Isomerism, coordination compounds

Valence-bond representation Molecular-orbital representation it bonding and multicenter it bonds Shapes of molecules Coordination compounds Isomerism Bonding in metals... 

Isomers are compounds with the same chemical composition but different structures, and the possibility of their occurrence in coordination compounds is manifest. Their importance in the early elucidation of the stereochemistries of complexes has already been referred to and, though the purposeful preparation of isomers is no longer common, the preparative chemist must still be aware of the diversity of the compounds which can be produced. The more important types of isomerism are listed below. 

The physical and chemical properties of complex ions and of the coordination compounds they form depend on the spatial orientation of ligands around the central metal atom. Here we consider the geometries associated with the coordination numbers 2,4, and 6. With that background, we then examine the phenomenon of geometric isomerism, in which two or more complex ions have the same chemical formula but different properties because of their different geometries. 

Alfred Werner s research on geometrically isomeric coordination compounds. G. B. Kaufmann, Coord. Chem. Rev., 1975, 15,1-92 (564). 

Development of Coordination Chemistry Since 1930 Coordination Numbers and Geometries Nomenclature of Coordination Compounds Cages and Clusters Isomerism in Coordination Chemistry Ligand Field Theory Reaction Mechanisms... 

Many complexes and coordination compounds exist as isomers, compounds that contain the same numbers of the same atoms but in different arrangements. For example, the ions shown in (13a) and (13b) differ only in the positions of the Cl ligands, but they are distinct species, because they have different physical and chemical properties. Isomerism is of more than academic interest for example, anticancer drugs based on complexes of platinum are active only if they are the correct isomer. The complex needs to have a particular shape to interact with DNA molecules. 

Figure 16.18 summarizes the types of isomerism found in coordination complexes. The two major classes of isomers are structural isomers, in which the atoms are connected to different partners, and stereoisomers, in which the atoms have the same partners but are arranged differently in space. Structural isomers of coordination compounds are subdivided into ionization, hydrate, linkage, and coordination isomers. 

FIGURE 16.18 The various types of isomerism in coordination compounds. 

Which of the following coordination compounds can have cis and trans isomers If such isomerism exists, draw the two structures and name the compound. 

One of the interesting aspects of the chemistry of coordination compounds is the possibility of the existence of isomers. Isomers of a compound contain the same numbers and types of atoms, but they have different structures. Several types of isomerism have been demonstrated, but only a few of the most important types will be described here. 

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... 

The ability of complexes to catalyze several important types of reactions is of great importance, both economically and intellectually. For example, isomerization, hydrogenation, polymerization, and oxidation of olefins all can be carried out using coordination compounds as catalysts. Moreover, some of the reactions can be carried out at ambient temperature in aqueous solutions, as opposed to more severe conditions when the reactions are carried out in the gas phase. In many cases, the transient complex species during a catalytic process cannot be isolated and studied separately from the system in which they participate. Because of this, some of the details of the processes may not be known with certainty. 

Effects of Isomerism Geometrical isomers of coordination compounds can exhibit different values of redox potential, as accounted for, in various cases, by simple djr orbital level splitting diagrams or by MO calculations. The dependence of the relative stability and redox behavior of the geometrical isomers on the electronic configuration of the metal is also... 

The compound [Mo(NO)(NH20)(NCS)2(phen)J on heating under reflux in acetophenone or on photolysis at 80 °C gives the five-coordinate, paramagnetic isomeric pair [Mo(NCS)2(NO)(phen)] (Table 4) a square pyramidal analogue [Mo(NCS)2(NO)(bipy)] was also obtained. In all compounds the NCS is N-bonded.51... 

In general the other types of isomerism for coordination compounds are less interesting than those discussed previously, but will be listed briefly to show the variety of possibilities. 

The present discussion of isomerism in coordination compounds is not, nor was it intended to be, comprehensive and exhaustive. The examples considered are an eclectic selection, and many important systems may have been neglected through ignorance. An obvious omission is any detailed consideration of polynuclear complexes139,256"259 and it is, of course, a quite arbitrary decision not to include any consideration of organometallic species. Other neglected issues, such as the development of a truly comprehensive system of stereochemical nomenclature, are perhaps not yet capable of solution. Nevertheless, it is to be hoped that the principal factors to be considered... 

In solution the photochromism of coordination compounds can result from photoinduced isomerization (e.g. metal dithizonates186-188), substitution (e.g. iron(II) phthalocyanine in the presence of benzyl isocyanide189) and oxidation—reduction (e.g. tris(diethyldithiocarbamato)-nickel(IV) bromide190). 

Photocalorimetry offers a convenient alternative to other methods of AH determination and, in some instances, may be the only practical method. The ligand substitution reactions of robust Werner-type complexes are a case in point. Conventional thermochemical measurements are complicated by the slowness of the substitution process and/or by competing reactions. Some of these same complexes, however, undergo clean photosubstitutions with high quantum yields and thus are excellent candidates for photocalorimetry. Examples include [Cr(NH3)6]3+, [Cr(CN)6]3-and [Co(CN)6]3-.192 Photocalorimetric measurements of AH have also been obtained for isomerization and redox reactions of coordination compounds.193194... 

Olefin isomerization -use of coordination compounds [COORDINATION COMPOUNDS] (Vol 7)... 

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