Didentate ligands

What happens if we look at the K2 or Ki, values for didentate ligands In general, the Kj values show stability patterns which closely parallel those for K. However, the values are different. Figure 8-17 presents K, data for transition-metal complexes of 1,10-phenanthroline and 1,2-diaminoethane (Eq. 8.14). 

Of course, the physical properties of the two enantiomers of a complex containing three chelating didentate ligands will be identical, and interactions with achiral reagents will also... 

Figure 2-7. Two representations of the two enantiomeric forms of a complex containing three chelating didentate ligands. The lower view emphasises the relationship between the donor atoms and the triangular faces of an octahedron. The labels A and A describe the absolute configuration of the complex.

Recent model studies strongly support the proposed mechanism. The first crystal structures of Fe(II) complexed to benzoylformate show that an a-keto acid can coordinate to the iron as either a monodentate or didentate ligand [236]. Exposure of these [Fe(II)(L)(bf)]+ complexes (L = tmpa or 6-Me3-tmpa) to 02 results in the quantitative conversion of benzoylformate to benzoic acid and C02, modeling the oxidative decarboxylation reaction characteristic of this class of enzymes. As with the enzymes, the use of 1802 in the model studies results in the incorporation of the label into the benzoate product. For [Fe(6-Me3-tmpa)(bf)]+, the rate of the oxidative decarboxylation increases as the substituent of the benzoylformate becomes more electron-withdrawing, affording a Hammett p of +1.07. This suggests that the oxidative decarboxylation involves a nucleophilic attack, most plausibly by the iron-bound 02, on the keto carbon of benzoylformate to initiate decarboxylation as proposed in Figure 27. 

Mixed donor didentate ligands containing selenium are less well known than their sulfur analogues. A typical example of an Se,N donor is (41), which coordinates as a didentate chelate another is (42), a selective analytical reagent for platinum group metals. The monoanionic (43) forms neutral ML2 complexes (M = Zn, Cd, Hg) with an overall N2Sc2 donor set, with the imine N selected as a much better donor than the alternate ether However, Se,0 donor chelation is displayed by the butane-2,4-dionate analogue... 

The N,P phosphine-oxazoline chelate (59) is chiral, and complexes can act as homogeneous catalysts for asymmetric synthesis the Ir(l) and Pd(II) complexes promote enantioselective olefin hydrogenation and allylic substitution respectively. An N,P analog of the N,N didentate ligand 2,2 -bipyridine is (60), the soft P donor helping to stabilize low-valent metals. Further, 2,2-bipyridine derivatives such as (61) can bind metals such as Ir and Ru as N,C chelates with one pyridine nitrogen rotated to the opposite side, away from the metal ion. 

Freedom to rotate about the C—C bond in diaminoethane permits cis or trans isomers, capable of chelation and bridging respectively (top). For rigid diaminobenzene (bottom), rearrangement is not possible, and the two isomers shown have exclusive, different coordinating functions as didentate ligands. 

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. 

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