Intermediate Bailar

Figure 12 Standard Gale diagrams for the triple dsd degenerate isomerization of an octahedron through a trigonal prism intermediate (Bailar or Ray and Dutt Twist)...

One important implication of the cage mechanism is that those reactions should be favored for which there is a high probability for the reactants to be in each other s vicinity or cage. Hence, the prevalence of aquation reactions and the importance of ion pairs as intermediates in a nation reactions. I think it is very interesting that John Bailar s examples of stereospecificitv involve rather drastic changes in the immediate environment around the complex since concentrated systems were used in the one set of cases, and dilute ones, in the other. 

Mark M. Jones There is one change that I could make in the paper. About ten years ago Daryle Busch and John Bailar (1) reported an intermediate type described under hydrolyses of Schiff bases, and I want to apologize for omitting it in the literature. 

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. 

The products of the substitution of a ligand G for ligand A of the species [M ABCDEF]" (in which M is a metal ion and A, B, C, D, E, and F are monodentate ligands or donor atoms of chelating ligands of an octahedral complex) are shown in Figure 1. Letters under the arrows indicate positions made adjacent by the loss of A, or the insertion position of G. The products are based on the principle that minimal atomic motion accompanies the attainment of the transition state, the reaction intermediates (if any), and the products. The results of a large number of studies support this principle, which was assumed by Werner, and which recently has been used by Pearson and Basolo (81) and by Kyuno, Boucher, and Bailar (67). 

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. 

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

Jahn-Teller distortions are also expected for trigonal prismatic complexes of the same metal ions. An important difference with the case of the octahedron is that, in the trigonal prism, the distortion consists in an elongation of two ligands in cis, rather than in trans, relative positions. Moreover, coordination spheres with intermediate degrees of Bailar rotation can present either cis or trans Jahn-TeUer distortions. 

The involvement of conjugate base species in such base hydrolysis reactions must be considered in arriving at a mechanistic path for the Bailar inversion. And even though a d and l inversion can be accommodated by an unsymmetrical trigonal bipyramidal conjugate base intermediate, " the concentration and silver ion dependence of the Bailar inversion require other explanations. Four such possibilities can be imagined for the concentration dependent reaction. 

The two popular mechanisms for the intramolecular rearrangement in octahedral complexes, known as Bailar twist and Ray-Dutt twist occurring through prismatic intermediates, are shown in Figure 11. 

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