Unsymmetrical chelate rearrangement

There are a large number of unsymmetrical chelates which present the feature of having both geometrical and optical isomers, as shown by the examples in Figure 4.7. The fac and mer isomers each have optical isomers thus, both racemization and geometrical isomerization can be observed in one system. This can serve to eliminate certain rearrangement processes. 

Table V represents the relationship betwenn half wave potentials and acidity for the binuclear complexes with a completely different sequence of protonated compounds. All of these structures are different from type I described above and they cannot be oxidized to the tin (IV) chelate without bond breaking and bond forming. This explains the higher stability of the binuclear chelates in acidic solution since their rates of oxidation are determined by the rates of rearrangement to the octahedral or pseudo-octahedral configuration (type I) of the mononuclear compound. Additional proof for this unsymmetric protonation sequence has been the isolation of several compounds which are presently being studied and one of which has been identified as H Y.SnCl, 2 H,0.

Figure 4.8. Products of the trigonal twist and rhombic bend rearrangements for an unsymmetrical tris chelate.

Rearrangements in tris-a-isopropyl- (44) and tris-a-isopropenyl-tropolonato-complexes of several metal(iii) cations have been probed by n.m.r. spectroscopy. Two intramolecular processes are likely for these tris-chelates of unsymmetrical ligands, A A inversion and cis trans isomerization. Both... 

---