodide anion. ... 

Another example are the triiodide anions 133). Here as weU as in the selenocyanates the distance depends on the coimterion and shows a similar fluctuation as do the trithiapentalenes concerning the interatomic distances... 

It may be helpful to consider a simple two-dimensional illustration - a linear triatomic molecule, such as the triiodide anion, which has been observed in very many different crystal environments. Assuming linearity (which holds in practice quite closely for triiodide ions in crystals), we completely describe the structure of any given example of a triiodide ion by the two interatomic distances, rfi and di- A collection of triiodide ions is then described by a collection of points in the plane spanned by the two corresponding basis vectors, pi and pi (Figure 1.5). It is evident that the points corresponding to observed structures lie close to a rather well defined curve. We can interpret this curve in different ways, some of which are described in later Chapters (see especially Chapter 5). For example, we can regard it as the reaction path for the exchange reaction I2 -I-1 I +12. 

Fig. 1.5. Correlation plot of interatomic distances in linear triiodide anions...

An alternative representation of the permutation groups can be given in configuration space. Here we need one dimension for each internal coordinate of interest, and this number may be greater than the number of elements (atomic labels) to be permuted. As we have seen, this is not the case for the simplest examples we have studied. For the linear triiodide anion we need a two-dimensional coordinate system, the interatomic distances a and b, and there are just two atomic labels of the terminal atoms for the triangular triiodide anion we need a three-dimensional space and there are three atomic labels. But for a tetrahedral MX4 molecule there are 10 distances (4M-X and 6X-X) that need to be considered and only four atomic labels. For this example, the permutations of the labels produce 24 equivalent points in the ten-dimensional space of the internal coordinates (which can be reduced to 9 dimensions if the redundancy among the X-X distances is taken into account see Section 1.3.1). 

Fig. 5.4. Correlation plot of interatomic distances in linear triiodide anions. The directions of the symmetric and antisymmetric stretch coordinates 9, = 5, = (d, and qi = Sj = (d, -di)/f2...

Fig. 5. Correlation plots of interatomic distances in linear triatomic systems. Left triiodide anions, center thiathiophthenes, and right O-H—O H-bonds. Adapted from H.-B. Biirgi, Angew. Chem., Int. Ed. 1975,14, 460.

Sensitivity for bromate determination has been improved by more than a factor of 10 through the use of a postcolumn derivatization reaction in which HI is generated in situ from K1 and reacts with bromate to form the triiodide anions (la ). In 2000, the U.S. EPA published Method 317.0, which uses postcolumn derivatization with o-dianisidine this reacts with the eluting bromate to form a chromophore, which is then measured with a UVA is detector. This method offers excellent limits of detection, below 1 p-g/L, for bromate, as well as for chlorite and chlorate. 

The triiodide anion, [13] , is a well-known species, yellow in dilute aqueous solution and brown when concentrated. Indeed, the formation of the triiodide anion underlies the solubility of elemental iodine in aqueous potassium iodide but not in pure water. 

Iodine forms the most extensive group of polyhalide anions, with triiodide, I3, being the best-known example. The formation of this anion is responsible for the increased solubility of I2 in water on addition of potassium iodide (KI). Triiodide is used extensively in analytical chem-isfly, many analytical procedures being based on the release or uptake of iodine and its subsequent titration with sodium thiosulfate (iodometry). The largest polyhalide anion known, Ijg, consists of an alternate arrangement of triiodide anions and neutral iodine molecules. 

The electrolysis of vinyl ethers in the presence of a supporting electrolyte either a tetraalkylammonium salt, an inorganic salt such as sodium perchlorate, or sodium tetraphenylborate readily leads to polymerization. In all cases, the mechanism of polymerization appears to be cationic, although different workers differ with respect to the precise steps involved. For example, Cerai and coworkers [128] have proposed that when tetra-M-butylammonium triiodide is used as the supporting electrolyte, the triiodide anion undergoes oxidation by the following anodic process, which generates elemental iodine ... 

The linear structure of the triiodide anion is in accord with VSEPR theory ideas. Other ions can be thought of as combinations of I , T, and groups held together by weak intermolecular forces. The structures of some of these anions are shown in Figure 18.4. 

J. Coupez-Courtot, and C. Madec, Bull. Soc. Chim. France, 4626 (1971). Solvation of the iodine molecule and the triiodide anion. 

Polyether compounds form stable adducts with iodine, a property which has led to their use as iodophores for bactericides on human and animal skin. For medical applications, iodide concentration is kept low, since the triiodide ion has little disinfecting activity compared to iodine. By contrast, iodine-based analytical methods for surfactant determination are usually conducted in the presence of a large excess of pxjtassium iodide. The likely mechanism consists of the K" -p)olyether complex associating with the large, lipophilic, triiodide anion. The absorbance maximum of the complex is shifted from that of triiodide... 

---