Structure of Interhalogen Compounds

Structures of Interhalogen Compounds and Polyhalides E. H. Wiebenga, E. E. Havinga, and K. H. Boswijk... 

Interhalogen compounds are compounds of two different halogens that have, with few exceptions, the general formula XX . Examples include BrCI, C1F, and IF5. Use I ewis structures to explain why n is always an odd number. 

The halogen pentafluorides and the [XeF5]+ cation have a square pyramidal configuration and any weak secondary bonds are found below the base of the pyramid and situated to avoid the axial, lone pair position. These contacts are much more significant for the xenon compounds than for the interhalogens, where they are so weak as to be virtually indistinguishable from normal intermolecular contacts, as seen in the structure of IF5.40... 

The interhalogen compounds obey the expectations based on the VSEPR theory, and typical structures are giver in Chapter 6. Ore 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). 

The structures of the interhalogen compounds are all known, to varying degrees of accuracy. These structures and the reasons for them in terms of the electronic configuration of the molecules30 are discussed in Chapter 4. 

CI2, Br2,12, and some interhalogen compounds XX form complexes with ammonia that are sufficiently stable to allow the structures to be determined in the gas phase. The structure of the complex H3NICI is shown in Fig. 18.6 [4]. The ICl molecule is situated on the threefold symmetry axis of the NH3 molecule, i.e. in the direction of the elecfron lone pair on the N atom. The N-I distance is 271 pm, 71 pm longer than the estimated single, covalent bond distance obtained from the MSS rule, but still 82 pm shorter than the sum of the van der Waals radii of N and 1. The experimental information needed for calculation of the dissociation energy of the complex is unfortunately not available, quantum chemical calculations have yielded dissociation energies ranging from 42 to 52 kJ mol [5,6]. 

The interhalogen compound BrFsfZ) reacts with antimo-ny(V) fluoride to form the salt (BrF2 (SbF6 ). Write the Lewis structure for both the cation and anion in this substance, and describe the likely structure of each. 

The halogens combine with each other to form four types of binary, neutral, interhalogen compounds XY (all possible combinations), XY3 (Y = F only), XY5 (Y = F only), and XY7 (X = I and Y=F only), where X is the heavier halogen (Table VI). The greater the electronegativity difference (Table HI) between the central atom X and the terminal atoms Y, the greater the total number of Y atoms that can be bound in the molecule. Thus, iodine can bind up to seven F atoms, but only a maximum of three Cl atoms or one Br atom. The structures of these molecules can generally be predicted by simple valence-shell electron-pair repulsion (VSEPR) theory, in which the bonded and... 

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