Shape polydentate ligands

There are more examples of a second type in which the chirality of the metal center is the result of the coordination of polydentate ligands. The easiest case is that of octahedral complexes with at least two achiral bidentate ligands coordinated to the metal ion. The prototype complex with chirality exclusively at the metal site is the octahedral tris-diimine ruthenium complex [Ru(diimine)3 with diimine = bipyridine or phenanthroline. As shown in Fig. 2 such a complex can exist in two enantiomeric forms named A and A [6,7]. The bidentate ligands are achiral and the stereoisomery results from the hehcal chirality of the coordination and the propeller shape of the complex. The absolute configuration is related to the handness of the hehx formed by the hgands when rotated... 

Complexes of arseniqlll). anrimony(III), (electron configuration = ( - I lead(II). and bismulh(lll) I(n - 21/14 (n - l)r/ 0/ 2] with polydentate ligands occupying six coordination sites have been found 10 have a stereochemically active lone pair. However, the dichotomy of behavior of the heavier elements that have a lone pair is reflected m Ihe crystal chemistry of Br - When forced into sites of high symmetry, the Bi,+ ion responds by assuming a spherical shape in crystals of lower symmetry the lone pair asserts itself and becomes stereochemically active. 

It is evident that the geometries of these 7-coordinated complexes are very much dependent on the constraints due to the shapes of the polydentate ligands. A more extreme example is the pentagonal bipyramidal coordination in complexes formed by the pentadentate ligand L, such as the cations in the salts [FeL(NCS)2]C104, (where the NCS groups occupy the apical positions with Fe—N, 2-01 A as compared... 

Further, some ligands contain groups with more than one heteroatom in principle capable of acting as a donor atom examples in the figure above are carboxylate (—COO ) and amido (—N —CO—). Coordination of both donors to the one metal ion may be forbidden by the shape of the ligand molecule, or else occurs under only special synthetic conditions. We shall explore polydentate ligands in more detail in Section 2.3. 

Examples of polydentate ligands of selected different basic shapes. 

The process of complexation of polydentate ligands involves an aspect of wrapping around the metal ions, usually with gross rearrangement of the ligand shape. 

Immediately above, we have introduced some of the issues that arise when a polydentate ligand is bound rather than simple monodentate or didentate ligands. Even stepping from two to three donors increases the options in terms of ligand shape (or topology), and this shape will affect the way a molecule may bind to a metal ion. Some shapes for potentially tridentate ligands appear in Figure 4.36. 

The straightforward synthetic routes to generate sophisticated star-shaped polydentate N-ligands 334 have allowed the preparation of molecular cages 335 with applications in catalysis or separation, coordination polymers with magnetic properties or host-guest cavities, and anion-hosting materials (Equation 71). 

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