Alkyl complexes thermodynamic properties

Table XVI summarizes all the ligand sequences observed for co-balt(III) complexes as a whole using the different experimental techniques. The direction of a sequence, as written in Table XVI, is determined by reference to the corrinoids in ground-state and thermodynamic properties and by obvious parallels in the case of kinetic properties. Examination of the results shows that a similar order of ligands is observed for all groups of complexes and at all levels. As a broad generalization, we can say that the ligands which contain the more electronegative donor atoms such as Cl, 0, N (except in NO ), and C in CN occur at one end, those with S, Se, I, H, and C toward the other, with the simple alkyls such as CHs together with NO at the extreme end.

The influence of steric effects on the thermodynamic parameters of extraction has been discussed in numerous publications (see the review in Ref. 12 and references therein). For a small series of selected extractants, a decrease of log Kex as a function of the length of alkyl substituents has been discovered.6970 However, in other cases, this dependence passes through a maximum or is not regular at all.71 The point is that the increase in the volume and branching of the substituents leads to some changes of molecular properties decrease of solubility in the aqueous phase, increase of steric hindrance upon the complexation, decrease of the extractant s aggregation, etc.12 Some of these factors strengthen extraction, but others weaken it. 

The r/ -acyl derivatives of transition metals are usually compounds with covalent properties and moderate to good solubilities in common organic solvents, and sometimes with sufficiently good volatility that they can be isolated and/or purified by recrystallization or by sublimation under reduced pressure. Sometimes care has to be taken since the acyl or aroyl complexes may be or may become thermodynamically unstable with respect to the corresponding alkyl or aryl compounds. Heating to elevated temperatures can trigger the reverse of reaction (b) to become a kinetically important path, when the compounds have been prepared by route (a) or when the carbonyl insertion (b) has been carried out at low temperature. 

A perspective report emphasised the key role of the application of pressure in kinetic studies in bringing clarity to understanding the mechanism of substitution reactions of cobalamins.193 The effect of various alkyl substituents in the trans position on the kinetic, thermodynamic and ground-state properties has been studied. Cobalamins featuring in these studies were cyanocobalamin (vitamin Bi2), aquacobalamin and the complex formed when the cyano or water ligand is replaced... 

There seems little doubt that 7r-complexes do not play a role in the ratedetermining step of acid-catalysed dehydration of 1,2-diphenylethanols and related substrates. However, in simple alkyl structures in which carbonium ion stability is lower, rr-complexes may offer an alternate pathway of lower energy. Certainly rr-complexes appear to account for the greater preference for the formation of c/5-but-2-ene rather than the thermodynamically more stable trans isomer from the dehydration of secondary butanol over solid catalysts ". Of course metal cations with a greater variety of orbitals do not really provide appropriate models for assessing the co-ordination properties of a proton. Considering all the available kinetic evidence the most plausible mechanism for dehydration can be described as (197), with route A for tertiary and secondary alcohols and route B for primary and secondary alcohols, viz. 

A series of related reports have appeared on the equilibria between five- and six-coordinate species and possible adduct formation in Co(iii) corrinoids, on the thermodynamic and kinetic properties for what is termed the base-on/ base-off equilibration of alkyl cobalamins, " and on the kinetics and thermodynamics of parallel equilibria of alkyl-13-epicobalamins. In the first report, the pressure dependence of the UV/visible spectra of the five-coordinate (yellow)/six-coordinate (red) equilibrium for both methylcobalamin and vinylcobinamide was obtained. Water is the ligand that converts the five- to a six-coordinate species. The reaction volumes were obtained from the pressure dependence of the equilibrium constant. The values of AF of —12.5 1.2 and —12.5 l.Ocm moF for the methyl and vinyl complexes are close to the values 13cm moF advocated and accepted for the displacement or... 

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