The formation of the sitting-atop (SAT) complexes of 5,10,15,20-tetraphenylporphyrin (H(2)tpp), 5,10,15,20-tetrakis(4-chlorophenyl)porphyrin (H(2)t(4-Clp)p), 5,10,15,20-tetramesitylporphyrin (H(2)tmp), and 2,3,7,8,12,13,17,18-octaethylporphyrin (H(2)oep) with the Cu(H) ion was spectrophotometrically confirmed in aqueous acetonitrile (AN), and the formation rates were determined as a function of the water concentration (Cw). The decrease in the conditional first-order rate constants with the increasing Cw was reproduced by taking into consideration the contribution of [Cu(H2O)(an)(5)](2+) in addition to [Cu(an)(6)](2+) to form the Cu(II)-SAT complexes. The second-order rate constants for the reaction of [Cu(an)(6)](2+) and [Cu(H2O)(an)(5)](2+) at 298 K were respectively determined as follows: (4.1 +/- 0.2) x 10(5) and (3.6 +/- 0.2) x 10(4) M-1 s(-1) for H(2)tpp, (1.15 +/-0.06) x 10(5) M-1 s(-1) and negligible for H(2)t(4-Clp)p, and (4.8 +/- 0.3) x 10(3) and (1.3 +/- 0.3) x 10(2) M-1 s(-1) for H(2)tmp. Since the reaction of H(2)oep was too fast to observe the reaction trace due to the dead time of 2 ms for the present stopped-flow technique, the rate constant was estimated to be greater than 1.5 x 10(6) M-1 s(-1). According to the structure of the Cu(II)-SAT complexes determined by the fluorescent XAFS measurements, two pyrrolenine nitrogens of the meso-substituted porphyrins (H(2)tpp and H(2)tmp) bind to the Cu(Il) ion with a Cu-N(pyr) distance of ca. 2.04 Angstrom, while those of the beta -pyrrole-substituted porphyrin (H(2)oep) coordinate with the corresponding bond distance of 1.97 Angstrom. The shorter distance of H(2)oep is ascribed to the flexibility of the porphyrin ring, and the much greater rate for the formation of the Cu(H)-SAT complex of H(2)oep than those for the meso-substituted porphyrins is interpreted as due to a small energetic loss at the porphyrin deformation step during the formation of the Cu(H)SAT complex. The overall formation constants, beta (n), of [Cu(H2O)(n)(an)(6-n)](2+) for the water addition in aqueous AN were spectrophotometrically determined at 298 K as follows: log(beta (1)/M-1) = 1.19 +/- 0.18, log(beta (2)/M-2) = 1.86 +/- 0.35, and log(beta (3)/M-3) = 2.12 +/- 0.57. The structure parameters around the Cu(II) ion in [Cu(H2O)(n)(an)(6-n)](2+) were determined using XAFS spectroscopy.