Symmetric isomers, illustration system

Polydentate ligands present particular problems, as a range of geometric isomers, often with enantiomers, may exist in principle. This is illustrated in Figure 4.34 for a simple system where a symmetrical tetradentate is bound. This can occur in three ways, two of which... 

In the absence of special symmetry, the phase mle requires a minimum of three components for a tricritical point to occur. Symmetrical tricritical points do have such s mimetry, but it is easiest to illustrate such phenomena with a true ternary system with the necessary symmetry. A ternary system comprised of a pair of enantiomers (optically active d- and /-isomers) together with a third optically inert substance could satisfy this condition. While liquid-liquid phase separation between enantiomers has not yet been found, ternary phase diagrams like those shown in figure A2.5.30 can be imagined in these diagrams there is a necessary symmetry around a horizontal axis that represents equal amounts of the two enantiomers. 

The general, 8-diketonate template has an overall lath-like structure, and subtle structural modifications can lead to crossover between smectic and columnar mesomorphism, or to structural change between discotic to calamitic shape, simply by changing the nature of the substituent and R (Figure 33). This is an important aspect of this type of complex, which would not be appreciated fully and understood if discussed separately. For instance, symmetrical systems (R = R, D2h) can have either disk-like or X-shaped structures, whereas the unsymmetrical compounds (R R ) can possess either zigzag- or cross-shaped structures (Figure 33). For the latter, both cis (C2v) and trans (C2h) isomers are also possible, and it is reasonable to think that mixtures are always present and that transicis interconversion occurs at elevated temperatures. This study is clearly a textbook case in addition to being a perfect illustration of induction, modification, or suppression of mesomorphism on complexation. 

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