Paramagnetic effects on the relaxation rate and shift difference of the O-17 nucleus of bulk water enable the study of water exchange mechanisms on transition metal complexes by variable temperature and variable pressure NMR. The water exchange kinetics of [Mn-II(edta)(H2O)](2-) (CN 7, hexacoordinated edta) was reinvestigated and complemented by variable pressure NMR data. The results revealed a rapid water exchange reaction for the [Mn-II(edta)(H2O)](2-) complex with a rate constant of k(ex) = (4.1 +/- 0.4) x 10(8) s(-1) at 298.2 K and ambient pressure. The activation parameters Delta H double dagger, Delta S double dagger, and Delta V double dagger are 36.6 +/- 0.8 kJ mol(-1), +43 +/- 3 J K-1 mol(-1), and +3.4 +/- 0.2 cm(3) mol(-1), which are in line with a dissociatively activated interchange (l(d)) mechanism. To analyze the structural influence of the chelate, the investigation was complemented by studies on complexes of the edta-related tmdta (trimethylenediaminetetraacetate) chelate. The kinetic parameters for [Fe-II(tmdta)(H2O)](2-) are k(ex) = (5.5 +/-0.5) x 10(6) s(-1) at 298.2 K, Delta H double dagger = 43 +/- 3 kJ mol(-1), Delta S double dagger = +30 +/- 13 J K-1 mol(-1), and Delta V double dagger = +15.7 +/- 1.5 cm(3) mol(-1), and those for [Mn-II(tmdta)(H2O)](2-) are k(ex) = (1.3 +/- 0.1) x 10(8) s(-1) at 298.2 K, Delta H double dagger = 37.2 +/- 0.8 kJ mol(-1), Delta S double dagger = +35 3 J K-1 mol(-1), and Delta V double dagger +8.7 +/-0.6 cm(3) mol(-1). The water containing species, [Fe-III(tmdta)(H2O)](-) with a fraction of 0.2, is in equilibrium with the water-free hexa-coordinate form, [Fe-III(tmdta)](-). The kinetic parameters for [Fe-III(tmdta)(H2O)](-) are k(ex) = (1.9 +/- 0.8) x 10(7) s(-1) at 298.2 K, Delta H double dagger = 42 +/- 3 kJ mol(-1), Delta S double dagger = +36 +/- 10 J K-1 mol(-1), and Delta V double dagger = +7.2 +/- 2.7 cm(3) mol(-1). The data for the mentioned tmdta complexes indicate a dissociatively activated exchange mechanism in all cases with a clear relationship between the sterical hindrance that arises from the ligand architecture and mechanistic details of the exchange process for seven-coordinate complexes. The unexpected kinetic and mechanistic behavior of [Ni-II(edta')(H2O)](2-) and [Ni-II(tmdta')(H2O)](2-) is accounted for in terms of the different coordination number due to the strong preference for an octahedral coordination environment and thus a coordination equilibrium between the water-free, hexaclentate [M(L)](n+) and the aqua-pentadentate forms [M(L')(H2O)](n+) of the Ni-II-edta complex, which was studied in detail by variable temperature and pressure UV-vis experiments. For [Ni(II)edta')(H2O)](2-) (CN 6, pentacoordinated edta) a water substitution rate constant of (2.6 +/- 0.2) x 10(5) s(-1) at 298. 2 K and ambient pressure was measured, and the activation parameters Delta H double dagger, Delta S double dagger, and Delta V dagger were found to be 34 +/- 1 kJ mol(-1), -27 +/- 2 J K-1 mol(-1), and +1.8 +/-0.1 cm(3) mol(-1), respectively. For [Ni-II(tmdta')(H2O)](2-), we found k = (6.4 +/- 1.4) x 10(5) s(-1) at 298.2 K, Delta H double dagger = 22 +/- 4 kJ mol(-1), and Delta S double dagger = -59 +/- 5 J K-1 mol(-1). The process is referred to as a water substitution instead of a water exchange reaction, since these observations refer to the intramolecular displacement of coordinated water by the carboxylate moiety in a ring-closure reaction.