Metal Complexes of Chiral Ligands

Bennett (1959) reported that bis (l) asparaginato copper II ) (Mll) is more stable than (d) (l) asparaginato copper II (Mdl). From a proton magnetic reso- 

The excess Gibbs energy of (MDD-fMLL) relative to Mdl was found to be 1.29 kJ mol i for the Coll compound and 2.04 kJ mol i for Nill, the former value being in very good agreement with that of McDonald and Phillips. 

More recently, Barnes and Pettit (1970), on the basis of calorimetric measurements of the bis-histidine complexes of Znll and Nill, have shown that the Mdl complexes are more stable than the Mdd or Mll. For example  

Histidine and asparagine function as tridentate chelates. In the asparagine complexes, the mixed (dl) form is the more stable while the reverse is true of the histidine complexes. Potentiometric studies of the complexes of various metals (Nill, Cull, Coll, Znll) with a wide range of amino adds all of which function as bidentate chelates have failed to reveal any differences in the stability of Mdl and Mll forms. (Ritsma et al. 1965 Gillard et al. 1966). The reason why the chirodiastaltic interactions should be so much more marked with tridentate than bidentate chelates calls for investigation in terms of the intramolecular force system. 

Molecular structures of cobalt II histidine complexes, reproduced with permission from Candlin and Harding (1970). Left-hand Mdl, with molecule I in the d form and molecule II in the l. Right-hand Mll. Nitrogen atoms are shaded oxygen atoms are shown heavier 

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The means by which metal complexes of chiral ligands block quadrants depends on the nature of the ligand and the metal-ligand adduct. In some cases, the stereogenic centers of the ligands exist in close proximity to the metal. In other cases, the stereogenic centers are located so far from the metal that it is not obvious how the effect of these distant stereocenters can be transmitted to the site of reaction. 

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