Stereochemical changes

The kinetic and mechanistic aspects of this general area tend to be strongly dependent on the particular system. This makes general treatments and explanations impossible, at least at the current stage of understanding. Various aspects of this area have been summarized in some general reviews. 

Ligands bonded to a metal can undergo a number of structural changes that do not involve complete breaking of the metal-ligand bond(s). Such processes are the subject of the following sections. 

Many chelate ligands have conformers that can interconvert For example, the conformers of ethylenediamine interchange by rotation about the carbon-carbon bond, as shown in the following structures  

Octahedral to square planar transformations have also been observed. These must involve a substitution as well because of the change in coordination number. Again Ni(II) complexes provide the most examples, with systems having a four-coordinate ligand in the square plane and two solvent molecules occupying the other octahedral positions, as shown in the following reaction  

These interconversions are quite rapid and have been studied by laser T-jump and ultrasonic relaxation. Kinetic studies suggest that several nickel(II)-salicylaldimine complexes favor ligand substitution through the square-planar isomer. 

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Stereochemical changes during octahedral substitution reactions. R. D. Archer, Coord. Chem. Rev., 1969, 4, 243-272 (148). 

There has been only one study concerned with stereochemical changes at an asymmetric metal. It dealt with the photochemical decarbonylation 208) ... 

By 1938, one fact was clearly established. Sweet compounds, unlike salty and sour compounds, are found in all classes of chemical compounds, including such inorganic salts as beryllium ( glucinium ) and lead salts. They are also found among compounds of all molecular shapes and sizes, and stereochemical changes may result in a very dramatic change in the taste, as seen in the gustatory differences between enantiomorphs. 

Fig. 6. Stereochemical changes accompanying the dissociative reaction MA BX MA4B MA4BY.

The main oxidation product from dibenzyl ether is benzaldehyde (up to 80% yield) with smaller amounts of benzyl alcohol and benzoic acid. The rates of oxidation are only slightly affected by major stereochemical changes, and it is considered that an outer-sphere oxidation of the ether is followed by radical breakdown, viz. 

The stereoselectivity of some Diels-Alder reactions was also strongly affected in water.26 At low concentrations, in which both components were completely dissolved, the reaction of cyclopentadiene with butenone gave a 21.4 1 ratio of endo/exo products when they were stirred at 0.15 M concentration in water, compared to only a 3.85 1 ratio in excess cyclopentadiene and an 8.5 1 ratio with ethanol as the solvent. Aqueous detergent solution had no effect on the product ratio. The stereochemical changes were explained by the need to minimize the transition-state surface area in water solution, thus favoring the more compact endo stereochemistry. The results are also consistent with the effect of polar media on the ratio.27... 

The reaction between a cation and carbanion can result in more a less tight association. Therefore stereochemical changes in the carbon species will take place. 

Table I. Rate Constants for Some Rapid Stereochemical Changes in Metal Complexes.

The photosensitized decomposition reduces the hydride shift, as may be expected for a triplet species. However, the stereochemical change in the cycloaddition is relatively small. Similieir results were obtained by Baer and Gutsche for the sensitized photolysis of o-n-butyl-phenyl-diazomethane 63... 

Ultrasonic absorption played a major historical role in an understanding of the mechanisms of metal complex formation. It has also been used to study stereochemical change in metal... 

The ability of certain metal complexes to exist in stereoisomeric forms, and particularly to interconvert, adds another dimension to the study of the mechanisms of their reactions. There are two aspects from which the phenomenon of stereochemical change may be regarded. 

The aquated Co(III) ion is a powerful oxidant. The value of E = 1.88 V (p = 0) is independent of Co(III) concentration over a wide range suggesting little dimer formation. It is stable for some hours in solution especially in the presence of Co(II) ions. This permits examination of its reactions. The CoOH " species is believed to be much more reactive than COjq Ref. 208. Both outer sphere and substitution-controlled inner sphere mechanisms are displayed. As water in the Co(H20) ion is replaced by NHj the lability of the coordinated water is reduced. The cobalt(III) complexes which have been so well characterized by Werner are thus the most widely chosen substrates for investigating substitution behavior. This includes proton exchange in coordinated ammines, and all types of substitution reactions (Chap. 4) as well as stereochemical change (Table 7.8). The CoNjX" entity has featured widely in substitution investigations. There are extensive data for anation reactions of... 

The incursion of inner-sphere pathways (for 1st two entries), solvation differences OJ, Oj) and stereochemical changes are the bases for the differences in theoretical and experimental rate constants. 

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