Optical isomerization coordination compounds

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. 

Octahedral coordination compounds having the general formula ML2X2, where L is a bidentate ligand and X is a halide, will also exhibit optical isomerism. Werner and his students were among the first chemists to synthesize an optically-active coordination compound. Specifically, Werner was able to resolve the optical isomers of ds-[CoCI(NH3)(en)2] , which are shown in Figure 15.17, by precipitation with silver d-a-bromocamphor- r-sulfonate. 

Oxalate (ox, 204 ) complexes of Cr° have been known since the very beginning of coordination chemistry. Thus, the resolution of chiral [Cr(ox)3] with strychninium counterion by Werner in 1912 prodnced the first optically active anionic coordination compound. There also exists a series of bis(oxalato) complexes of the type [Cr(ox)2X2] + , where X can be any of a variety of nentral donors (e g. H2O, NH3, etc.) or anionic ligands (e g. SCN, N3, etc.). These compounds have been used to study the mechanism of cis/trans isomerization and racemization of optically active octahedral coordination compounds. 

From studies of isomerizations and racemizations of optically active octahedral coordination compounds, Fujiwara and Bailar [78] concluded that none of the several mechanisms proposed was capable of accounting for the observations for all of the reactions investigated. 

When a species cannot be superimposed on its mirror image the two forms are known as enantiomers or optical isomers. Most examples with coordination compounds have chelating (e.g. bidentate) ligands (see Topic E3 ). Structures 10 and 11 show respectively the delta and lambda isomers of a tris(chelate) complex, with the bidentate ligands each denoted by a simple bond framework. As discussed in Topic C3. optical isomerism is possible only when a species has no improper symmetry elements (reflections or inversion). Structures 10 and 11 have the point group D3, with only C3 and C2 rotation axes. 

Coordination compounds may display geometric and/or optical isomerism. 

Molecules that show optical activity have no plane of symmetry. The commonest case of this is in organic compounds in which a carbon atom is linked to four different groups. An atom of this type is said to be a chiral centre. Asymmetric molecules showing optical activity can also occur in inorganic compounds. For example, an octahedral complex in which the central ion coordinates to six different ligands would be optically active. Many natur y occurring compounds show optical isomerism and usually only one isomer occurs naturally. 

Unlike four-coordinated systems, six-coordinated complexes afford many examples of optical isomerism. These are very common among compounds or ions of the type [M(AA)3]. For example, the optical isomers of trioxalatochromate(lll) are complexes XX and XXL... 

Several types of isomerism, other than geometrical and optical, are known for coordination compounds. These are often unique to this class of compound. Specific examples are given below to represent each type. In general, the nature of the isomerism is sufficiently obvious from the examples that no lengthy discussion is required. 

Complex ions consist of a metal ion surrounded by ligands. The donor atoms in the ligands each contribute an electron pair to the central metal ion in a complex. Coordination compounds may display geometric and/or optical isomerism. 

As an example of optical isomerism (see Section 4.4) in coordination compounds. Figure 15.12 shows the cis and trans isomers of dichlorobis(ethylenediamine)cobalt (111) ion and their mirror images. The trans isomer and its mirror image are superim-posable, but the cis isomer and its mirror image are not. The cis isomer and its mirror image are, therefore, optical isomers. Unlike geometric isomers, optical isomers have identical physical and chemical properties, except for the way in which they interact with polarized light and in the way they react with other chiral molecules. 

The instances of geometric isomerism also multiplied with the realisation that cis and trans isomerism was to be found in double-bonded nitrogen compounds, and also in saturated cyclic compounds. Werner s work on inorganic coordination compounds provided further instances of both geometrical and optical isomerism (Chapter 12). 

Coordination compounds exhibit isomerism, in which two compounds have the same composition but different structures. The types of isomerism exhibited by coordination compounds are described, including structural isomers, geometric isomers, and optical isomers, which are two isomers of a com-poimd that are mirror images of one another. 

Coordination compounds can exhibit constitutional isomerism (coordination and linkage) and stereoisomerism (geometric and optical). 

The occurrence of enantiomers (optical isomerism) is concerned with chirality, and some important terms relating to chiral complexes are defined in Box 19.3. Enantiomers of a coordination compound most often occur when chelating ligands are involved. Figure 19.13a shows [Cr(acac)3], an octahedral tris-chelate complex, and Fig. 19.13b shows cw-[Co(en)2Cl2]", an octahedral bis-chelate complex. In this case, only the cw-isomer possesses enantiomers the tra 5-isomer is achiral. Enantiomers are distinguished by using the labels A and A (see Box 19.3). 

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