Oxidative addition (OxAd) reactions Rh(LL)(PPh3)(CO) + CH3I, where LL
is a bidentate monoanionic ligand with O,O or O,N donor atoms, yield isomeric
rhodium(III) octahedral methyl carbonyl complexes or square pyramidal acetyl
complexes, for example:

I LL = Hexafluoroacetylacetonate: single cis-(CH3, I) isomer
II LL = Cupferronate: two cis-(CH3, I) isomers
III LL = 8-Oxyquinolate: a mixture of cis- and trans-(CH3, I) isomers
IV LL = Dimethylketoiminate: acetyl complex
Our recent X-ray data on the complex I in combination with known structures
of II [1], trans-(III [2], and IV [3] considered in parallel with NMR spectra make
it possible to propose the reliable NMR criteria for determining structures of
compounds formed in the course of the OxAd process. Application of these
criteria suggests a general mechanistic scheme for a wide series of OxAd
reactions Rh(LL)(PPh3)(CO) + CH3I:

The experimentally observed proportion of cis and trans isomers in each
specific case is governed by relative rates of the reactions 1, 2, and 3.
The sequence of reactions 1-2-3 seems to be an interesting example of
the non-dissociative isomerization of octahedral complexes via two-step methyl
migration: from the axial position at the metal center to carbonyl carbon and then
back to the metal center, but in the equatorial position.
Literature:
1. S.S.Basson et al., Inorg. Chim. Acta 128 (1987) 31;
2. Van Aswegen et al., Transition Metal Chem. 16 (1991) 369;
3. L.J.Damoense et al., Rhodium Express, 14 (1995) 4
Supported by the “Universities of Russia” Program, grant UR.05.01.032.