Racemization unsymmetrical chelates

There are a large number of unsymmetrical chelates which present the feature of having both geometrical and optical isomers, as shown by the examples in Figure 4.7. The fac and mer isomers each have optical isomers thus, both racemization and geometrical isomerization can be observed in one system. This can serve to eliminate certain rearrangement processes. 

Among the commonest types of diketo complex are those with the stoichiometries M(diketo)3 and M(diketo)2. The former all have structures based on an octahedral disposition of the six oxygen atoms. The tris-chelate molecules then actually have D3 symmetry and exist as enantiomers. In cases where there are unsymmetrical diketo ligands (i.e., those with R R") geometrical isomers also exist, as indicated in (21-XXVIII). Such compounds have been of value in investigations of the mechanism of racemization of tris-chelate complexes, which are discussed in detail in Section 21-17. 

The case is selected in which the ligands are unsymmetrical so as to also show whether geometrical isomerization accompanies optical inversion. The twist mechanisms have been suggested by some authors as possible modes of racemization. For some jS-diketonates and other chelates inferential evidence has suggested that isomerizations occur principally by bond rupture plus bond reorganization rather than by bond reorganization without bond... 

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