7-Membered chelate ring

Potentiometric titration and chromatographic behavior on CM-Sephadex of copper-chelated and metal-free BLM indicate that a deprotonated functional group must occupy one of the coordination sites. The amide nitrogen of IV is located in a favorable position for the coordination site to form 5- and 6-membered chelate rings. Therefore, the deprotonated amide nitrogen of IV can be inferred to be ligated. 

The geometry of a chiral 6-membered chelate ring is not conducive to effective asymmetric catalysis. Consider square-planar complexes of d[, l-2,3-diphenyl-l,3 bis (diphenylphosphino) propane (24), which are presumed to exist in chair conformation (Scheme 6) with rapid ring inversion. The close approximation to a-symmetry in the environment of the metal suggests that there will be little discrimination between the diastereomeric modes of binding of a prochiral bidentate ligand, since substituents on the olefin experience similar steric interactions in both isomers. The expectation of low selectivity is borne out in practice, for in some cases enamide complexes derived from this phosphine exist in two diastereomeric forms (Figure 3). 

Small metal ions prefer 6-membered chelate rings and large metal ions prefer 5-membered chelate rings. This applies to macrocyclic and cryptate complexes as well as open-chain chelating ligands. 

The formation of bis-chelated complexes is most favourable for phenyl-substituted diphosphines which can form either 5- or 6-membered chelate rings. 

The VCO2 value (1581 cm ) for Cu(eddp)(H20), where eddp = ethylene-diamine-iVW -di-3-propionate, is consistent with the carboxylate forming a 6-membered chelate ring. SERS data for picolinic acid adsorbed on silver include VgC02 at 1379... 

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