Isomerizations coordination isomerism

Hydrate Isomerism, Coordination Isomerism, and Polymerization Isomerism... 

Ionization isomerism Hydration isomerism Coordination isomerism Linkage isomerism Polymerization isomerism... 

Figure A3.4.6. Potential energy along the reaction coordinate r for an unimolecular isomerization (left) and a...

Figure A3.8.1 A schematic diagram of the PMF along the reaction coordinate for an isomerizing solute in the gas phase (frill curve) and in solution (broken curve). Note the modification of the barrier height, the well positions, and the reaction free energy due to the interaction with the solvent.

Abrash S, Repinec S and Hochstrasser R M 1990 The viscosity dependence and reaction coordinate for isomerization of c/s-stilbene J. Chem. Phys. 93 1041-53... 

INORGANIC COMPLEXES. The cis-trans isomerization of a planar square form of a rt transition metal complex (e.g., of Pt " ) is known to be photochemically allowed and themrally forbidden [94]. It was found experimentally [95] to be an inhamolecular process, namely, to proceed without any bond-breaking step. Calculations show that the ground and the excited state touch along the reaction coordinate (see Fig. 12 in [96]). Although conical intersections were not mentioned in these papers, the present model appears to apply to these systems. 

Diacetoxylation of various conjugated dienes including cyclic dienes has been extensively studied. 1,3-Cyclohexadiene was converted into a mixture of isomeric l,4-diacetoxy-2-cyclohexenes of unknown stereochemistry[303]. The stereoselective Pd-catalyzed 1,4-diacetoxylation of dienes is carried out in AcOH in the presence of LiOAc and /or LiCI and beiizoquinone[304.305]. In the presence of acetate ion and in the absence of chloride ion, /rau.v-diacetox-ylation occurs, whereas addition of a catalytic amount of LiCl changes the stereochemistry to cis addition. The coordination of a chloride ion to Pd makes the cis migration of the acetate from Pd impossible. From 1,3-cyclohexadiene, trans- and ci j-l,4-diacetoxy-2-cyclohexenes (346 and 347) can be prepared stereoselectively. For the 6-substituted 1,3-cycloheptadiene 348, a high diaster-eoselectivity is observed. The stereoselective cij-diacetoxylation of 5-carbo-methoxy-1,3-cyclohexadiene (349) has been applied to the synthesis of dl-shikimic acid (350). 

Coordination sterochemistry (including various forms of isomerization) is an area of significant research interest. This aspect of coordination is important for stereospecific catalytical appHcations. 

In simple chemical systems, it is often possible to make a good first guess at the dominant reaction pathway [25-28]. An example of such a reaction is the chair-to-boat isomerization in cyclohexane. In that pathway, a clever combination of two torsion angles provides an excellent reaction coordinate for the isomerization reaction [29,30]. 

Fig. 13.9. Schematic reaction profile for the EZ isomerization of stilbene. The reaction coordinate 6 is the torsion angle about the double bond. [From FI. Meier, Angew. Chem. Int. Ed. Engl. 31 1399 (1992)]. Reproduced by permission of Wiley-VCH.

The structure of the isomeric benzo-l,2,3-thiadiazole 11.30 is unknown, but the 1 1 adduct with AsFs (11.31) has been structurally characterized. The AsFs molecule is coordinated to the carbon-bonded nitrogen atom. Cycloocteno-l,2,3-selenadiazole is an effective source of selenium for the production of semi-conductors such as cadmium selenide." ... 

To the structural generalizations on carborancs (p. 183) can be added the rule that, in metallocarboranes, the M atom tends to adopt a vertex with high coordination number. M occupancy of a low CN vertex is not precluded, particularly in kineiically controlled syntheses, but isomerization to more stable configurations usually results in the migration of M to high CN vertices. 

Ambidentate ligands possess more than 1 donor atom and can coordinate through either one or the other. This leads to the possibility of linkage isomerism (p. 920). The commonest examples are the ions NO2 (p. 463) and SCN (p. 325). Such ligands can also coordinate via both donor sites simultaneously, thereby acting as bridging ligands. 

Four-coordinate complexes provide good examples of the early use of preparative methods for establishing stereochemistry. For complexes of the type [Ma2b2], where a and b are unidentate ligands, a tetrahedral structure cannot produce isomerism whereas a planar structure leads to cis and trans isomers (see below). The preparation of 2 isomers of [PtCl2(NH3)2], for instance, was taken as good evidence for their planarity. ... 

Isomers are compounds with the same chemical composition but different structures, and the possibility of their occurrence in coordination compounds is manifest. Their importance in the early elucidation of the stereochemistries of complexes has already been referred to and, though the purposeful preparation of isomers is no longer common, the preparative chemist must still be aware of the diversity of the compounds which can be produced. The more important types of isomerism are listed below. 

In principle this type of isomerism (also known as polytopal isomerism) is possible with any coordination number for which there is more than one known stereochemistry. However, to actually occur the isomers need to be of comparable stability, and to be separable there... 

Violet, easily hydrolysed, PdFp is produced when Pd [Pd F(sl s refluxed with SCF4 and is notable as one of the very few paramagnetic compounds of Pd. The paramagnetism arises from the configuration of Pd which is consequent on its octahedral coordination in the rutile-type structure (p. 961). The dichlorides of both Pd and Pt are obtained from the elements and exist in two isomeric forms which form i.s produced depends on the exact experimenial conditions used. The more usual a form of PdCb is a red material with... 

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