The dynamic processes occurring in the triangular clusters [Re 3 (μ-H) 3 (μ-pz-κN 1 :κN 2 )(CO) 10 ] − (pz = pyrazolate, 4), [Re 3 (μ-H) 2 (μ-pydz-κN 1 :κN 2 )(CO) 10 ] − (pydz = pyridazine, 5) and [Re 3 (μ-H) 3 (μ-pydz-κN 1 :κN 2 )(CO) 10 ] (6), have been investigated by 1 H and 13 C NMR. In the pyrazolate derivative 4 the exchange (k ≈ 1 s −1 at 320 K) between the two carbonyls in the trans-diaxial apical positions has been recognized, and its activation parameters, in C 2 D 2 Cl 4 , have been determined (E a = 68(3) kJ mol −1 ). The exchange has been attributed to the rotation of the apical H 2 Re(CO) 4 fragment with respect to the Re 2 (μ-pz) basal fragment, a process analogous to that previously observed in the unsaturated dianion [Re 3 (μ-H) 3 (CO) 10 ] 2− (2) and in the monoanion [Re 3 (μ-H) 3 (μ–NC 5 H 4 –κN 1 :κC 6 )(CO) 10 ] − (1), containing a bridging orthometallated pyridine ligand. The vertex rotation was not observed in the pyridazine derivatives 5 and 6. An explanation for this different behaviour is presented, based on the view of the fluxional clusters 1, 2 and 4 as adducts between the apical and basal moieties (π- or σ-complexes). The ΔG # 312K value here measured in acetone for the σ-complex 4 (77 kJ mol −1 ) is very similar to that previously determined for the other σ-complex 1 (ΔG # 305K = 76 kJ mol -1 ) and significantly higher than the values measured for the π-complex 2 (ΔG # 260K = 60 kJ mol −1 ). The di-hydrido derivative 5 shows a different much faster dynamic process, namely the hopping of one hydride between the two lateral edges, affording a pseudo C s symmetry in the molecule. The process has been monitored by both 1 H and 13 C analysis, affording quite similar activation parameters (E a = 44(1) and 45(1) kJ mol −1 , respectively, in THF-d 8 ), that did not significantly change in CD 2 Cl 2 solution, in agreement with an intramolecular process.