Anionic ligands effects

An anionic ligand effect in iron porphyrin complex-catalyzed competitive epoxidations of cis- and frawi-stilbenes by BU4NHSO5 has been studied in detail . Oxidation of the thiophosphite to the thiophosphate in solid phase oligonucleotide synthesis using BU4NHSO5 was also reported. ... 

Anionic ligand effect on the nature of epox-idizing intermediates in iron porphyrin complex-catalyzed epoxidation reactions. Inorg. Chem. 41, 3647-3652. 

The above substitution effects appear to be independent of the nature of the ligand (16) since the benzyl compounds behave similarly, Table XI. It would appear from these observations that the introduction of anionic ligand would be sufficient to increase activity of transition metal alkyl compounds for polymerization. This, however, is probably an oversimplifica-... 

Additional bonds are thus donor bonds, and to accept electron pairs from neutral and anionic ligands, zinc uses the two remaining 4p orbitals to form sp2 and sp3 hybrids. In the absence of steric effects, discrete, homoleptic, anionic tri- and tetraorganozinc compounds (zincates) have almost always ideal trigonal-planar and tetrahedral geometries, respectively. 

The stability of crown-ether complexes depends on several factors these include cavity size of the ligand, cation diameter, spatial distribution of ring binding sites, the character of the hetero-atoms, the presence of additional binding sites and the type of solvent used. In apolar solutions it also depends on the nature of the anion. The effects of these parameters will be illustrated in the next sections. 

In Sect. 5.2, the strong 7r-donor ability of methoxide and fluoride has been elaborated. These two ligands effect a push-pull effect on the nitrosonium ion bound in Os(OEP)NO(OMe) [31c] and Os(OEP)NO(F) [3Id], as indicated by the low NO-stretching frequencies of the NO ion as compared with the dinitrosyl Os(OEP)(NO)2 [31e] and the perchlorato complex, 0s(0EP)N0(0C103) ([3If], Table 11). Thus, the a/7r-donor balance for the coordinated anions decreases in the series OMe > F > NO > OClOf. [31c] and [3Id] can be vaporized at 200°C/10-6 Torr in a mass spectrometer, while the dinitrosyl [31e] decomposes above 100 °C. This demonstrates the push-pull effect also in a chemical sense. 

Amazingly, there is no systematic cis effect of the ligand L trans to carbon monoxide in the carbonylosmium(II) porphyrins Os(OEP)CO(L) [30a-30g], (Table 10). In the nitrosyl derivatives, Os(OEP)NO(L), the anionic ligand Le exerts only a small effect (Table 11) the wavelengths of the a-bands increase in the series [31c] < [3Id] <... 

The effect of anion binding on k is not nearly as great. The data in Fig. 6 show that for both metals, there is less than a 50% decrease in the thermal ET rate constant between hybrids containing neutral and anionic ligands. 

Metal-substituted hemoglobin hybrids, [MP, Fe " (H20)P] are particularly attractive for the study of long-range electron transfer within protein complexes. Both photoinitiated and thermally activated electron transfer can be studied by flash excitation of Zn- or Mg-substituted complexes. Direct spectroscopic observation of the charge-separated intermediate, [(MP), Fe " P], unambiguously demonstrates photoinitiated ET, and the time course of this ET process indicates the presence of thermal ET. Replacement of the coordinated H2O in the protein containing the ferric heme with anionic ligands (CN , F , Nj ) dramatically lowers the photoinitiated rate constant, k(, but has a relatively minor effect on the thermal rate, kg. 

The main usefulness of Eh-pH diagrams consists in the immediacy of qualitative information about the effects of redox and acid-base properties of the system on elemental solubility. Concerning, for instance, cerium, figure 8.20 immediately shows that, within the stability field of water, delimited upward by oxidation boundary curve o and downward by reduction boundary curve r, the element (in the absence of other anionic ligands besides OH groups) is present in solution mainly as trivalent cerium Ce and as soluble tetravalent hydroxide Ce(OH)2. It is also evident that, with increasing pH, cerium precipitates as trivalent hydroxide Ce(OH)3. 

Of the various factors that cause redox disequilibria, the most effective are biologic activity (photosynthesis) and the metastable persistence of covalent complexes of light elements (C, H, O, N, S), whose bonds are particularly stable and difficult to break (Wolery, 1983). For the sake of completeness, we can also note that the apparent redox disequilibrium is sometimes actually attributable to analytical error or uncertainty (i.e., difficult determination of partial molalities of species, often extremely diluted) or even to error in speciation calculations (when using, for instance, the redox couple Fe /Fe, one must account for the fact that both Fe and Fe are partly bonded to anionic ligands so that their free ion partial molalities do not coincide with the bulk molality of the species). 

Alkyl halides, hydroperoxide synthesis, 327-8 Alkyl hydroperoxides anion ligands, 114-19 covalent radii, 114, 118-19 dihedral angles, 119 geometric parameters, 115-8 tetrahedral distortion, 119 artemisinin formation, 133-4 chlorotriorganosilane reactions, 779-83 crystal structure, 105-14 anomeric effect, 110-11 geometric parameters, 106-9 hydrogen bonding, 103-5, 111-14 tetrahedral distortion, 110 determination, 674... 

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