Bailar twists

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. 7 Definition of the chelating bite angle a. Large angles a facilitate Ray-Dutt twists with rhombic (C2U transition states, whereas small angles a favor Bailar-twists with trigonal (D3h) transition states...

Fig. 22 Four synchronous Bailar-twists and six ligand atropenantiomerization processes are required for the enantiomerization of (AAAA)-32 to (AAAA)-32 [122]...

As one of the special features, MOMEC has a plane twist function. This has been included to limit the tetrahedral twist in four-coordinate compounds, where 1,3-nonbonded interactions lead to a preference for a tetrahedral arrangement (see Section 3.6). That is, the plane twist potential can be used to induce a square-planar arrangement or, using constraints, any intermediate structure can be enforced. The same potential can in principle be used for other structural features (see Fig. 17.14.1), such as the Bailar twist of six-coordinate complexes or for computing the rotational barrier of metallocenes. However, at present it has only been implemented in MOMEC for the tetrahedral twist and no parameters have been included as yet. 

Fig. 14. A clockwise (CW) Ray-Dutt twist followed by a CW Bailar twist, applied schematically to a model complex like 30-32. Each of the two processes exchanges diastereomers, but the combined action is a topomerization equivalent to a (X, Cl)-1,2 shift. The eye drawn near one of the structures shows which triangular face was selected for the second twist process.45 Reproduced with permission from the American Chemical Society.

There are two pathways without bond rupture that have been widely discussed. One is the trigonal, or Bailar, twist and the other is the rhombic, or Ray-Dutt, twist, shown in Fig. 1-12 (a) and (b), respectively. Twist processes are, of course, not confined to chelate complexes. 

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. 

It will be noted that the difference in racemization rates are very considerable, and it has been suggested [44] that this arises from two factors low barriers to the Bailar twist in d° and d ° systems, and flexibility in the ligand—models of 9 suggest that it is impossible to attain the trigonal prismatic transition state of the Bailar twist. 

Table 14. Calculated moments of inertia for the bailar twist...

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).

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