Reaction of methyl iodide with square planar [kappa(2)-Tp*Rh(CO)(PMe3) 1a (Tp* = HB(3,5-Me(2)pz)(3)) at room temperature affords [kappa(3)-Tp*Rh(CO)(PMe3)(Me)] 2a, which was fully characterized by spectroscopy and X-ray crystallography. The pseudooctahedral geometry of cationic 2a, which contains a kappa(3)-coordinated Tp* ligand, indicates a reaction mechanism in which nucleophilic attack by Rh on Mel is accompanied by coordination of the pendant pyrazolyl group. In solution 2a transforms slowly into a neutral (acetyl)(iodo) rhodium complex [kappa(3)- Tp*Rh(PMe3)(COMe)1] 3a, for which an X-ray crystal structure is also reported. Kinetic studies on the reactions of [kappa(2)-Tp*Rh(CO)(L)] (L PMe3, PMe2Ph, PMePh2, PPh3, CO)] with Mel show second-order behavior with large negative activation entropies, consistent with an S(N)2 mechanism. The second-order rate constants correlate well with phosphine basicity. For L = CO, reaction with Mel gives an acetyl complex, [kappa(3)-Tp*Rh(CO)(COMe)]. The bis(pyrazolyl)borate complexes [kappa(2)-Bp*Rh(CO)(L)] (L = PPh3, CO) are much less reactive toward Mel than the Tp* analogues, indicating the importance of the third pyrazolyl group and the accessibility of a theta coordination mode. The results strengthen the evidence in favor of an S(N)2 mechanism for oxidative addition of Mel to square planar d(8) transition metal complexes.