Racemization trigonal twist mechanism

Bailar, J. C. Jr. (1958). /. Inorg. Nucl. Chem. 8, 165. The paper describing trigonal twist mechanisms for racemization. 

This is particularly true now with the recent discovery of trigonal prismatic structures for some 6-coordinated metal complexes (21). Surely the proper metal-ligand combination should give a system in which the difference in energy between an octahedral and a trigonal prismatic structure is small. As a result, racemization may take place by a trigonal twist mechanism. Twisting processes of the square-planar-tetrahedral type are known to occur readily in certain nickel(II) complexes (20). 

There have been several attempts to provide a theoretical framework for describing geometrical isomerization and racemization. Vanquickenbome and Pierloot used ligand field theory to calculate the electronic energies of the intermediates pr osed in the dissociative and trigonal twist mechanisms for low-spin d systems. 

Bailar realized that racemization reactions could occur without the dissociation of ligands. He proposed a mechanism involving the twisting of one triangular octahedral face of A-[M(AA)3] by 120 about the C3 axis to form A-[M AA), This is known as the Bailar or trigonal twist A rhomboid twist is known as the Ray-Dutt twist In some cases racemization results from ligand dissociation. 

Such a dissociative mechanism is rare, and kinetic data are usually consistent with an intramolecular process, e.g. for [Cr(ox)3], [Co(ox)3] (low-spin) and [Fe(bpy)3] " (low-spin), the rate of racemization exceeds that of ligand exchange. Two intramolecular mechanisms are possible a twist mechanism, or the cleavage and refoimation of the M—L bond of one end of the bidentate ligand. Alternative twist mechanisms (the Bailar and Ray-Dutt twists) for the interconversion of enantiomers of M(L—L)3 are shown in Fig. 26.9. Each transition state is a trigonal prism and the... 

Both mechanisms proceed via trigonal prismatic transition states. In the Bailar twist, all three chelating rings remain equivalent throughout the racemization, whereas in the case of the Ray-Dutt twist, the ligands are grouped into non-equivalent pairs of one and two ligands, respectively. The... 

Figure 9. Intramolecular racemization of [M (A A) 3] by the following mechanisms (A) chelate ring opening-closing) B) rhombic twist) and (C) trigonal...

Preliminary studies have shown that d-[Co(en)3]l3 cannot be racemized by application of high pressure (3900 atm over a period of 70 hr). However, this compound does undergo racemization when heated, perhaps indicating a dissociative mechanism in this case. A more detailed study of the thermal racemization of ti-[Co(en)3]l3 is underway. Studies are also underway on racemization of t/-ds-[Co(en)2(NH3)2]Cl3. If this later compound racemizes by a dissociative mechanism, anation would be expected. Racemization without anation could be interpreted as evidence for an intramolecular mechanism, possibly the trigonal (Bailar) twist. 

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