Color coordination compounds

Simple nickel salts form ammine and other coordination complexes (see Coordination compounds). The octahedral configuration, in which nickel has a coordination number (CN) of 6, is the most common stmctural form. The square-planar and tetrahedral configurations (11), iu which nickel has a coordination number of 4, are less common. Generally, the latter group tends to be reddish brown. The 5-coordinate square pyramid configuration is also quite common. These materials tend to be darker in color and mostiy green (12). 

Dyes and Pigments. Several thousand metric tons of metallated or metal coordinated phthalocyanine dyes (10) are sold annually in the United States. The partially oxidized metallated phthalocyanine dyes are good conductors and are called molecular metals (see Semiconductors Phthalocyanine compounds Colorants forplastics). Azo dyes (qv) are also often metallated. The basic unit for a 2,2 -azobisphenol dye is shown as stmcture (11). Sulfonic acid groups are used to provide solubiHty, and a wide variety of other substituents influence color and stabiHty. Such complexes have also found appHcations as analytical indicators, pigments (qv), and paint additives. 

Na[AuClJ, per mole of silver haHde. Coordination compounds are used as emulsion stabilizers, developers, and are formed with the weU-known thiosulfate fixers. Silver haHde diffusion transfer processes and silver image stabilization also make use of coordination phenomena. A number of copper and chromium azo dyes have found use in diffusion transfer systems developed by Polaroid (see Color photography, instant). Coordination compounds are also important in a number of commercial photothermography and electrophotography (qv) appHcations as weU as in the classic iron cyano blueprint images, a number of chromium systems, etc (32). 

The properties of copper(Il) are quite different. Ligands that form strong coordinate bonds bind copper(Il) readily to form complexes in which the copper has coordination numbers of 4 or 6, such as tetraammine copper(Tl) [16828-95-8] [Cu(NH3)4], and hexaaquacopper(Il) [14946-74-8] [Cu(H,0),p+ ( see Coordination compounds). Formation of copper(Il) complexes in aqueous solution depends on the abiUty of the ligands to compete with water for coordination sites. Most copper(Il) complexes are colored and paramagnetic as a result of the unpaired electron in the 2d orbital (see Copper... 

The copper(I) ion, electronic stmcture [Ar]3t/ , is diamagnetic and colorless. Certain compounds such as cuprous oxide [1317-39-1] or cuprous sulfide [22205-45 ] are iatensely colored, however, because of metal-to-ligand charge-transfer bands. Copper(I) is isoelectronic with ziac(II) and has similar stereochemistry. The preferred configuration is tetrahedral. Liaear and trigonal planar stmctures are not uncommon, ia part because the stereochemistry about the metal is determined by steric as well as electronic requirements of the ligands (see Coordination compounds). 

Most coordination compounds are brilliantly colored, a property that can be explained by the crystal field model. 

The striking feature of many coordination compounds is that they are colored or paramagnetic or both. How do these properties arise To find out, we need to understand the electronic structures of complexes, the details of the bonding, and the distribution of their electrons. 

The most intriguing results were obtained for cobalt(III) chloride. By 1890, several ammonia compounds of C0CI3 had been isolated. These coordination compounds differed in several of their properties, the most striking of which were their beautiful colors. At the time, the formulas of these cobalt complexes were written as follows ... 

Reactions in which isomerization of coordination compounds occur in solutions are common, and some reactions of this type in solid complexes have been studied. Generally, there is a change in color of the complex as the crystal field environment of the metal ion changes. Accordingly, some of the color changes that occur when complexes are heated may indicate isomerization, but very few geometrical isomerization reactions in solid complexes have been studied in detail. One such reaction is... 

Transition metal complexes, catalysts based on, 20 193 Transition-metal compounds cause of color, 7 326t, 328-331 Transition-metal coordination compounds, 7 582-584... 

Weroer. in formulating his ideas about the structure of coordination compounds, had before hint facts such as the following. Four complexes of cobaltflll) chloride with ammonia had been discovered and named according to their colors ... 

Different historians ascribe different dates to the discovery of the first coordination compound. Perhaps the earliest known of all coordination compounds is the bright-red alizarin dye, a calcium aluminum chelate compound of hydroxyanthraquinone. It was first used in India and known to the ancient Persians and Egyptians long before it was used by the Greeks and Romans. Joseph s coat of many colors may possibly have been treated with it. 

Another candidate for the first coordination compound is Prussian Blue, potassium iron(III) hexacyanoferrate(II), a complex of empirical formula KCN-Fe(CN)2- Fe(CN)3. It was first obtained accidentally in 1704 by Diesbach, a manufacturer of artist s colors from Berlin. Initially it was described9 as a nontoxic pigment suitable for oil colors, but its method of preparation was kept secret, probably because Diesbach wished to benefit monetarily from his discovery. 

In the early days of coordination chemistry, coordination compounds were named after their discoverer or according to their color. Now, we use systematic names that specify the number of ligands of each particular type, the metal, and its oxidation state. Before listing the rules used to name coordination compounds, let s consider a few examples that will illustrate how to apply the rules ... 

Y" h Laurence Poncini and Franz L. >UI Wimmer, "Color Classification of Coordination Compounds/ J. 

Copper complex formation. Add a few drops of aqueous copper(n) sulphate solution to an aqueous solution of the amino acid. A deep blue coloration is obtained. The deep blue copper derivative may be isolated by boiling a solution of the amino acid with precipitated copper(n) hydroxide or with copper(n) carbonate, filtering and concentrating the solution. These blue complexes are coordination compounds of the structure ... 

Structural isomers also exist for some coordination compounds. One example is a set of compounds in which a ligand in one isomer may occupy a position outside the coordination sphere in another isomer, such as [Co(NH3)5Br]S04 and [Co(NH3)5S04]Br. These two compounds are structural isomers, each recognizable by its own color and complete set of distinctive properties. 

Prussian blue — iron(III) hexacyanoferrate(II) is the archetype of sparingly soluble mixed valence polymeric metal hexacyanometalates with the formula Me Me(N) [Me c (CN)6] with (i), (N), and (C) indicating the position in the crystal lattice, where (i) means interstitial sites, (N) means metal coordinated to the nitrogen of the cyanides, and (C) means metal ions coordinated to the carbon of the cyanides. It is one of the oldest synthetically produced coordination compounds and was widely used as pigment in paints because of the intensive blue color. The compound has been studied extensively by electrochemical and other methods. The importance of Prussian blue in electrochemistry is related to the fact that it has two redox-active metal centers and that it has an open structure that allows small cations to... 

The chemistry of cobalt involves mainly its +2 and +3 oxidation states, although compounds containing cobalt in the 0, +1, or +4 oxidation states are known. Aqueous solutions of cobalt(II) salts contain the Co(H20)62+ ion, which has a characteristic rose color. Cobalt forms a wide variety of coordination compounds, many of which will be discussed in later sections of this chapter. Some typical cobalt compounds are listed in Table 20.8. 

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