Trigonal twist mechanism

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

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

It is useful to note that the same issue does not arise in complexes that have a horizontal symmetry plane. There, a wedge geometry allows L and hj to attain equivalent positions above and below that plane. These conceptual issues have been addressed in the case of oxorhenium(V) complexes by two experimental studies, each of which supports intervention of intermediate(s) that undergo turnstile or trigonal twist mechanisms. In so doing, L and L7 attain equivalent or at least interchangeable positions. These studies are the subjects of the next two sections. 

Fig. 7. Site rearrangement scheme of a M(A-B )3 type complex for the trigonal twist mechanism or averaging set Ag36 ...

Several important results have come from the M(dtc)3 studies in addition to the operation of the trigonal twist mechanism. First, the rate of enantiomerization is found to be greatly dependent on metal ion with the following order In, Ga, V >... 

The most noteworthy observation with the M(RT)3 complexes in addition to operation of the trigonal twist mechanism for several is the suprisingly low activation energies for inversion of the Co(III) complexes. These were the first nonrigid Co(III)... 

D3 symmetry and they are at or near the D3j limit or along the D3ti D3h reaction path.75) This observation has obvious importance with respect to the trigonal twist mechanism. 

An electrochemical study has shown reversible reduction to Mn" and oxidation to Mnlv species and a H NMR study shows that isomerization, of the compounds of unsymmetrical ligands such as MePhdtc, is slow on the NMR time scale at - 60°C and occurs via a trigonal twist mechanism rather than a dissociative one. 

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). 

As part of his general investigation of the [MLs] system, Kepert has also modeled this trigonal twist mechanism for [M(bidentate)3] polyhedra [24, 28]. The... 

The best available evidence45 for a trigonal twist mechanism has been obtained for the mixed ligand complex (21-LII). It is proposed that the... 

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