Bailar twist mechanism

In the absence of a dissociative process, the remaining mechanistic options are only intramolecular nondissociative of the pseudorotation family. The nondissociative site-exchange reactions commonly discussed for octahedral complexes are the Ray-Dutt [10] and Bailar twist mechanisms [11], by which three ligands are rotated relative to the other three via a trigonal prism transition state. None of these processes effects a topomerization in one step. At least two consecutive such twists are necessary in order to bring about the observed topomerizations. However, with these mechanisms it would be difficult to rationalize the observation of two distinct barriers in 1-4. 

The racemization of dinuclear triple-stranded helicates has been studied. These complexes adopt either AA or AA configurations arising from the tris-bidentate coordinated metal centers. A mononuclear tris-catecholate complex ML3 racemizes via a Bailar twist mechanism. Linked tris-catecholate metal centers would be expected to racemize at the same rate as that of the mononuclear complex in the absence of mechanical coupling. However, when two tris-catecholate complexes are linked in a helicate, the racemization of the M2L3 structure (from AA to AA) slows by a factor of one hundred, while racemization of the four tris-catecholate metal centers in an M4L6 tetrahedron M4L6 is not observed. Although the components of these assemblies are labile, the chiral tetrahedral structure displays structural inertness. 

Fig. 5.14 Bailar twist mechanism for the racemisation of an octahedral tiis-chelate complex via a trigonal prismatic transition state or intermediate.

FIGURE 20.11 The Bailar top) and Ray-Dutt (bottom) trigonal twist mechanisms. 

If forming the transition state by a twist mechanism leads to some expansion of the lattice, it would be expected that a higher lattice energy would hinder racemization. In a very detailed study (Kutal and Bailar, 1972) of the racemization of ( + )-[Co(en)3]X3-nH20 (where X = Cl-, Br, I-, or SCN ), it was found that the rate varied with the nature of the anion. The rate was found to decrease in the order... 

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

The question might be asked Are there similar mechanisms for changing the configuration of molecules without breaking bonds in molecules with coordination numbers other than 3 and 5 The answer is yes. One of the most important series of inorganic compounds consists of six-coordinate chelate compounds exemplified by the tris(ethylenediamine)cobalt(lIl) ion. Because of the presence of the three chelate rings, the ion is chiral and racemization can take place by a mechanism that is closely related to atomic inversion or Berry pseudorotaiion (the mechanism for six-coordination is termed the Bailar twist see Chapter 13k... 

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... 

Fig. 25.8 Twist mechanisms for the interconversion of A and A enantiomers of M(L—L)3i (a) the Bailar twist and (b) the Ray-Dutt twist. The chelating L—L ligands are represented by the red lines (see also Box 19,2).

Fujiwara and Yamamoto have studied the racemization of anhydrous compounds of the type [M(phen)3]X2 and [M(bipy)3]X2 where M = Fe or Ni and X = d , Br, 1 or C104 . The rates of racemization of the nickel compounds were found to depend on the identity of the anion racemization rates for the iron compounds were anion-independent. However, the kinetic analysis has been called into question. If valid, the kinetic anion effect for the nickel compounds follows an order that is opposite that found by Kutal and Bailar for the racemization of [Co(en)3]X3 salts C104 < 1 < Br < Cr. It was proposed that the iron compounds racemize by a twist mechanism and that the nickel compounds racemize by nucleophilic attack by lattice anions to form a seven-coordinate transition state. 

The Bailar Twist provides a mechanism for the racemisation of tris chelate complexes via the formation of a trigonal prismatic intermediate. Dissociation of a chelating ligand does not occur in such a rearrangement. The mechanism is shown diagramatically in Fig. 5.14. 

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