Optical isomerism Octahedral complexes

In complexes of chelates there are a number of types of isomerism which may occur. In a tris(ethylenediamine) octahedral complex two optically active isomers occur (often denoted A and A). 

A few cases of optical isomerism are known for planar and tetrahedral complexes involving unsymmetrical bidentate ligands, but by far the most numerous examples are afforded by octahedral compounds of chelating ligands, e.g. [Cr(oxalate)3] and [Co(edta)] (Fig. 19.13). 

Optical isomers are special kinds of stereoisomers they are non-superimposable mirror images of each other (Fig. 16.27). Both geometrical and optical isomerism can occur in an octahedral complex, as in [CoCI2(en)2]+ the trans isomer is green (14a) and the two alternative cis isomers (14b) and (14c), which are optical isomers of one another, are violet. 

This kind of isomerism occurs when a compound can be represented by two asymmetrical structures, one of which is the mirror image of the other. It is common in octahedral complexes involving bidentate groups for, unless bidentate groups are present, there is always a plane of symmetry in complexes of this shape. The cation of [CoCl2(en)2]Cl has two optically active cis-forms (Fig. 289(a) and (b)) in addition to an inactive trans-form (Fig. 289(c)). [Co(cn)3]Br3 has also been resolved Fig. 290(a) and (b) show the two enantiomorphs of the terpositive cation. 

The number of possible diastereomers depends on the variety of ligands and sometimes requires use of the one-letter code (cis/trans is noted c/t). This nomenclature may be applied to square planar complexes and to square planar pyramidal and octahedral complexes, but not to tetrahedral complexes where a given position is equivalent to any other. Moreover, geometric isomerism often implies the existence of optical isomerism. 

The existence of a carbon atom bound to four different groups is not the strict condition for optical activity. The essential point is that the molecule should be asymmetric. Inorganic octahedral complexes, for example, can show optical isomerism. It is also possible for a molecule to contain asymmetric carbon atoms and still have a plane of symmetry. One structure of tartaric acid has two parts of the molecule that are mirror images, thus having a plane of symmetry. This (called the meso-form) is not optically active. See also resolution). 

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. 

Figure 22.11 Optical isomerism in an octahedral complex ion. A, Structrre I and its mirror image, structure II, are optical isomers of C(s-[Co(en)2Cl2] . Rotating structure I gives structure III, which is not the same as structure II. (The curved wedges represent the bidentate...

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). 

Compounds with bidentate ligands and monodentate increase the potential for isomerism in octahedral complexes. Figure 23.16 shows the isomers of the dichloro-bis(ethylenediamine)cobalt (III) ion, CoCl2(en)2. Isomer A is the trans isomer (it has a green color). Both B and C are cis isomers (both have a violet color). Yet isomers B and C are not identical molecules. They are enantiomers, or optical isomers that is, they are isomers that are nonsuperimposable mirror images of one another. 

Octahedral isomers containing a bidentate ligand can form a pair of complexes that are mirror images of each other (Figure 13.24). These molecules are known as enantiomers. This form of isomerism is called optical isomerism (Chapter 20) because the two forms will rotate plane-polarized light in equal but opposite directions. 

The previous examples demonstrate optical isomerism in octahedral complexes. Tetrahedral complexes can also exhibit optical isomerism, but only if aU four coordination sites are occupied by different ligands. Square planar complexes do not normally exhibit optical isomerism as they are superimposable on their mirror images. 

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