AbstractThe benzoylformyl Pd(II) complex, Pd(PPh3)2(Cl)(COCOPh), thermally decomposes to the corresponding benzoyl complex by the loss of CO. The predominant route of decarbonylation is led by a reversible dissociation of a phosphine ligand. The disappearance of the benzoylformyl complex in solutions follows first order kinetics not only in the existence of excess PPh3but also in the absence of added PPh3. Through the treatments of both preequilibrium and the steady state approximations to the kinetic data, the rate constants of the intramolecular acyl migration, k1and k2; as well as the equilibrium constant and the individual rate constants of the reversible phosphine dissociation step, K, kdand kd, were determined in CHCl3. The activation parameters for kd, beingΔH‡= 25.4 Kcal Mol−1,ΔS‡= 15.9 eu,ΔG‡= 20.7 Kcal Mol−1;and for k−d, beingΔH‡= 13.0 Kcal Mol−1,ΔS‡= −7.9 eu,ΔG‡= 15.4