Trigonal twist

Fig. 13. Transition-state structures for pseudorotation by the trigonal twist (M3) and rhombic twist (M3) mechanism. According to Ref. [93]...

Density functional theory (B3LYP) has been used to investigate the mechanism of the cis-trans isomerization in Ir(H)2(CO)(C6H3 CH2P(H2) 2).300 The preferred mechanism involves two consecutive trigonal twists in which the complex passes through a distorted octahedral intermediate, as shown in reaction Scheme 19. 

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. 6.4. Trigonal twist angles 0 for transition metal hexaamine cage complexes plotted as a function of the d-electron occupancy d"[1 1.

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)... 

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