Labile systems

A comparison with the glycine work suggests that it is the nitrogen coordination rather than the carboxylate coordination which accelerates the water substitution of the metal complex. It appears that three nitrogens coordinated to Ni(II) gives a labile system which is diminished by having four or five nitrogens coordinated. 

The substitution reaction in which a molecule of solvent replaces one of the ligands represents one of the most commonly and conveniently studied processes in coordination chemistry. In labile systems, analysis of the relaxation kinetics in the complex formation studies will, of course, give the rate constants for the solvolysis as well as those for the complex formation. In inert systems... 

This area has attracted a great deal of interest and many reviews have been published.17 373-379 The most recent, and probably the largest,17 deals primarily with labile systems and should provide an excellent source of references for that area. The literature up to and including 1975 is covered and so this review is too early to include the important contributions of Merbach and coworkers which are discussed in Section 7.1.2. 

Occasionally the amount of solvento complex present is too great for the stationary state approximation to be valid and the analysis of the rate constants will be more complicated.441 A rapidly established equilibrium between the substrate and the solvento complex is not uncommon in labile systems with low concentrations of the nucleophile and leads to a rate law of the type... 

Chelate complexes with two ethylenediamine rings in a cis configuration lack a plane of symmetry and thus have the potential to be separated into enantiomeric (A, A) (12) forms. Inert cis-bis(en) complexes of Co111,247 Crmn or Rh111248 can be resolved by the method of racemic modification 249 or using chromatographic techniques,35 but labile systems, such as Ni(en)2+, which occasionally crystallize in one chiral form,220 rapidly racemize in solution. 

In more recent years attention has turned from studying the equilibria of binary metal-amino acid complexes to that of ternary complex formation in aqueous media, particularly to complexes of the type (aa)—M11—L, where L is some other ligand or a different amino acid to (aa), and M11 is a kinetically labile metal ion. Ternary complexes involving kinetically inert metal ions, e.g. Co,w and Pt", are more well known since they can be separated from mixtures and studied in isolation. Such is not the case with the labile systems. Because of the facile nature of their equilibria they must be studied in situ (claims regarding the separation of labile species by chromatographic procedures... 

The ability of metal ions to catalyze the hydrolysis of peptide bonds has been known for 50 years, while the catalytic effect on the hydrolysis of amino acid esters was highlighted in the 1950s. As Hay and Morris point out in their review,76 the major problem with the kinetically labile systems is determining the nature of the reactive complex in solution. Such problems generally do not arise in the more inert systems and consequently reactions involving Co111 have been the more popular for study. 

Some evidence234 for Zn—OH attack in anhydride hydrolysis has been obtained using the complex (65) (Section 61.4.11) but the evidence is not definitive, and other mechanisms could apply. Large rate enhancements occur in the Zn11- and Cu -promoted hydrolysis of the lactam (66) (Section 61.4.10). Rates increase commensurate with the ionization of a metal-bound water molecular and sigmoidal pH-rate profiles are observed. Rate enhancements of 9 x 105 and 1 x 103 occur with (66)—Cu—OH and (66)—Zn—OH compared with the free ligand. A number of other reactions which are believed to proceed via M—OH species, in kinetically labile systems, are considered in Section 61.4.3. 

From the preceding discussions, it is evident that, in all systems studied, and, in particular, in higher plants, attempts to synthesize cellulose in vitro have met with only limited success this therefore leads to the conclusion that, for poorly understood reasons, the cellulose synthetase complex is a highly labile system. As a conclusion to this article, it may prove useful for future research to discuss possible reasons for this apparent lability. 

Here, the i conformers each of the oxidized and the reduced forms are related by the 2 (i-1) equilibrium constants Kt and K, respectively, and by the i redox potentials Ef. A quantitative analysis of the redox potential in the square scheme of Eq. 11.4 requires a knowledge of all equilibrium constants. For labile systems this is only possible when theoretical methods can be applied. Molecular mechanics has been used in this context to calculate the conformational equilibria and then to predict the electrochemical behavior of [Co(sep)]3+/2+11511, [Co(dien)2]3+/2+11511 and [Co (S)-pn 3]3+/2+13451 (sep is defined in Table 11.1, dien in Table 8.1, pn in Table 8.2). 

Thus, whilst there may be some use for electronic spectra in confirming structure of Mn11 compounds,40 it is not likely to be great. Many more spectra of known species will need to be accumulated for assessment, and a major difficulty with these labile systems will be to ensure that we know structure in solution first before we start trying to assign spectra. 

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