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Scheme I b' a'(diphos)Ir(CH3) and for (C8H12)[(C6H5)(CH3)2P]2Ir(CH3) are entirely consonant with the mechanism suggested by Shapley and Osborn (see Scheme I) in which there are successive "pseudorotations" about Pt, R, and P2. For equilibration of vinyls 1 and 2 to occur, the reaction must proceed from ground state a, through the high-energy intermediates b and b', back to ground state a'. In the high-energy inter-mediate, a methyl group occupies an equatorial position and the two phosphine groups occupy an equatorial and an axial position. Since ground state a is of higher energy for the diphos compound, due to extra strain associated with a small equatorial P-Ir-P' angle, the diphos compound is expected (as observed) to undergo the intramolecular interconversion a -* a' at a greater rate than the analogous bis-(monodentate)phosphine complex in which the P-Ir-P' angle is not strained by chelation and is closer to the ideal equatorial value of 120°.Further information on these systems is expected to be obtained from current X-ray structural analyses on the species (C8H12)[( C6Hs)2P(CH2)nP(C6H5)2]Ir(CH3) in which n has values other than 2. (The case with n = 2, of course, is covered by the present article.) Registry No. (C8H12)(diphos)Ir(CH3), 36974-34-2. Acknowledgments. S. A. B. gratefully acknowledges the receipt of an NSF predoctoral fellowship. This work was generously supported by the National Science Foundation through Grants GP-26293 and GP-33018 (to M. R. C.).