Solids geometrical isomerization

Reactions in which isomerization of coordination compounds occur in solutions are common, and some reactions of this type in solid complexes have been studied. Generally, there is a change in color of the complex as the crystal field environment of the metal ion changes. Accordingly, some of the color changes that occur when complexes are heated may indicate isomerization, but very few geometrical isomerization reactions in solid complexes have been studied in detail. One such reaction is... 

Owing to the relative rigidity of the carbon-nitrogen double bond, oximes can exist in two discrete geometrically isomeric forms the E or anti isomer anti-E-217) and the (Z) or syn isomer (syn-Z-217) (equation 70). In solid state, both oximes show high configurational stability and discrete existence. In solution, equilibrium between both isomers is rapidly established, favouring the thermodynamically most stable isomer " ... 

The chemical structure of a polymer determines whether it will be crystalline or amorphous in the solid state. Both tacticity (i.e., syndio-tactic or isotactic) and geometric isomerism (i.e., trans configuration) favor crystallinity. In general, tactic polymers with their more stereoregular chain structure are more likely to be crystalline than their atactic counterparts. For example, isotactic polypropylene is crystalline, whereas commercial-grade atactic polypropylene is amorphous. Also, cis-pol3nsoprene is amorphous, whereas the more easily packed rans-poly-isoprene is crystalline. In addition to symmetrical chain structures that allow close packing of polymer molecules into crystalline lamellae, specific interactions between chains that favor molecular orientation, favor crystallinity. For example, crystallinity in nylon is enhanced because of... 

As this position is fixed in space isomeric compounds are possible in which the position of two of the elements or groups linked to the doubly bound carbon atoms are reversed, as in (A) and (B) above. Two stereo-isomeric compounds should therefore be possible according to such a space arrangement and the two isomeric crotonic acids may thus be explained. This kind of stereo-isomerism is termed geometric isomerism. Without taking up in detail the proofs as to which of the two stereo-chemical formulas applies to each of the two crotonic acids, we may simply state the fact, that the properties of the solid or ordinary crotonic acid prove that it must be represented by formula (A), above, in which the methyl and carboxyl groups are 12... 

There are two types of geometrical isomerism associated with octahedral species. In EX2Y4, the X groups may be mutually cis or trans as shown for [SnF4Me2] (1.26 and 1.27). In the solid state structure of [NH4]2[SnF4Me2], the anion is present as the trans- sors tx. 

Jeevarajan AS, Kispert LD and Piekara-Sady L (1993b) An ENDOR study of carotenoid cation radicals on silica-alumina solid supports. Chem Phys Lett 209 269-274 Jeevarajan AS, Wei CC and Kispert LD (1994a) Geometrical isomerization of carotenoids in dichloromethane. J Chem Soc Perkin Trans 2 861-869... 

Cobaltate, tris(l, 2-ethanediamine)-racemization solid state, 466 Cobaltate, tris(oxalato)-racemization solid state, 467 Cobaltates sepulchrates, 22 Cobalt complexes geometric isomerism, 11 hexaammine... 

MisceUaneous.—The phosphorescence and fluorescence characteristics of solid camphorquinone have been reinterpreted. The remarkable stability to light and heat of (565), unlike the diketone or imino-ketone analogues, is attributed to the consumption of the considerable energy which is absorbed in geometrical isomerizations of the imine groups. ... 

Linkage isomerization to the N-bonded isothiocyanato-complex /w j-[Co(acac)2-(NCS)py] was observed to have a half-life of ca. 4 h in chloroform solution. In the solid state linkage isomerization at 341 K has a half-life of ca. 12 h, and this is followed by geometric isomerization with a half-life of ca. 2 days. The latter process is suspected to be catalysed by cobalt(ii). 

There are numerous complexes of Cr(III) their stability is related to the (t2g) configuration. The chelate effect confers extra stability on the oxalato-(ethanedioato-) complexes. When water in the hexaaqua ion is replaced by three oxalate ligands, a trisoxalato-complex is formed in solution and can be readily prepared as a solid potassium salt. This exhibits optical isomerism. Geometrical isomerism is encountered in the cis- and tram- bisoxalato-complexes. The former exhibits optical isomerism. Although no solid containing the monooxalato-cation has been obtained, there is spectral evidence of its formation in solution. 

Geometrical Isomerization. The l,l-Dimethyl-/i -allyl group in [(A -l,l-Me2C8H3)M(PF3)3] (M = Co or Rh) (51) isomerizes to the 1,2-dimethyl isomer on heating to 333 K. The mechanism probably involves a 1,4-hydrogen shift to give (53) with the intermediacy of (52). In the solid state [Co(diphos)2-CI]+ exists in two isomeric forms which equilibrate immediately upon separate... 

Thus, such substitution reactions proceed both in the solid state and in solution in the manner predicted by the trans-effect. Peyrone s rule (Chapter 1) sometimes appears to be violated since isolated products may have undergone geometric isomerization subsequent to the initial thermal transformation. 

The cis-trans isomerization of square-planar transition metal complexes, in particular those of platinum(II) and palladium(II), can be understood by comparison of the trans-effects of the individual ligands (Chapter 1). This approach predicts the favored direction of isomerization for a large group of complexes with ligands of different types. In this Section examples of geometric isomerization which are not adequately explained by the trans-effect are considered and it is shown that solid state effects can influence the outcome of such reactions. 

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