Labile metals

The speciation scheme of Batley and Florence requires eight measurements on four samples. After removing insoluble particulates by filtration, the solution is analyzed for the concentration of anodic stripping voltammetry (ASV) labile metal and the total concentration of metal. A portion of the filtered solution is passed through an ion-exchange column, and the concentrations of ASV metal and total metal are determined. A second portion of the filtered solution is irradiated with UV light, and the concentrations of ASV metal... 

Step in Solution of ASV- Labile Metal of Total Metal... 

The mechanism of octahedral complex formation by labile metal ions. D. J. Hewkin and R. H. Price, Coord. Chem. Rev., 1970, 5, 45-73 (177). 

On this basis = 0.0170 sec , = 0.645 sec , and K = 0.739 mole.P at 25 °C. The corresponding activation parameters were determined also by Es-penson. By a method involving extrapolation of the first-order rate plots at various wavelengths to zero time, the absorption spectrum of the intermediate was revealed (Fig. 1). Furthermore, the value of K obtained from the kinetics was compatible with that derived from measurements on the acid dependence of the spectrum of the intermediate. Rate data for a number of binuclear intermediates are collected in Table 2. Espenson shows there to be a correlation between the rate of decomposition of the dimer and the substitution lability of the more labile metal ion component. The latter is assessed in terms of the rate of substitution of SCN in the hydration sphere of the more labile hydrated metal ion. 

The lack of structural detail for interactions involving labile metals, particularly the Group I ions, is problematic and here model compounds are proving useful. A range of binding modes for Na+ and K+ ions involving both T-02 and T-04 have been realized with Nl-alkylated thymi-... 

On the other hand, labile metal ions, for which 98% plus reactions should be easily obtained, often undergo side reactions and depolymerizations (the reverse reactions) are favored from entropy considerations. 

One method of solving the kinetics dilemma is well known in coordination chemistry that is, start with a labile metal ion and render it inert during the course of the synthetic reaction. We have accomplished this in the case of zirconium(IV) by starting with tetrakis(salicylaldehydo)zirconium(IV), which is quite labile, and polymerization with 1,2,4,5-tetraaminobenzene in a Schiff-base condensation reaction in situ (6). The polymeric product contains a "double-headed" quadridentate ligand, which is much more inert to substitution. However, 1,2,4,5-tetraaminobenzene has become very expensive. Therefore, the synthesis of a zirconium polymer with 3,3, 4,4 -tetraaminobiphenyl (commercially 3,3 -diami nobenzidine) with zirconium salicylaldehyde, Zr(sal)4 (7) has been undertaken as shown below ... 

It should be noted that the basic reactions used to prepare phthalocyanine derivatives today are fundamentally those developed by Linstead and coworkers in the 1930s [52-54]. Due to the large number of substituted phthalocyanines described in the literature, space limitations mean that a detailed review of synthetic aspects cannot be provided here. The following discussion is concerned with the synthesis of lanthanide phthalocyanines via (i) template tetramerization of phthalonitrile with lanthanide salts, (ii) direct metalation of the metal-free ligands by the salts or (iii) metal exchange of a labile metal ion or ions for a lanthanide. 

In addition to the discussed cyclotetramerizations, direct metallation of the metal-free ligands or metal exchange of a labile metal ion or ions for one held more robustly, the desired complexes may be prepared by the direct substitution, exchange or modification of substituents on preformed phthalocyanine derivatives. However, a review of works carried out on these types of transformations lies out of the scope of this chapter. 

It is interesting to compare this behaviour with the situation which occurs when a relatively inflexible ligand interacts with a very labile metal ion (that is, one in which the rate of solvent exchange is very high). For... 

Because of differing sensitivities and the natural levels of free metal or the anodic scanning voltammetric labile metal, cadmium, and copper in seawater are analysed using a 10 minute plating time, a -1.0V plating potential, and scanning in 6.67 mV/s increments. Zinc determinations can be made on a fresh aliquot of sample to eliminate any possible effects due to Cu-Zn inter-... 

Batley and Matousek [390,778] examined the electrodeposition of the irreversibly reduced metals cobalt, nickel, and chromium on graphite tubes for measurement by electrothermal atomisation. This method offered considerable potential for contamination-free preconcentration of heavy metals from seawater. Although only labile metal species will electrodeposit, it is likely that this fraction of the total metal could yet prove to be the most biologically important at the natural pH [779]. 

