Trigonal prismatic transition state

Since a trigonal-prismatic transition state with greater steric hindrance must be traversed, it thus becomes understandable why the compound with the very bulky tricyclohexylphosphine ligand isomerizes more slowly than does the corresponding triethylphosphine complex. 

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... 

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. 

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

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