Halides of the elements

The preparation and structural characterization of the ions HX2 has been an important feature of such work/ As expected, these H-bonded ions are much less stable than Hp2 though crystalline salts of all three anions and of the mixed anions HXY (except HBrI ) have been isolated by use of large counter cations, typically Cs+ and NR4+ (R = Me, Et, Bu ) — see pp. 1313-21, of ref. 23 for further details. Neutron and X-ray diffraction studies suggest that [C1-H - C1] can be either centrosymmetric or non-centrosymmetric depending on the crystalline environment. An example of the latter mode involves interatomic distances of 145 and 178 pm respectively and a bond angle of -168 (Cl- -Cl 321.2pm).(  

A detailed discussion of individual halides is given under the chemistry of each particular element. This section deals with more general aspects of the halides as a class of compound and will consider, in turn, general preparative routes, structure and bonding. For reasons outlined on p. 805, fluorides tend to differ from the other halides either in their method of synthesis, their structure or their bond-type. For example, the fluoride ion is the smallest and least polarizable of all anions and fluorides frequently adopt 3D ionic structures typical of oxides. By contrast, chlorides, bromides and iodides are larger and more polarizable and frequently adopt mutually similar layer-lattices or chain structures (cf. sulfides). Numerous examples of this dichotomy can be found in other chapters and in several general references.Because of this it is convenient to discuss fluorides as a group first, and then the other halides. 

Colton and J. H. Canterford, Halides of the First Row Transition Elements, Wiley, London, 1969, 579 pp. Halides of the Second and Third Row Transition Elements, Wiley, London, 1968, 409 pp. 

Structural Inorganic Chemistry, 5th edn. pp. 407-44, Oxford University Press, Oxford, 1984. 

The Halogens Fluorine, Chlorine, Bromine, Iodine and Astatine 

The binary halides of the elements span a wide range of stoichiometries, stmcture types and properties which defy any but the most grossly oversimplified attempt at a unified classification. Indeed, interest in the halides as a class of compound derives in no small measure from this very diversity and from the fact that, being so numerous, there are many examples of well-developed and well-graded trends between the limiting cases. Thus the fluorides alone include OF2, one of the most volatile molecular compounds known (bp —145°), and Cap2, which is one of the least-volatile ionic compounds (bp 2513°C). Between these extremes of discrete molecules on the one hand, and 3D lattices on the other, is a continuous sequence of oligomers, polymers and extended layer lattices which may be either predominantly covalent [e.g. CIP, (MoPs)4, 

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The Halides of the Elements—Double Halides —Endothermic Combinations—Hydrolysis —Oxidation and Reduction—Mass-Action. 

The characteristic of the halides of the elements of the lithium group is that they are all soluble white salts, crystallising in cubes. In dilute solution they are all ionised, and even in strong solution a large percentage of ions are present. Hence they all react as metal ions and as halogen ions. Thus, for instance, with silver nitrate, which is the usual test for ionic chlorine, the following reaction takes... 

Sodium has been widely used in the synthesis of various organo-metals and organo-metalloids, by causing the halides of the elements to react with organic halogen compounds. The following are examples of this type of reaction ... 

Table A selection of oxides and halides of the elements So-Cu. X represents any halogen unless...

This tendency is greater for the halides of the elements in the +4 oxidation state than it is for those of the +2 state owing to the greater Lewis acid strength of the EX4 compounds. 

The halides of the elements of the second and higher groups of the periodic classification are evidently not ionic compounds in view of the improbability of the formation of poly-charged ions such as 326... 

E9.12 The covalent character increases with increasing oxidation state of the metal thus the covalent character is increasing in order Crp2 < CrFj < CrF. Refer to Section 9.8(c) Halides of the elements. 

The element indium is to be obtained by electrolysis of a molten halide of the element. Passing a current of 3.20 A for a period of 40.0 min forms 4.57 g of In. What is the oxidation state of indium in the halide melt ... 

For a few elements, electrodeless discharge lamps are to be preferred over hollow cathode lamps. This applies for volatile elements such as As. Se, and Te. In the hollow cathode lamps of these relatively volatile elements self-absorption at low discharge currents may be considerable, and self-reversal may even occur. This is not the case with electrodeless discharge lamps. The latter consist of a quartz balloon in which the halide of the element is present. The analyte spectra are excited with the aid of a high-frequency (MHz) or microwave field (GHz), which may be supplied through an external antenna. 

The halides of the elements have been surveyed throughout our tour of the representative elements. Common methods to prepare them include (1) direct reaction of the elements, (2) reactions of oxides or hydroxides with hydrogen halides, (3) reactions of oxides or lower halides with covalent fluorides, and (4) halogen exchange reactions. Nonmetal halides usually hydrolyze to form the corresponding hydroxide, oxide, or oxoacid. Pseudohalides are anions that resemble halides in their chemical behavior. 

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