Polyhalide anions

Typical examples of linear (or nearly linear) triatomic polyhalides are in Table 17.15 the structures are characterized by considerable variability of interatomic distances and these distances are individually always substantially greater than for the corresponding diatomic interhalogen (p. 825). Note also that for 

836 The Halogens Fluorine, Chlorine, Bromine, Iodine and Astatine Ch. 17 

Penta-atomic polyhalide anions [XY4] favour the square-planar geometry (D4h) as expected for species with 12 valence-shell electrons on the central atom. Examples are the Rb+ and Cs+ salts of [C1F4], and KBrp4 (in which Br-F is 189 pm and adjacent angles F-Br-F are 90 ( 2°). The symmetry of the anion is slightly 

Hepta-atomic polyhalide anions are exemplified by BrFe (K+, Rb+ and Cs+ salts) and IFfi- (K+, Cs+, NMe4+ and NEu+ salts). The 

V-) apcd I sdiion in MC4NU1 i(h th largtri caiion bti (o-benzyldloxiniato) bis p-piculmet iron fUl)] ihc scqueiKe of imeraiomic liSiUnce w b 277, J(M, 

IF5 is unusual as an interhalogen in forming adducts with both XeF2 and XeF4  

XeFi + 2IF5 XeF2.2IFs room temp 

It should be emphasized that the reactivity of IF5 is mild only in comparison with the other halogen fluorides (p. 830). Reaction with water is extremely vigorous but the iodine is not reduced and oxygen is not evolved  

Boron enflames in contact with IF5 so do P, As and Sb. Molybdenum and W enflame when heated and the alkali metals react violently. KH and CaC2 become incandescent in hot IF5. However, reaction is more sedate with many other metals and non-metals, and compounds such as CaCOa and Ca3(P04)2 appear not to react with the liquid. 

IF7 is a stronger fluorinating agent that IF5 and reacts with most elements either in the cold or on warming. CO enflames in IF7 vapour but NO reacts smoothly and SO2 only when warmed. IF7 vapour hydrolyses without violence to HIO4 and HF with small amounts of water at room temperature the oxyfluoride can be isolated  

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Much of the current research into halogen bonding involves organohalogen or inorganic halide acceptors. However, the dihalogens (X2) and inter halogens (XY) continue to attract attention. This review will focus specifically on structural and theoretical studies of such systems over the past decade. While polyhalide anions are closely related to the neutral donor-acceptor concepts that are the focus here, they will be discussed only when necessary to elaborate on the central theme of the review. 

Numerous cases have been described elsewhere in which NO+ is produced, but the C1F2 represents an anion of an interhalogen. Polyhalide anions are not rare because when iodine is dissolved in a solution containing I-, the I3 ion is formed. 

Dioximes form bis complexes (35) where the ligand normally chelates through the nitrogen atoms.251-253 Detailed studies have been made of the partially oxidized compounds obtained from reaction of bis(diphenylglyoximato)- and bis(benzoquinone dioximato)-palladium with iodine254,255 and bromine.256 These complexes contain polyhalide anions and possess the high anisotropic electrical conductivity associated with many complexes which crystallize as stacked molecular units.257... 

Fluorine forms polyhalide anionic complexes including IFBr, IF Clt. BrFV. and lFf". which occur in salts of the higher alkalies, as well as catioits such as BrF .. which occurs in such salts as BrF ShF, BrFiAuFj, BrFvSOiF. etc. The dilluuriodatc ion. KFFj is also known in such salts as KIO F . 

Structural studies show thin ihc complex HICIi 4II () contains a positive irn alent iodine nioni, anti ihc planar ion ICH is one of the most stahle of Ihc polyhalide anions, as might he expeeied from consideration ot ihe large size ol the iodine atom and the small size of the chlorine atoms. 

The halides participate as terminal and bridge ligands and can be included in inner and external spheres of this kind of complex. A few examples of compounds with such terminal ligands are the boron ammines [BX3 NH3], [BX2(NH3)2]X, BX(NH3)3]X2, and [B(NH3)4]X3 (X = F, Cl, Br, I). Halides form part of many mixed-ligand carbonyl complexes, for example, Fe(CO)4I2 and polyhalide anions with various elements [4], In this respect, the structure of the anion [Re2Clx]2 provides important information the Re — Re bond is present and the chlorine atoms have a bridge function [4]. 

The product, N0C1F2, is essentially ionic, and it contains the NO+ (nitrosyl) cation and the C1F2 anion. This polyhalide anion is similar to several others that are known. For example, I3 results when I2 is dissolved in a solution that contains KI ... 

The large size of the Cs+ and Rb+ ions frequently allows them to form ionic salts with rather unstable anions, such as various polyhalide anions and the superoxides already mentioned. 

Cation formation is not restricted, as normally observed, to association with metallic species. It can also occm among nonmetals and even the most typical ones such as halogens or interhalogens, which are usually more noted for halide and polyhalide anion formation. 

The triatomic polyhalide anions have a linear structure (7), the pentaatomic ones [XY4] are square planar (8) with a D4h symmetry, while the heptaatomic [XFe] have a distorted octahedral structure. The [IFg]" ion has a square antiprismatic structure. 

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