The water exchange reactions on Cr(NH3)(5)OH23+ Cr(NH2CH3)(5)OH23+, trans-Cr(NH3)(4)(OH2)(2)(3+), trans-Cr(NH3)(4)(NH2CH3)OH23+, Ru(OH2)(6)(3+), and Cr(OH2)(6)(3+) (the latter having been reinvestigated with an improved model) have been studied with ab initio quantum chemical calculations on the hydrated ions, whereby the solvent was treated as a dielectric continuum. The computations on RU(OH2)63+ have been performed because of similarities in the electronic structures of the transition states [cis-Cr(OH2)(5)...(OH2)(2)(3+)](double dagger) and [cis-Ru(OH2)(5)...(OH2)(2)(3+)](double dagger). The water exchange reaction on Cr(NH2CH3)(5)OH23+ proceeds with a dissociative activation, which is due to the bulky NH2CH3 ligands. As in the analogous case of Rh-III, the calculations did not allow the distinction of the dissociative interchange (Id) from the dissociative (D) mechanism. All of the other reactions follow the associative interchange (I,) mechanism with retention of the configuration. The M ... Q bond lengths in the transition states for this interchange mechanism increase with increasing basicity of the "spectator" ligands, whereas the effect of the ligand trans to the exchanging H2O molecules is much larger than that of the cis ligands. The increase of the M ... O bond lengths is paralleled by a corresponding variation of the experimental activation volumes. Finally, the calculations showed that putative square pyramidal pentacoordinated intermediates, for example Cr(NH3)(5)(3+) and Cr(NH2CH3)(5)(3+), exhibit activation energies for the rearrangement via a trigonal bipyramidal transition state that are considerably higher than those found for water addition. The situation is similar for Ru-III, and the activation energy for the corresponding process of Rh-III is higher. Thus, all thermally activated substitution reactions on Crm, Rum, and Rh-III proceed with retention of the configuration.