Ligands lower acids

As with EDTA, which we encountered in Chapter 9, o-phenanthroline is a ligand possessing acid-base properties. The formation of the Fe(o-phen)3 + complex, therefore, is less favorable at lower pH levels, where o-phenanthroline is protonated. The result is a decrease in absorbance. When the pH is greater than 9, competition for Fe + between OH and o-phenanthroline also leads to a decrease in absorbance. In addition, if the pH is sufficiently basic there is a risk that the iron will precipitate as Fe(OH)2. 

Calculation based on humic cation-exchange capacity data obtained by Stuermer and Harvey (1974, 1977) fulvic acid (FA) concentration in Sargasso Sea = 0.134 mg 1" (49.98% C) equivalent weight = 473 g the FA carbon represents 5.1% of the DOC it is assumed that recovery of FA is 100%, and that FA is the only metal-binding ligand. Lower recoveries and/or the presence of other non-humic ligands would tend to increase values shown. 

The observed trends in pK with L variation in Table 9.1 are consistent with the trend that lower values of 1/2, i e., greater electron density at M, correspond to lower acidity. Thus the auxiliary ligand set plays a major role in determining acidities, as already noted above. For the [CpRuHjCPP)]system for both classical or nonclassical complexes, acidity increases with decreasing basicity of the diphosphines dmpe < dppp < 2PPh3 < dppe < dtfpe. Substitution of PPhj by CO decreases pX of related classical hydrides by 5-8 units [HCo(CO)4 and... 

Suzuki and Suga reported the use of clays as solid acids to support and activate metallocene catalysts for olefin polymerization. They were able to use much less alkylaluminmn cocatalyst relative to solution polymerization conditions. The clays were slurried with AlMeg in toluene, then treated with a solution containing zirconocene dichloride, II, and AIMeg. The metallocenium cation was presumed formed via abstraction of chloride and/or methyl ligands by acidic sites on the surface of the clay, and the low basicity of the clay smface was proposed to stabilize the coordinatively unsaturated cation. Propylene was copolymerized with 250 psi ethylene at 70°C. For acid-treated KIO montmorillonite, an activity of 3300 X 10 kg polymer/(g Zr h) was obtained. Catalysts based on vermiculite, kaolin, and synthetic hectorite all showed lower but still appreciable activities. In this brief report, the Al/Zr ratio was not specified, and the clay dispersion was not reported. 

The equilibrium constant (K) for the addition of benzoic acid to Vaska s type compounds depends on the electron density at iridium. Thus electron attracting ligands lower the basicity and electron donating ones raise it (Order of K where X = I > Br > Cl L = Me, > Me,PPh > MePPh, > PPh,). 

Although phosphinite- and NHC-based iridium catalysts show very similar enantiose-lectivities in the hydrogenation of various olefins, replacement of the phosphinite group by an A-heterocyclic carbene (NHC) unit results in particularly effective catalysts which are much better suited for the hydrogenation of acid-sensitive substrates beeause of the lower acidity of iridium hydride intermediates produced. The new NHC-pyridine ligands are also likely to prove useful for other applications in asymmetric catalysis." ... 

Metal-complexed ligand radicals are formed in the oxidation of iron(II) diimine complexes with cerium(IV) in sulfate media. The reaction with /r/5-[2-pyridinal-a-methyl(methylimine)]iron(n) ([Fe(pmm)3], studied at lower acid concentrations (< 4 M H2SO4) is a complex process involving formation of Fe(II)- and Fe(III)-ligand oxidized species, a total of 10-11 mol of oxidant being required for reaction with each iron(II) complex. The [Fe(pmm)3] species undergoes intramolecular reductions to Fe(II) and a radical which may be further... 

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