Isomerization tetrahedral forms

Selenosulfate is an analogue of thiosulfate wherein one of the S atoms is replaced by a Se atom. Thiosulfate and selenosulfate anions are known to have tetrahedral structure as constituting the S and Se analogues, respectively, of the sulfate anion. The isomeric thioselenate anion SSeO " is not produced by the reaction of sulfur with selenite nor is the selenoselenate ion 86203 formed from selenium and selenite. Actually, SSeOj may be produced as a metal salt by boiling an aqueous solution of selenite with sulfur, but in aqueous solution thioselenates are not stable and isomerize to selenosulfates. 

These results are consistent with active sites consisting of highly distorted octahedral WOx clusters on Zr02. Acid sites formed by these octahedral WO surface species are more effective isomerization sites than previously reported tetrahedral WOx species on AI2O3 [17], possibly because of the ability of WOx clusters to form metastable proton-containing complexes during catalytic isomerization reactions. 

Reaction of 2a with Cl2BCH2SiMe3 yields the compound 3a [18]. When the symmetrically substituted 3a is reacted with lithium in diethyl ether, the unsymmetri-cally substituted folded 4b [18] is obtained (Scheme 3.2-3). Formation of 4b can be explained by a rapid isomerization of the first formed 4a via the distorted tetrahedral transition state 5a. 

To illustrate what is meant by optical isomerism, consider lactic acid (2-hydroxypropanoic acid). It contains a tetrahedral carbon atom (shown in black) with four different groups attached. As a result, lactic acid exists as two distinct isomeric forms. 

This is the first coordination number io be discussed that has an important place in coordination chemistry. It is also the first for which isomerism is to be expected. The structures formed with coordination number 4 can be convenienlly divided into tetrahedral and square planar forms allhough intermediate and distorted structures are common. 

Tetrahedral complexes du not exhibit geometrical isomerism. However, they are potentially chiral just as is tetrahedral carbon. The simple form of optical isomerism exhibited by most organic enantiomers, namely four different substituents, is rarely observed because substituents in tetrahedral complexes are usually too labile10 for the complex to be resolved, i.e., they racemize rapidly. However, an interesting series of cyclopentadienyliron phosphine carbonyl compounds (see Chapter 15 for further... 

Nickel(II) complexes display a variety of equilibria which involve spin state changes. Planar four-coordinate complexes are invariably diamagnetic. These can undergo an intramolecular isomerization to paramagnetic tetrahedral four-coordinate species. Alternatively, the planar complexes can coordinate additional ligands to form five- and six-coordinate paramagnetic complexes. The additional ligand molecules can be Lewis bases in solution, or solvent molecules, or, in par-... 

Cis-trans isomerization can take place either photochemically or in the dark, but the reaction pathways are quite different. In the light-induced process the reaction goes through a tetrahedral intermediate formed from the triplet excited state, whereas the dark reaction involves a dissociation of the complex, followed by recombination. In the latter case the presence of free glycine is demonstrated by the use of radioactive tracers no free glycine appears in the photochemical reaction. 

On the basis of his discoveries, Pasteur postulated that optical isomerism had to be related to the molecular dissymmetry of substances such that nonidentical mirror-image forms could exist. However, it remained for van t Hoff and Le Bel to provide, almost simultaneously, a satisfactory explanation at the molecular level. In his first published work on tetrahedral carbon van t Hoff said .. . it appears more and more that the present con-... 

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