Halogens in positive oxidation states

Several trends are noticeable from the data in the table. The bond strengths of the interhalogens are clearly related to the difference in electronegativity between the component halogen atoms, as expected on the basis of Pauling s ideas on ionic character (Chapter 3). Furthermore, the tendency to form the higher fluorides and chlorides depends upon the initial electronegativity of the central alom. Only iodine forms a heplafluoride or a trichloride. Not shown in Table 17.2 (except indirectly by computation from the values) is the instability of certain lower oxidation states to disproportionation  

This tendency towards disproportionation is common among the lower fluorides of iodine and bromine. The behavior of the four iodine fluorides presents a good picture of the factors important in the relative stabilities. Both IF and IF, tend to disproportionate (the former to the extent that it cannot be isolaied). not because of weakness in 

Exactly the same result is obtained if the initial electronegativity of the central halogen is assumed to be higher in a higher oxidation stale, and Ax and ionic resonance energy are lower. The. same arguments apply equally well to all of the oxidation slates  

Note that the more ionic the bcxid. the less important the apparent distinction between 3-c-d-e bonding and VBT becomes, because the weaknesses of both methods of modeiing the bonding decrease when one goes towards the limit of a purely ionic bond.  

The inierhalogen compounds obey the expectations based on the VSEPR theory, and typical structures are given in Chapter 6. One compound not included there is the dimeric iodine trichloride, in which the iodine atom of the monomeric species appears to act as a Lewis acid and accept an additional pair of electrons from a chlorine atom (Fig. 17.5). 

4 Values in parentheses are bond energies front Appendix E (kJ mol-1). 

24 Since the most stable isotope of astatine has a half-life of only 8,3 hours, the chemistry of this halogen has not been studied extensively. In the following discussion generalities made about the halogens may or may not Include astatine. In the present instance AlBr and AlCI have been prepared. See the discussion of astatine chemistry later in this chapter. 

Case II. Sx is similar in XY and XY, and since Sx = 3fiY irtii,.Je. Ym, h1,i ic 35Y,rihj, dc. This becomes an increasingly good approximation as Y becomes larger with lower values of both a and b, and the large, soft, central atom becomes smaller and harder. In this case Ihe monohalide is favored. 

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