Semi-labile complexes, lability

For an excess of ligand over metal, [L] will be equal to the total concentration of ligand in the bulk solution ([L]t), whereas 8 will depend on the hydrodynamic conditions (Section 3.2). It follows that the lability criterion (A8 or Ar0) is -C 1 for inert complexes and close to 1 for semi-labile complexes. In the case of semi-labile complexes, the overall flux of M to the microorganism is larger than that given by the diffusion of inert complexes (cf. equation (55)), but smaller than that due to the diffusion of fully labile complexes (cf. equation (57)), due to partial dissociation of the complexes. 

At the outset of this discussion, it is perhaps worth noting that there is a considerable difference between the lability of groups on silicon toward redistribution catalyzed by acids or bases (see Section I) and transition metal complexes. Thus, oxy substituents are classified as "labile and hydrogen as semi-labile toward acid- or base-catalyzed redistribution, whereas the reverse is usually the case with transition metal-catalyzed redistributions. Thus, these two sets of catalysts are complementary in their capacity to labilize the various substituents on silicon. 

It is apparent from the examples in this section that the lability of the groups on silicon is greatly dependent on the catalyst. With conventional acid or base catalysts, SiO— is classed as a labile ligand, SiH as semi-labile, and Si—R (R = alkyl or aryl) as nonlabile (/). However, with low-valent complexes of the group VIII metals, SiH is the most labile ligand, and SiO— and Si—R appear to have comparable reactivities. Hence, these two sets of catalysts types are complementary in their capacity to redistribute ligands on silicon. 

In the final section of this chapter, we shall attempt to give a brief rationalization of the regularities and peculiarities of the reactions of non-labile complexes which have been discussed in the previous sections. The theoretical framework in which the discussion will be conducted is that of molecular orbital theory (mot). The MOT is to be preferred to alternative approaches for it allows consideration of all of the semi-quantitative results of crystal field theory without sacrifice of interest in the bonding system in the complex. In this enterprise we note the apt remark d Kinetics is like medicine or linguistics, it is interesting, it js useful, but it is too early to expect to understand much of it . The electronic theory of reactivity remains in a fairly primitive state. However, theoretical considerations may not safely be ignored. They have proved a valuable stimulus to incisive experiment. 

Although autoxidation of Ru(sar) + has similar characteristics in acidic solution, in base hydrogen atom transfer from Ru(sar) + to O2 leads to a deprotonated Ru(III) species which is oxidized to relatively stable Ru" (sar-2 H+) + Ref. 175. The strong deviation from linearity for semi-log plots, with a large excess of O2, is removed when Fe(II) is added. This suppresses the step and doubles the rate. Compare Sec. 2.2.1(b). The value of k can be assessed as 1.3 x 10 M s Ref. 176. The behavior of pentacyanoruthenium complexes has been compared with the iron analogs. Substitution in M"(CN)5L" with both M = Fe and Ru is dissociative, with decreased lability for tbe Ru(II) species. Table 8.10. 

The primary products from autoxidation are hydroperoxides, which are often simply referred to as peroxides. Peroxides are odorless and colorless, but are labile species that can undergo both enzymatic and nonenzymatic degradation to produce a complex array of secondary products such as aliphatic aldehydes, alcohols, ketones, and hydrocarbons. Many of these secondary oxidation products are odiferous and impart detrimental sensory attributes to the food product in question. Being able to monitor and semi-quantitate the development of peroxides by objective means (e.g., PV determination) over time is important for food scientists who want to characterize the quality of an oil or a lipid-containing food product, even though the peroxides themselves are not directly related to the actual sensory quality of the product tested. 

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