Ray/Dutt twists

Dunham and Abbott have studied ligand exchange in cis- and trans-[Pt(ox)2(H20)2] and shown that it occurs too slowly to be involved in the formation of metal-metal bonded polymers.507 They also observed a trans to cis isomerization that occurred without ligand exchange and proposed a Bailar or Ray-Dutt twist process.507... 

An NMR investigation of water exchange at [Pt(H20)2(oxalate)2] is relevant to the mechanism of formation of one-dimensional mixed valence oxalatoplatinum polymers. In fact the rate constant for this presumably dissociative (AS = + 42 JK mol-1) reaction is considerably too low for water loss to be, as recently proposed, the first step in formation of these polymers. The mechanism of trans to cis isomerization for this oxalate complex, and for its (2 -methyl)malonate analogues, is intramolecular (Bailar or Ray-Dutt twist), since there is no concurrent incorporation of labeled solvent (177). 

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... 

Fig. 7 Definition of the chelating bite angle a. Large angles a facilitate Ray-Dutt twists with rhombic (C2U transition states, whereas small angles a favor Bailar-twists with trigonal (D3h) transition states...

There are two pathways without bond rupture. One is the trigonal or Bailor twist and other is rhombic, or Ray-Dutt, twist shown in figure below. Some cis-M(CO)4(PR3)2 complexes are believed to isomerise in... 

Fig. 14. A clockwise (CW) Ray-Dutt twist followed by a CW Bailar twist, applied schematically to a model complex like 30-32. Each of the two processes exchanges diastereomers, but the combined action is a topomerization equivalent to a (X, Cl)-1,2 shift. The eye drawn near one of the structures shows which triangular face was selected for the second twist process.45 Reproduced with permission from the American Chemical Society.

Rate law, for nucleophilic substitution, 540-543 Ray, P. C., 556 Ray-Dutt twist, 556 Rayleigh, Lord, 825 Reaction rates influenced by acid and base, 553-555... 

Fig. 25.8 Twist mechanisms for the interconversion of A and A enantiomers of M(L—L)3i (a) the Bailar twist and (b) the Ray-Dutt twist. The chelating L—L ligands are represented by the red lines (see also Box 19,2).

Bailar realized that racemization reactions could occur without the dissociation of ligands. He proposed a mechanism involving the twisting of one triangular octahedral face of A-[M(AA)3] by 120 about the C3 axis to form A-[M AA), This is known as the Bailar or trigonal twist A rhomboid twist is known as the Ray-Dutt twist In some cases racemization results from ligand dissociation. 

Figure 1.10 Gale diagrams for the triple dsd degenerate isomerization of an octahedron through a trigonal prismatic intermediate corresponding to the Bailar or Ray/Dutt twists.

Such a dissociative mechanism is rare, and kinetic data are usually consistent with an intramolecular process, e.g. for [Cr(ox)3], [Co(ox)3] (low-spin) and [Fe(bpy)3] " (low-spin), the rate of racemization exceeds that of ligand exchange. Two intramolecular mechanisms are possible a twist mechanism, or the cleavage and refoimation of the M—L bond of one end of the bidentate ligand. Alternative twist mechanisms (the Bailar and Ray-Dutt twists) for the interconversion of enantiomers of M(L—L)3 are shown in Fig. 26.9. Each transition state is a trigonal prism and the... 

Figure 11 Ray-Dutt twist (top) and Bailar twist (bottom) in the isomerization of an octahedral complex.

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. 

---