Mechanistic Investigation on the Water Substitution in the h5-Organometallic Complexes Cp*Ir(H2O)32+ and Cp*Rh(H2O)32+

The substitution of water in the half-sandwich complexes Cp*Rh(H2O)32+ and Cp*Ir(H2O)32+ (Cp* = η5-pentamethylcyclopentadienyl anion) by Cl-, Br-, I-, SCN-, py-CN (4-cyanopyridine), py-nia (nicotinamide), py (pyridine), TU (thiourea), and DMS (dimethylsulfide) was studied by stopped-flow spectroscopy at variable concentration, temperature, and pressure. The proton dissociation constants of the triaqua complexes, pKa = 6.47 (for rhodium) and pKa = 3.86 (for iridium), as well as the equilibrium constants for the formation of the dinuclear species (Cp*M)2(μ-OH)3+ were obtained by spectrophotometric titrations. The equilibrium constants K1 for the formation of the monosubstituted complexes Cp*M(H2O)2L+/2+, as determined for anionic and neutral ligands L, lie in the range 102−105 M-1 and follow the sequences K(Cl-) < K(Br-) < K(I-) and K(py-CN) < K(py-nia) < K(py) < K(TU,DMS). Assuming the Eigen−Wilkins mechanism for the formation of the monosubstituted complexes, second-order rate constants kf,1 were correcte...