Chelate rings isomers

The effect of /rfive-coordinate aluminum alkyls have been discussed.94 Studies have also focused on the equilibrium between four-coordinate and five-coordinate isomers in compounds of the type [R2Al /i-0(GH2) ER1x ]2 (n = Z, 3 ER1x = OR1, SR1, NRX2) (shown in Figure 7(h) for n = Z). Factors that control this equilibrium and hence the coordination around the aluminum include the steric bulk of the substitutent at the aluminum (R), and the Lewis base donor R1, the basicity of the neutral donor group ER1, and the chelate ring size (as determined by n).95... 

Inversion at the N center is coupled to conformational changes in a chelate ring. The kinetics of inversion at asymmetric N centers in complexes of tetraaza linear or macrocyclic ligands have received scant attention. There are five configurational isomers of the planar complex Ni([14]aneN4) +, Sec. 3.1.1. The interconversions between such structures are base catalyzed with second-order rate constants covering a small range from 1.2 x 10 to 2.4 x 10 M- s- Refs. 108-110. 

All these methods have found applications in theoretical considerations of numerous problems more or less directly related to solvent extraction. The MM calculated structures and strain energies of cobalt(III) amino acid complexes have been related to the experimental distribution of isomers, their thermodynamic stability, and some kinetic data connected with transition state energies [15]. The influence of steric strain upon chelate stability, the preference of metal ions for ligands forming five- and six-membered chelate rings, the conformational isomerism of macrocyclic ligands, and the size-match selectivity were analyzed [16] as well as the relation between ligand structures, coordination stereochemistry, and the thermodynamic properties of TM complexes [17]. 

Experimentally based intuitive arguments have been presented to arrive at a regional rule for optical activity of d-d transitions of conformational isomers of octahedral metal complexes. Conformational preferences for chelate rings formed by 1,3-pn in its octahedral mono, bis, and tris metal complexes have been studied by calculation of the conformational energies. In all cases, the chair conformation was found to be the most stable. The lowest energy pathway for converting from one chair configuration into another has a barrier to activation of about 7 kcal mol Conformational types of metal-edta complexes have been studied. ... 

It is not necessary to have three chelate rings present. The cation dichloro-bis(ethylenediamine)coball(lI() exists as two geometric isomers, cis and irans The j... 

One or more factors may be operating simultaneously to provide a delicate balance of counterpoising effects. An interesting series of compounds illustrates the competing effects in linkage isomers of square planar palladium(II) complexes (Fig. 12.46a-d).87 The six-membered chelate ring in Figure 12.46c allows an essen-... 

Tbe dissymmetry would thus be lost, and when the chelate ring reforms, it would have a 50-50 chance of producing either the A or A isomer (Chapter 12). Since the rate-determining step in this mechanism is the dissociation of the ligand, the rate of racemization (kr) would have to equal the rate of dissociation (kj). For example, tris(pbenanthroline)rnckel(II) racemizes at the same rate (kr — 1.5 x 104 s l) as it dissociates (kj = 1.6 x 104 s l), which implies dissociation is part of the racemization mechanism. If racemization takes place faster than dissociation (as it does, e.g., for tris(phefianthroline)iron(lI) k,. — 6.7 x 1CT4 s 1 and kd = 0.70 x JO 4 s1], ibis mechanism can be eliminated. 

Specifically since there are three chelate rings, each of which can potentially be either lei or ob, these two isomers should be labeled as lei and obj since it is possible to have mixed ob-lel complexes. 

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