The substitution of water in the half-sandwich complexes Cp*Rh(H2O)(3)(2+) and Cp*Ir(H2O)(3)(2+)(Cp* = eta(5)-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, pK(a) = 6.47 (for rhodium) and pK(a) = 3.86 (for iridium), as well as the equilibrium constants for the formation of the dinuclear species (Cp*M)(2)(mu-OH)(3)(+) were obtained by spectrophotometric titrations. The equilibrium constants K-1 for the formation of the monosubstituted complexes Cp*M(H2O)(2)L+/2+, as determined for anionic and neutral ligands L, lie in the range 10(2)-10(5) 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 k(f,1) were corrected for outer sphere complex formation and for statistical factors to obtain rate constant k(i)' for the interchange step. The interchange rates k(i)' are nearly independent of the nature of L and very close to the rate of water exchange (k(ex)(Rh) (1.6 +/- 0.3) x 10(5) s(-1) and k(ex)(Ir) = (2.5 +/- 0.08) x 10(4) s(-1)). In all cases, i.e., for M = Rh and Ir and for L = anionic or neutral, the volume of the transition stale is larger than that of the triaqua species. These findings support the operation of an I-d mechanism without excluding a D mechanism. For a given ligand L, the substitution of another water molecule in the complexes Cp*M(H2O)(2)L+/2+ is by 1 order of magnitude slower than the substitution of the first water molecule in the triaqua species Cp*M(H2O)(3)(2+), as verified, for example, by k(f,l) = 2.61 x 10(3) and k(f,2) = 3.09 x 10(2) M-1 s(-1) for M = Ir and L = py.