Shuman and Michael [10] applied a rotating disk electrode to the measurement of copper complex dissociation rate constants in marine coastal waters. An operational definition for labile and non-labile metal complexes was established on kinetic criteria. Samples collected off the mid-Atlantic coast of USA showed varying degrees of copper chelation. It is suggested that the technique should be useful for metal toxicity studies because of its ability to measure both equilibrium concentrations and kinetic availability of soluble metal. 

In spite of the above mentioned Co(EII) compounds, kinetically labile metal complexes may provide fast product/substrate exchange and some of these systems show real catalytic activity. In native dinuclear phosphatases Mg(II), Mn(II), Fe(II/III), or Zn(II) ions are present in the active centers. Although the aqua complexes of the weakest Lewis acids, Mg(H) and Mn(II), show measurable acceleration of e.g. the transesterification of 2-hydroxypropyl p-nitrophenyl phosphate HPNP, [Mn(II)] = 0.004 M, kobs/ uncat = 73 at pH 7 and 310 K, [38] or the hydrolysis of S -uridyluridine (UpU) [39], only a few structural [40] but no functional phosphatase-mimicking dinuclear complexes have been reported with these metal ions. 

Marcus theory, first developed for electron transfer reactions, then extended to atom transfer, is now being applied to catalytic systems. Successful applications to catalysis by labile metal ions include such reactions as decarboxylation of oxaloacetate, ketonization of enolpyru-vate, and pyruvate dimerization (444). 

The bioaffinity parameter a basically reflects the free metal ion concentration, whereas the limiting flux ratio b reflects the total labile metal species concentration. Due to the complexation, the ratio a/b thus changes by a factor (1 +sKcl) in spherical geometry, while the factor (1 I sKc ) (l I Kc ) is required for planar geometry [26]. 

Puy, J., Galceran, J., Salvador, J., Cecilia, J., Diaz-Cruz, J. M., Esteban, M. and Mas, F. (1994). Voltammetry of labile metal macromolecular systems for any ligand-to-metal ratio, including adsorption phenomena - the role of the stability constant, J. Electroanal. Chem., 374, 223-234. 

Filella, M., Buffle, J. and van Leeuwen, H. P. (1990). Effect of physico-chemical heterogeneity of natural complexants Part I. Voltammetry of labile metal-fulvic complexes, Anal. Chim. Acta, 232, 209-223. 

Less labile metal ions can be used to control the levels of biologically active ligands in the body. Thus Fe(III) in sodium nitroprusside delivers NO to tissues and is used for the treatment of hypertension and control of blood pressure. The possibility arises of utilizing Ru(III) to scavenge NO in the treatment of septic shock. 

There is a major problem in the interpretation of the results when labile metal ion complexes are used as promoters. It is difficult to characterize the reactant species and therefore to distinguish between the following modes of attack by OH (and HjO) ... 

If a hydroxyester function is incorporated into a potential chelating system then the ability even of a labile metal ion to catalyze the hydrolysis can be assessed. The 8-hydroxyquinoline 17 and 2,2 -phenanthroline 18 frameworks have proved popular for this purpose. 

Data for water exchange with Fe(H20)j+ are shown in Table 4.1. " The value for AV indicates an interchange dissociative mechanism, which is also reflected in data for the reaction of Fe + with tpy AV = -1-3.5 cm mol , ) and other ligands. " One of the earliest studies of substitution in a labile metal ion was of the reaetion of Fe + with bpy and phen in acid solution (Sec. 2.1.4). 

The Ag+ ion is labile. Even with cryptands, which react sluggishly with most labile metal ions, Ag reacts with a rate constant around 10 M s (in dmso). The higher stability of Ag(I) complexes compared with those of the main groups I and II resides in much reduced dissociation rate constants. Dissociation tends to control the stability of most metal cryp-tand complexes. Silver(I) is a useful electron mediator for redox reactions since Ag(I) and Ag(II) are relatively rapid reducers and oxidizers, respectively. Silver(I) thus promotes oxidation by sluggish, if strong, oxidants and catalyses a number of oxidations by S20 in which the rate-determining step is... 

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