The formation of a sitting-atop (SAT) complex of Cu(II) ion with 5,10,15,20-tetraphenylporphyrin (H(2)tpp) in acetonitrile has been observed, and the kinetic parameters for the formation were determined as follows : k(so) (3.6 +/- 0.1) x 10(5) mol(-1) dm(3) s(-1) at 25.0 degrees C, Delta H(so)double dagger = 56 +/- 5 kJ mol(-1), and Delta S(so)double dagger = 46 +/- 19 J mol(-1) K-1. The H-1 NMR spectrum of the SAT complex (Cu(H(2)tpp)(2+)) indicated that two pyrrolenine nitrogens coordinate to the Cu(II) ion and that two protons bound to the pyrrole nitrogens remain. The protons were abstracted by the addition of pyridine (py) as the Bronsted base to give the Cu(tpp) metalloporphyrin. In the presence of py, the product for the reaction of the Cu(II) ion with H(2)tpp was Cu(tpp) instead of the SAT complex. The observed conditional rates for the formation of Cu(H(2)tpp)(2+) and Cu(tpp) were interpreted by the contribution of Cu2+, Cu(py)(2+), and Cu(py)(2)(2+) species, and the second-order rate constants of the SAT complex formation were k(s1) = (3.5 +/- 0.3) x 10(4) mol(-1) dm(3) s(-1) for Cu(py)(2+) and k(s2) = 90 +/- 2 mol(-1) dm(3) s(-1) for Cu(py)(2)(2+). Deprotonation rates were measured by following the reaction between the SAT complex and py as a function of the py concentration, and the second-order rate constant was determined to be (2.3 +/- 0.1) x 10(2) mol(-1) dm(3) s(-1). The present kinetic results have indicated that the SAT complex exists during the course of the metalation process and that the SAT complex formation is a rate-determining step.