Ligand acidity term

Discuss the binding of metal ions and of ligands in terms of the Lewis-acid-base theory. 

If the inner sphere coordination is a highly dominant term in the lattice energy of salts of a metal ion then the crystal structures should be just the packing of complexes of a fixed M-dependent structure. For example metal hydrates of SO4- give compounds [M(H20)6]n+ SO4 n H20 for Mg2+, Ni2+, Al3+, Cr3+, Co3+, Mn2+ etc. and similar hydrates appear in halides, and salts of other strong acid oxy-anions, e.g. nitrates. Even when water molecules do not occupy all sites around M the octahedron is maintained [Exceptions such as Co(II) and Zn(II) giving M(0)4 polyhedra are easily explained on the basis of polarisation (ligand-field) terms but these are still central field effects, i.e. 

J0rgensen proposed the principle of symbiosis with respect to hard and soft acid-ba.se behavior. This rule of thumb states that hard species will tend to increase the hardness of the atom to which they are bound and thus increase its tendency to attract more hard species. Conversely, the presence of some soft ligands enhances the ability of the central atom to accept other soft ligands. In terms of the electrostatic versus covalent picture of Pearson s hard and soft or Drago s and... 

Due to the relative ease of synthesis of the phosphoramidites, it is simple to introduce variations within the ligand, especially the substitution on the nitrogen atom. In turn, this allows for rapid screening of ligands to hnd the best one for a specific substrate. The screen can be used to not only increase enantioselectivities but also to turn over numbers and frequencies. In a number of cases, the piperidine analogue (20b) has been found to be superior for a-amino acid and ester production over MonoPhos The best ligands in terms of enantioselectivity are 19c ... 

It remained for G.N. Lewis (2), M.L. Huggins (3), and their contemporaries to interpret the primary and secondary valences of Werner in terms of the newly emerging electron patterns which were being used to explain "valence." Primary valences were normal covalent bonds (one electron from each bonded atom) and secondary valences were coordinate covalent bonds (two electrons for bond formation supplied by the ligand). The terms Lewis base and Lewis acid entered the literature. The Werner and Lewis-Huggins descriptions of stmcture and bonding were excellent for the metal-ammines, halo complexes of metals, and related species. Classical Werner coordination compounds fit the Wemer-Lewis description, and there were many of such compounds. 

Based on the classification of cations and ligands proposed by Ahrland, Pearson (Pearson 1963, Pearson 1966) developed the type (a) and type (b) by explaining the differential complexation behavior of cations and ligands in terms of electron pair-donating Lewis bases and electron pair-accepting Lewis acids ... 

Yamamoto developed a conceptually new class of chiral boron-Lewis acids 174 (Equation 20) that can readily be assembled from a tetraphenol ligand and B(OMe)3 [93, 94]. These were shown to be especially effective with a wide range of a-substituted enals in enantioselective Diels-Alder reactions. One illustrative example is the cycloaddition between aldehyde 173 and cyclopentadiene to give 175 in 99% ee, > 99 1 exo/endo selectivity, and quantitative yield [93]. Yamamoto suggested that these catalysts 174 were Bransted-assisted chiral Lewis acids (termed BLAs), in which the phenol proton activates the enal substrates by hydrogen bonding, as depicted in the proposed transition state assembly 176. 

A more eflicient and general synthetic procedure is the Masamune reaction of aldehydes with boron enolates of chiral a-silyloxy ketones. A double asymmetric induction generates two new chiral centres with enantioselectivities > 99%. It is again explained by a chair-like six-centre transition state. The repulsive interactions of the bulky cyclohexyl group with the vinylic hydrogen and the boron ligands dictate the approach of the enolate to the aldehyde (S. Masamune, 1981 A). The fi-hydroxy-x-methyl ketones obtained are pure threo products (threo = threose- or threonine-like Fischer formula also termed syn" = planar zig-zag chain with substituents on one side), and the reaction has successfully been applied to macrolide syntheses (S. Masamune, 1981 B). Optically pure threo (= syn") 8-hydroxy-a-methyl carboxylic acids are obtained by desilylation and periodate oxidation (S. Masamune, 1981 A). Chiral 0-((S)-trans-2,5-dimethyl-l-borolanyl) ketene thioketals giving pure erythro (= anti ) diastereomers have also been developed by S. Masamune (1986). 

Gold Compounds. The chemistry of nonmetallic gold is predominandy that of Au(I) and Au(III) compounds and complexes. In the former, coordination number two and linear stereochemistry are most common. The majority of known Au(III) compounds are four coordinate and have square planar configurations. In both of these common oxidation states, gold preferably bonds to large polarizable ligands and, therefore, is termed a class b metal or soft acid. 

Adsorption of Metal Ions and Ligands. The sohd—solution interface is of greatest importance in regulating the concentration of aquatic solutes and pollutants. Suspended inorganic and organic particles and biomass, sediments, soils, and minerals, eg, in aquifers and infiltration systems, act as adsorbents. The reactions occurring at interfaces can be described with the help of surface-chemical theories (surface complex formation) (25). The adsorption of polar substances, eg, metal cations, M, anions. A, and weak acids, HA, on hydrous oxide, clay, or organically coated surfaces may be described in terms of surface-coordination reactions ... 

This shows that the pM value of the solution is fixed by the value of K and the ratio of complex-ion concentration to that of the free ligand. If more of M is added to the solution, more complex will be formed and the value of pM will not change appreciably. Likewise, if M is removed from the solution by some reaction, some of the complex will dissociate to restore the value of pM. This recalls the behaviour of buffer solutions encountered with acids and bases (Section 2.20), and by analogy, the complex-ligand system may be termed a metal ion buffer. 

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