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The reaction of the σ-bonded (PCP)Pd-Me complex (PCP = 2,6-bis[(di-tert-butylphosphino)methyl]phenyl) with CO₂ is first-order in palladium and first-order in CO₂ with a rate constant k_s = 8.9 ± 0.8 M⁻¹ s⁻¹ at 353 K. Activation parameters are ΔH^⧧ = 73 ± 7 kJ/mol and ΔS^⧧ = −118 ± 19 J/K mol. Based on this and theoretical calculations we propose an S_E2 mechanism where the coordinated methyl group attacks a completely noncoordinated carbon dioxide molecule in a bimolecular reaction. The PCPPd−crotyl complex was synthesized in an 65:35 E:Z mixture, and it was shown to react with CO₂ to give the complex PCPPd−O(CO)CH(CH₃)CHCH₂ as a single isomer, where the former γ-carbon has been carboxylated. Theoretical calculations again suggest an S_E2 mechanism with a noncoordinated carbon dioxide reacting with the terminal carbon on the allyl group, forming an η²-bonded olefin complex as an …