Geometrical and Optical Isomerism

This is by far the most studied of the isomerisms, with those of square planar and octahedral complexes receiving the major attention. As we shall see, developments in the preparation and separation of isomeric forms and the clever use of nmr techniques have played a major part in the increased understanding of the field. 

---

Although both geometrical and optical isomerism are in principle possible in seven-coordinate complexes, no examples are known. Note, for example, that the py3tren complexes must be optically active (see Fig. 12 35b), subject of course to kinetic stability with respect to racemizciion. 

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. 

Configurational centres impose a rigid shape on sections of the molecule in which they occur. However, their presence gives rise to geometric and optical isomerism. Since these stereoisomers have different shapes, biologically active stereoisomers will often exhibit differences in their potencies and/or activities (Table 2.1). These pharmacological variations are particularly likely when a chiral centre is located in a critical position in the structure of the molecule. The consequence of these differences is that it is now necessary to make and test separately all the individual stereoisomers of a drug. 

Camacho A. D., Pierce H. D., Jr and Borden J. H. (1993) Geometrical and optical isomerism of pheromones in two sympatric Dryocoetes spp. (Coleoptera Scolytidae), mediates species specificity and response level. J. Chem. Ecol. 19, 2169-2182. 

Alfred Werner was born in 1866 and died from arteriosclerosis in 1919, He started as an organic chemist and finished his chemical career in 1915 as one of the foremost inorganic chemists,. He won the Nobel Prize in 1913, During a period of two and a half decades he published 174 papers and supervised the work covered by 200 doctoral dissertations, Werner was the founder of coordination chemistry. He rejected the then prevailing concept formulated by Kekule) that the valence of an element is invariable and introduced instead the notion of principal and auxiliary valence. He also formulated the concept of coordination number, Werner used both the inductive and the deductive methods of reasoning. Most of his predictions on geometrical and optical isomerism were verified by experiment. 

In three-, four-, five-, and six-membered ring systems, geometric and optical isomerism is possible since the rigidity of the ring prevents free rotation around its bonds. Werner s resolution provided a method for determining the configuration of such cyclic compounds. Although he was able to resolve the trans isomer, all his attempts to resolve the cis isomer proved fruitless, in accordance with his expectations. 

An interesting series of chiral 77-allyl complexes have recently been prepared and resolved (60, 172). Studies of the epimerization and syn-and anticonfigurations of these complexes have elucidated the mechanisms of their geometric and optical isomerization. 

The study of isomerism depends on kinetic factors limiting the rate of interconversion, and in the 3d series is confined to complexes of Cr111 and Co111 Coordination, linkage, geometrical, and optical isomerism are possible. 

Linkage Isomerism In addition to the geometric and optical isomerism discussed previously, there is... 

All available structural data are consistent with the octahedral geometry expected for hexacoordinate main group elements. Many examples of both geometric and optical isomerism have been recognized, and some interest attaches to the factors determining the relative stability of geometric isomers. 

Trimethylene as an intermediate for the geometrical and optical isomerizations has been calculated by a large number of investigators [38—46]. Our results on the geometrical and the optical isomerizations were similar to the other calculations. 

Geometrical and optical isomerism are discussed in Section 3.3. The naming of isomeric materials is included at this point for completeness. Geometrical isomers are generally named by the use of the terms cis to designate adjacent (90° apart) positions and trans for opposite (180° apart) positions. It is occasionally necessary to use a number system to designate the position of... 

If a 1-substituted butadiene is polymerized by 1,4-addition a ditactic macromolecule can be formed which has the elements of geometrical and optical isomerism. Two such examples are structures (XXI) and (XXII) for 1,3-pentadiene (piperylene). Both have been experimentally realized by Natta et al. (1961c, 1963a). 

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). 

The mechanisms of geometrical and optical isomerization already discussed also apply to organometallic compounds, but these show some additional unique rearrangement processes. This area has been reviewed by Mann. ... 

---