Ligand approach

As six ligands approach a central metal ion to form an octahedral complex, they change the energies of electrons in the d orbitals. The effect (Figure 15.10, p. 419) is to split the five d orbitals into two groups of different energy. 

Dissociation constant, the ratio of the rate of offset of ligand away from a receptor divided by the rate of onset of the ligand approaching the receptor. It has the units of concentration and specifically is the concentration of ligand that occupies 50% of the total number of sites available for ligand binding at equilibrium (see Affinity). 

When ligands approach a central atom or ion, the following energetic contributions become effective ... 

Nevertheless, there remains a plethora of substrates that have not yet been studied using the monodentate ligand approach. In the application of asymmetric hydrogenation, it is very important to go beyond the benchmark substrates, and several studies have already shown that the scope of enantioselective hydrogenation might be much broader than was originally assumed. 

Suppose the first subunit, the ligand, approaches to bind to the second subunit (Fig. 4.21). Its binding energies to L and H are... 

Note the appearance of in but <2aa in The reason is that a ligand approaching the first (or third) subunit sees an equilibrium concentration, of L and H, different from a ligand approaching the second subunit. These also differ firom the equilibrium concentrations of L and H for an isolated subunit, for which the binding constant is... 

Now consider the formation of an octahedral complex such as [N1(H20)J +. Think of six water ligands approaching the Nl + ion along the x-, y- and z-axes. The electrons in the d orbitals of the nickel ion that lie along the axes will be repelled by the electrons of the approaching ligands. 

Asymmetric homogeneous catalysis generally requires chiral ligands. Approaches to chiral NHCs have focused on the generation of chiral centers either in the 4- and 5-position of imidazolidinium salts 71 or in the a-position of the nitrogen substituents for imidazolium salts 72. 

Choudhary, S. Morrow, J. R. Dynamic acylhydrazone metal ion complex libraries A mixed-ligand approach to increased selectivity in extraction. Angew. Chem. Int. Ed. 2002, 41, 4096-4098. 

The typical strategy used to prepare these dendrimers is the so-called complexes as metals and complexes as ligands approach,59 which has enabled the construction of species containing 4, 6, 10, 13, and 22 metal-based units. 

Goldberg, S. and Traina, S.J. (1987) Chemical modelling of anion competition on oxides using the constant capacitance model-mixed-ligand approach. Soil Sci. Soc. Am.J., 51,... 

A chimeric ligand approach (Fig. 26) was used to combine the properties of FQ and those of thiosemicarbazones (TSC) [133]. 

Han H, Janda KD, Multipolymer-supported substrate and ligand approach to the Sharpless asymmetric dihydroxylation, Angew. Chem., Int. Ed. Engl., 36 1731-1733, 1997. 

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