Geometrical and Optical Isomerism in Octahedral Systems

The interconversion of geometrical isomers (cis, trans, fac, mer) and the racemization of optical isomers (X, A) can proceed by two general mechanisms, ligand dissociation or intramolecular rearrangement. 

The diffetentiation of the dissociative and intramolecular mechanisms is usually based on a comparison of the rates of ligand dissociation (exchange or solvolysis) and racemization. If the rates are quite similar, dissociation is assumed, whereas if racemization is much faster, then an intramolecular mechanism is operating. However, this involves the implict assumption that the initial chelate ring opening step is rate limiting for both fnocesses. 

For the tris(oxalato)rhodium(III) ion in water, Damrauer and Milbum have studied the kinetics of racemization, aquation and exchange of the oxalate oxygens. The racemization and aquation have acid-dependent pseudo-first-order rate constants given by 

The oxygen exchange is first-order in [H ] and the inner, coordinated oxygens are exchanged much more slowly than the outer oxygens. The kinetic results are summarized in Table 4.1. 

Comparisons of volumes of activation in Table 4.2 from the work of Lawrance and Stranks provide some further insights. The authors suggest that the small values of AV for Ni(II) and Cr(III) imply an intramolecular 

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