Tetrahedral complex optical isomerism

Tetrahedral complexes of Be(II), B(III) and Zn(II) with asymmetrical bidentate ligands also exhibit optical isomerism. A complex shown in Figure 42, bis(benzoylacetonato)beryUium(II), [Be(C6H5COCHCOCH3)2] is an important example of this class. 

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). 

Tetrahedral complexes du not exhibit geometrical isomerism. However, they are potentially chiral just as is tetrahedral carbon. The simple form of optical isomerism exhibited by most organic enantiomers, namely four different substituents, is rarely observed because substituents in tetrahedral complexes are usually too labile10 for the complex to be resolved, i.e., they racemize rapidly. However, an interesting series of cyclopentadienyliron phosphine carbonyl compounds (see Chapter 15 for further... 

Cobalt(II) forms more tetrahedral complexes than any other ion except zinc(II). Draw the structure(s) of the tetrahedral complex [CoCl2(en)]. Could this complex exhibit geometric or optical isomerism If one of the CD ligands is replaced by Br, what kinds of isomerism, if any, are possible in the resulting compound ... 

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 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. 

The most common coordination number is six and such complexes have an octahedral structure. The next most common four-coordinated systems have either tetrahedral or square planar structures. Other complexes are known having different coordination numbers and structures. The stereochemistry of metal complexes is a fascinating subject. Several different types of isomeric structures are possible and have been demonstrated in these systems. For our purpose here it is sufficient to cite examples of geometrical (ds-trans) and optical isomerism. This can readily be iUustrated by the cis (III) and trans (IV) isomers of QCo(en)2Cl2]+. Note that the... 

In 1936, Mills and Clark published their findings on new thio-salts of Hg, Cd and Zn. They proposed a tetrahedral conformation and discovered a specific and reversible isomerization between two crystal forms. They noted that optical activity could possibly exist in these complexes and suggested it as the most probable explanation for the existence of the two modifications. The next, larger step in main group dithiolate chemistry came only in 1960, when Gilbert and Sandell prepared Mo(tdt)3 as the first neutral tris-dithiolate. While main groups dithiolates are still of broad interest, most of the recent work in this area has focused on structural questions. 

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