Mechanisms for water exchange from the bioxo-capped M-M-bonded trinuclear clusters, [M-3(mu(3)-O)(2)(mu-O2CCH3)(6)(OH2)(3)] (2+) [M = Mo(IV) and W(IV)], were investigated using high-pressure O-17 NMR and compared to our previous work on a similar Rh(III) trimer. Reaction rates decrease by more than a factor of 2 when pressure is increased from 6 to 250 MPa for the Mo(IV) trimer, while exchange rates increase by less than a factor of 1.2 (10-229 MPa) for the W(IV) trimer. From the pressure dependence of the reaction rate, activation volumes (Delta V-double dagger) were calculated to be Delta V-double dagger = +8.0 (+/- 0.4) cm(3) mol(-1) and Delta V-double dagger = -1.9 (+/- 0.2) cm(3) mol(-1) for the Mo(IV) cluster and W(IV) cluster, respectively, which is the largest difference (similar to 10 cm(3) mol(-1)) in activation volumes for any pair of 4d-5d (and 3d-4d) transition metal species located within the same group of the periodic table. If we interpret these activation volumes in terms of Swaddle's semiempirical model, which he established for simple octahedral monomers (Associative (A) = Delta V-double dagger approximate to -13; Interchange (I) = Delta V-double dagger approximate to 0; or Dissociative (D) = Delta V-double dagger approximate to + 13), our results suggest that water exchange follows a dissociative-interchange (Id) mechanism for the Mo(IV) cluster and an associative-interchange (Ia) activation mode for the W(IV) trimer. These volumes exhibit a unique changeover in the water-exchange mechanism despite considerable similarities in molecular structure and reactivity. This changeover could provide a standard for computational simulations of ligand-exchange pathways in molecules that are more complicated than monomers.