The mechanism of the [PdL4]-catalyzed couplings between R-OTf (R= pentahalophenyl; L= PPh3, AsPh3) and Sn (CH CH2) Bu3 has been studied. The addition of LiCl favors the coupling for L= AsPh3 in THF but retards it for L= PPh3. Separate experiments show that for L= AsPh3, LiCl accelerates the otherwise very slow and rate-determining oxidative addition of the aryl triflate to [PdL4], leading to trans-[PdRClL2]. Therefore, the overall process is ...