Earlier’ we have described a series of pentacoordinate Iri complexes of the type, RIr(COD)P, , (R = H, CH,; COD = 1,Scyclooctadiene; P = tertiary phosphine) in which rapid intramolecular rearrangement is found to occur. In order to obtain a more secure assignment of the ground state geometry for such complexes and to clarify the mechanism for the exchange (ina L = phosphine or arsine; see Table 1) were prepared by reacting [Rh(diene)Cl] z with methyl lithium in toluene in the presence of a stoichiometric amount of L. The reactions were initiated at -78’ under an atmosphere of nitrogen and the yellow crystalline compounds isolated at O”. The proton NMR spectra of these complexes (CHaCla as solvent) were measured from -9S” to +50° (Table 1). Above +35” several complexes showed irreversible changes in the NMR spectrum indicating chemical reaction to be occurrinm. The low temperature limiting spectra show the rhodium bound methyl as a triplet of doublets in the phosphine complexes (I-III, V, VI) indicating coupling of the methyl to resRh and equal coupling to two 31P nuclei. In addition, in the PMe,Ph and AsMe,Ph complexes (I, IV, V, VII) two diastereotopic methyl resonances are observed. Further, the low temperature liniming spectra of all complexes show two separate vinylic resonances for the diene ligandm, but, significantly, only one reasonably