Trigonal twist isomerism mechanism

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. 

There are numerous other examples of stereochemical correspondence in propeller molecules. The propellane 19> 1e has only one energetically reasonable isomerization mechanism available a twisting motion about the C3 axis which corresponds to the three-ring flip as well as to the trigonal twist rearrangement (Fig. 5). Similarly, only one isomerization mechanism (stereochemically correspondent in the permutational sense to the zero-ring flip) is energetically reasonable for tri-o-thymotide (Id). 8,20)... 

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. 

There have been several attempts to provide a theoretical framework for describing geometrical isomerization and racemization. Vanquickenbome and Pierloot used ligand field theory to calculate the electronic energies of the intermediates pr osed in the dissociative and trigonal twist mechanisms for low-spin d systems. 

Figure 7.4. Syn-syn, anti-anti exchange (a) the angular twist mechanism of cis-trans isomerization and (b) Berry s pseudorotation mechanism of the exchange of equatorial and axial ligands in the trigonal bipyramid.

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