Isomers coordination isomerism

Zn(NH3)4JCuCl4] can display coordination isomerism. Another isomer is OlfNHjJZnClj... 

An pFl-dependent coordination isomerism has been observed for the tumor targeting rhenium(V) 0x0 complex [ReO(DMSA)2] (DMSA = 2,3-mercaptosuccinate). In solution the crystallo-graphically characterized syn,endo-isomQx (87a) slowly isomerizes into the anti- (87b) and the iyn,exo-isomers (87c) which has consequences for the biodistribution of the potential pharmaceutical (Scheme 10). The conversion rate decreases with increasing pH suggesting an acid catalyzed reaction. The syn,exo-comy> ex is favored in alkaline solutions (pH > 8.4) which can be understood in terms of the repulsions between the deprotonated carboxylic groups and the 0x0 ligand. 

Compounds having the same numbers and types of atoms but different structures are called isomers. Coordination compounds exhibit several of types of isomerism, and the study of these various types of isomers constitutes one of the interesting and active areas of research in coordination chemistry. Because so much of coordination chemistry is concerned with isomeric compounds, it is essential that a clear understanding of the various types of isomerism be achieved before a detailed study of structure and bonding in complexes is undertaken. Although the possibility of a substantial number of types of isomerism exists, only the more important types will be discussed here. 

The first type of structural isomerism we will consider is coordination isomerism, in which the composition of the complex ion varies. For example, [C NH-d.sSO Br and [Cr(NH3)5Br]S04 are coordination isomers. In the first case S042- is coordinated to Cr3 +, while Br acts as the counter ion in the second case the roles of these ions are reversed. 

Coordination isomerism can occur in compounds containing both complex cations and complex anions. Such isomers involve exchange of ligands between cation and anion, that is, between coordination spheres. 

Stereoisomers are compounds that are made up of the same types and numbers of atoms bonded together in the same sequence but with different spatial arrangements. There are two types of stereoisomers geometric isomers and optical isomers. Coordination compounds may exhibit one or both types of isomerism. Note, however, that many coordination compounds do not have stereoisomers. 

Another classical example is nitrite ion, which offers N or O atoms as donors. This example has been deeply studied, and the way it behaves is fairly well understood. The O-bound isomer converts (isomerizes) to the thermodynamically stable N-bound isomer, sometimes even in the solid state, by an intramolecular process (without the ligand departing the coordination sphere) in inert complexes. Another feature of ambidentate ligands is that they can display a tendency to bridge between two metal ions, with each of the two different donor atoms attached to one of two metal ions. 

Coordination isomerism is another relatively rare, though well-known, phenomenon, exemplified in the compounds [Cr(NH3)6][Co(CN)6]/[Cr(CN)6][Co(NH3)6] and [Cr(en)20x]-[Cr(ox)2en]/[Cr(en)3][Cr(ox)3] (ox = 204 ). It is related to the structural isomerism that might be expected to be encountered in polynuclear systems (Figure 3). Although examples of such isomer types are known, e.g. the chiral cobalt(III) hexol , [Co Co(OH)2(NH3) 4]3] , and the chromium(III) rhodoso ion with the alternative achiral structure shown in Figure 3, no isomeric pairs have been thoroughly characterized. 

The definition of coordination isomerism depends on the context. Historically, a complete series of coordination isomers required at least two metals. The ligandimetal ratio remains the same, but the ligands attached to a specific metal ion change. For the empirical formula Pt(NH3)2Cl2, there are three coordination isomer possibilities that contain Pt(II). 

A special type of coordination isomerism is one that involves the different placement of ligands in a bridged complex. This is sometimes called coordination position isomerism, and an example is provided by the isomers... 

Three mechanisms that are consistent with this rate law have been proposed and are shown in Scheme 16.22. All three mechanisms generate a hydroxyethyl intermediate, but the three differ m the mechanism by which this intermediate is formed. The first mechanism iiwolves formation of a palladium hydroxide, accompanied by isomerization of the trans isomer to the cis isomer. The isomerization occurs via aquation of a chloride ligand and the formation of ds-[(C2H )Pd(OH)2Cl]" as an intermediate, followed by insertion of olefin into the metal-hydroxide bond. The second involves rate-determining external attack of hydroxide on the coordinated olefin, and the tliird involves external attack of water on the coordinated olefin, followed by proton transfer and rate-determining dissociation of chloride. 

Werner also described and named the various other types of isomerism observed in coordination compounds. These are ionisation isomerism, displayed by [Co(NH3)5C1]S04 and [Co(NH3)5S04]Cl hydrate isomerism, for example [Cr(H20)5]Cl3 and [Cr(H20)5Cl]Cl2.H20 and coordination isomerism, which occurs when both cation and anion are complexes, for example [Co(NH3)5][Cr(CN)6] and [Cr(NH3)5][Co(CN)6] Werner also explained one case of what is now known as linkage isomerism in [Co(NH3)5N02]Cl2 and [Co(NH3)50NO]Cl2, where the NO2 ligand has two possible sites of attachment to the metal atom. It was many years before other linkage isomers were prepared. 

Four-coordinate complexes provide good examples of the early use of preparative methods for establishing stereochemistry. For complexes of the type [Ma2b2], where a and b are unidentate ligands, a tetrahedral structure cannot produce isomerism whereas a planar structure leads to cis and trans isomers (see below). The preparation of 2 isomers of [PtCl2(NH3)2], for instance, was taken as good evidence for their planarity. ... 

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. 

In principle this type of isomerism (also known as polytopal isomerism) is possible with any coordination number for which there is more than one known stereochemistry. However, to actually occur the isomers need to be of comparable stability, and to be separable there... 

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