Complexes with solvents

Finally it should be said that o-/p-ratios can be considerably influenced by the solvent in which the reaction is carried out. This can arise from changes in the relative stabilisation by solvent molecules of the transition states for o- and p-attack, but it may also involve the actual attacking electrophile being different in two different solvents the species actually added complexing with solvent molecules to form the electrophile proper—a different one in each case. This almost certainly occurs in halogenation without Lewis acid catalysts, e.g. in the chlorination of toluene at 25°, where f0./fp. ratios between 0-75 and 0-34 have been observed depending on the solvent. 

The equilibrium constant of hexaphenylethane dissociation, in striking contrast to the rate constant for dissociation, varies considerably with solvent. The radical with its unpaired electron and nearly planar structure probably complexes with solvents to a considerable extent while the ethane does not. Since the transition state is like the ethane and its solvation is hindered, the dissociation rate constants change very little with solvent.12 13 From an empirical relationship that happens to exist in this case between the rate and equilibrium constants in a series of solvents, it has been calculated that the transition state resembles the ethane at least four times as much as it resembles the radical. These are the proportions that must be used if the free energy of the transition state in a given solvent is to be expressed as a linear combination of the free energies of the ethane and radical states.14... 

The simplest reactions to study, those of coordination complexes with solvent, are used to classify metal ions as labile or inert. Factors affecting metal ion lability include size, charge, electron configuration, and coordination number. Solvents can by classified as to their size, polarity, and the nature of the donor atom. Using the water exchange reaction for the aqua ion [M(H20) ]m+, metal ions are divided by Cotton, Wilkinson, and Gaus7 into four classes ... 

Table 15. Cation-solvent complexes with solvent molecules other than water... 

The hydrocarboxylation can take place by insertion of the alkene into a metal-hydride bond followed by CO insertion and finally reaction of the acyl complex with solvent as illustrated in equation (36). Alternatively, a transition metal-carboxylate complex can be generated initially. Insertion of the alkene into the metal-carbon bond of this carboxylate complex followed by cleavage of the metal-carbon bond by solvent completes the addition, as shown in equation (37). Both sequences provide the same product. 

It should also be taken into account that a lot of synthetic processes in coordination chemistry are carried out in solvating media. So, the reaction schemes (1.1)—(1.8) should include solvating reagents and consider the possibility of formation of adducts of final complexes with solvent molecules [Scheme (1.9)] ... 

Lansky and Weiss [122] proposed a novel model by considering the reaction of the undissolved solute with the free solvent yielding the dissolved drug complexed with solvent ... 

The crown ether has two effects on KMn04 first, it makes KMn04 much more soluble in benzene second, it holds the potassium ion tightly, making the permanganate more available for reaction. Chemists call this a "naked anion" because it is not complexed with solvent molecules. 

Changes in the observed CD of a complex with solvent have been observed where solvation of the chiral ligand is unimportant, but where NMR and, in some cases, Raman spectroscopic evidence shows that the non-chiral ligands in the complex are being solvated to varying degrees. Two general effects of this have been observed ... 

The selectivity of chlorination is influenced by solvents. For example, the chlorination of 2,3-dimethylbutane shows increased preference for the tertiary position in benzene. " The complexation with solvent attenuates the reactivity of chlorine atoms. Halogenated solvents also give evidence of complex formation. Brominated solvents lead to greater selectivity. " ... 

Grignard reagents are known to exist as an equilibrium mixture of the organomagnesium halide (RMgX) and the diorganomagnesium (R Mg) forms and often exist is dimers or higher oligimers (all of which arc further complexed with solvent), l- or... 

The effects of solvent on charge-transfer maxima have been reviewed by Murrell and Reichardt , the latter in connection with empirical measures of solvent polarity. The rules of thumb are as follows For complexes of the non-ionic type, the charge-transfer maximum should be red shifted in polar solvents, blue shifted in non-polar solvents. For ionic complexes, the opposite behavior is anticipated. However, a recent attempt to correlate absorption maxima of complexes with solvent polarity has failed, and it appears there may be exceptions to the general rules. 

In the formation of adducts, or even molecular complexes, with solvent or acid molecules, the peak at the highest m/z ratio may not correspond to the molecular ion species ([M -I- H] in positive mode and [M — H] in negative mode). Instead, [2M + H]+ or [2M - H] may be formed. This issue may be corrected by an increase in cone voltage, which diminishes formation of both adducts and complexes. ... 

Influence of single and mixed solvents on the rate of electrode reactions, electrochemical properties of organic redox couples and the complex formation of metal ions by macrocyclic ligands were studied. In addition, variation of the stability constants of complexes with solvent composition and modeling of probable structures of coordination compounds by computer simulation were investigated. 

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