Halogen complexes coordination compounds

An extensive review55 of the chemistry of halogen complexes containing carbon ligands, the hypervalent halogen compounds , has been published recently. This is written from an organic chemical viewpoint and includes discussion of the organic chemistry involved in reactions of these compounds. Only aspects directly related to coordination chemistry will be mentioned here. 

Although there are authoritative and extensive reviews of the chemistry of the halogens available, these usually cover the complete chemistry of the elements, and are not restricted to coordination chemistry.1"4 Halogens are ubiquitous as ligands, in complexes with a wide variety of oxidation states for the central atom involved. The difficulty in this particular area of coordination chemistry is to draw a line between those compounds considered as coordination compounds and those more appropriately treated as collections of ions. 

Coordination compounds of vanadium(V) also catalyze peroxidative halogenation reactions where the reactive oxidant is a monoperoxo complex of a mononuclear vanadium compound (Figure 6) [11,22,92-95,99]. 

The halide ligands, and particularly those of chloride, are the most propagated ones in modern coordination chemistry. Their complex compounds are represented in classic [89-91] and modern editions [1, vol. 2 3-5,32,73,92]. Coordination compounds of practically all metals and halogens have been synthesized and characterized. Presentation of the enormous amount of material available for the complexes of this type is beyond the limits of the present book. 

Other examples of reported molecular complexes are tabulated in Refs. 202,506, and 507. However, it is necessary to note that the participation in these syntheses of acids, ammonium salts, halogens, and alkyl halides [506,507,551,562] provokes doubts in respect of determining the role of the electrochemical processes, since the reagents above can themselves dissolve elemental metals in various liquid media, forming coordination compounds (Sec. 3.4.3). 

Complex compounds obtained by oxidative dissolution of elemental metals in the systems M°-ligand-solvent-HX (or halogen-substituted hydrocarbon), are represented by all types of complexes [509]. However, their ratio is not equal they are mainly molecular complexes and metal chelates the % -complexes and di(poly)-nuclear coordination compounds are not numerous. A large number of molecular (neutral and ionic) complexes, chelates, and their adducts, mostly with N-bases, are tabulated in monographs [201,202]. 

Since there are a number of complexes that might conceivably be formed from most combinations of reagents (Exercise 8), the obtaining of a desired complex in substantial yield is in many cases an art. In identifying coordination compounds of cobalt, halogen atoms held in the complex ion are distinguished from anionic halides by treatment with Ag+. This precipitates anionic chloride, bromide, and iodide immediately, but eom-plexed halide precipitates only slowly. Thus, with the purpose compound... 

As a first approximation, the reactions of pyridines with electrophiles can be compared with those of trimethylamine and benzene. Thus, pyridine reacts easily at the nitrogen atom with reagents such as proton acids, Lewis acids, metal ions, and reactive halides to form salts, coordination compounds, complexes, and quaternary salts, respectively. Under much more vigorous conditions it reacts at ring carbons to form C-substitution products in nitration, sulfonation, and halogenation reactions. 

Other scientists soon used X-ray diffraction to confirm the octahedral configuration of the six halogen atoms in similar hexacoordinate complexes. Wyckoff also confirmed Werner s view that the molecules of water and of ammonia in most crystalline hydrates and ammines are associated with the metallic atom in the same way as the coordinated atoms and radicals in a complex anion. Since the early 1920s, the stractures of numerous coordination compounds of various coordination numbers have been determined by X-ray diffraction. 

Adducts of the l,2-dithiole-3-thione with silver nitrate and copper dichloride have been described, and for various l,2-dithiole-3-thiones, adducts containing iron, cobalt, nickel, and copper have been studied in more detail. The ligands (L) used are either unsubstituted l,2-dithiole-3-thione or its methyl- or aryl-substituted derivatives. If X represents a halogen atom, the following types of coordination compounds have been characterized CuXLj and CuXLa, FeXjLj, CoXaL, and NiX Lj. Complexes of titanium(III), tin(IV), antimony(III) and (V), and bismuth(III) have also been studied. ... 

Usually, Au(I) exists only in insoluble compounds or complexed species. For example, the ions AuXj (X = CN, I, Nj, SCN) are well known. The majority of other Au(I) complexes are also linear and tend to be of the LAuX type, where L is a neutral donor such as R3P or RjS, and X is halogen, SCN , or RS (R = alkyl group). With higher concentrations of RP3 ligands, tetrahedral coordination compounds such as... 

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