Metal-ligand stoichiometry

It is possible to obtain fairly reliable values for many complexes of 1 1 (metal ligand) stoichiometry by using an extended empirical equation in which the dielectric constant is dependent on the cationic charge (21). For the complexation reaction ... 

Otsuka et al. (107) describe [Ni(CNBu )2], as a reddish brown microcrystalline substance, which is extremely air-sensitive. It can be recrystallized from ether at —78°C, and is soluble in benzene in the latter solution the infrared spectrum (2020s, br, 1603m, 1210m) and proton NMR (three peaks of equal intensity at t8.17, 8.81, and 8.94) were obtained. Neither analytical data nor molecular weight is available on this complex. The metal-ligand stoichiometry is presumably established by virtue of the molar ratio of reactants and by the stoichiometries of various reaction products. 

The data of Loukidou et al. (2004) for the equilibrium biosorption of chromium (VI) by Aeromonas caviae particles were well described by the Langmuir and Freundlich isotherms. Sorption rates estimated from pseudo second-order kinetics were in satisfactory agreement with experimental data. The results of XAFS study on the sorption of Cd by B. subtilis were generally in accord with existing surface complexation models (Boyanov et al. 2003). Intrinsic metal sorption constants were obtained by correcting the apparent sorption constants by the Boltzmann factor. A 1 2 metal-ligand stoichiometry provides the best fit to the experimental data with log K values of 6.0 0.2 for Sr(II) and 6.2 0.2 for Ba(II). 

Copper(II)-aminoglycoside complexes are often isolated as monomeric species over a wide pH range. The coordination complex is usually formed in 1 1 metal ligand stoichiometry, and 1 2 metal ligand stoichiometric complexes are rare. ... 

Aromatic polyalcohols act as strong coordinating agents and Table 17 summarizes reported formation constants. The complexes are quite stable this behaviour has been used for the qualitative and quantitative determination of vanadium (e.g. refs. 494 and 495). At pH 3-4, an initial vanadyl catechol complex slowly converts to a tris complex.496 In fact complexes with 1 3 metal-ligand stoichiometry have been isolated (see below), but since in the equilibrium (30) no protons are consumed or liberated, [VO(cat)2]2- and [V(cat)3]2 are not distinguishable by potentiometric studies. 

In addition to the presence of one of five MPT derivatives, cofactor variants differ in metal-ligand stoichiometry, coordination number, and coordination geometry. The cofactor variations include (1) the numbers of bound MPT derivatives, either one or two (2) the presence or absence of oxido and/or sulfido coordination (3) the presence or absence of bound protein ligands (either serine, cysteine, or selenocysteine) and (4) the presence or absence of coordinated water, hydroxido, or hydrosulfido ligands. These distinctions are discussed in detail in Section IV. 

The interaction of the linearly linked tris-cyclam derivative 13 with Ni(II), Cu(II), Zn(II), Cd(II), and Pd(II) has been investigated [32], As for the above tri-branched systems, all five metals yield solid complexes in which the metal ligand stoichiometry is 3 1 with, for Cu(II), a spectrophotometric titration also confirming the formation of a complex of this stoichiometry in acetonitrile. Cyclic voltammograms of both the Ni(II) (low-spin) and Cu(II) complexes both yield evidence for the presence of M(II)/M(III) as well as M(I)/M(II) couples in acetonitrile. 

Twenty years ago the main applications of electrochemistry were trace-metal analysis (polarography and anodic stripping voltammetry) and selective-ion assay (pH, pNa, pK via potentiometry). A secondary focus was the use of voltammetry to characterize transition-metal coordination complexes (metal-ligand stoichiometry, stability constants, and oxidation-reduction thermodynamics). With the commercial development of (1) low-cost, reliable poten-tiostats (2) pure, inert glassy-carbon electrodes and (3) ultrapure, dry aptotic solvents, molecular characterization via electrochemical methodologies has become accessible to nonspecialists (analogous to carbon-13 NMR and GC/MS). 

Numerous crystal structural analyses of complexes with the 1 1 metal ligand stoichiometry have been reported. These are discussed in the appropriate sections. A wide variety of geometries are adopted, but it is clear that the essentially planar terdentate ligand imposes a steric requirement such that uncommon geometries are favored. In particular, the distorted trigonal-bipyramidal (Fig. 9a), square-pyramidal (Fig. 9b), and pentagonal-bipyramidal geometries (Fig. 9c) are commonly encountered. 

In a related investigation to that just discussed, the tetramethyl-substituted quaterpyridine 37, which was designed to promote co-ordination in a twisted configuration, yielded complexes with 1 1 and 2 1 (metal ligand) stoichiometries with manganese(II) and cobalt(II). With copper(I), a dimeric helical complex of... 

A binuclear palladium(II) complex with a metal ligand stoichiometry of 2 1 was also obtained. ... 

The cyclohexane-containing system (54) forms a sandwich complex with K+ but of 2 2/ metal ligand stoichiometry, i.e. two molecules of (54) coordinate simultaneously the two cations... 

Scheme 3.9 Solvent effects on the metal/ligand stoichiometry and C-H activation.

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