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The reactions of the hydrido complexes [Re 2 H 2 (μ-H)(CO) 8 ] − ( 1 ) and [Re 4 H 2 (μ-H) 3 (CO) 16 ] − ( 2 ) with 0.5 equivalents of AgOTf (OTf − = CF 3 SO 3 − ) afforded rapidly and selectively the novel cluster anions [Ag(μ-H) 4 {Re 2 (μ-H)(CO) 8 } 2 ] − ( 3 ) and [Ag(μ-H) 4 {Re 4 (μ-H) 3 (CO) 16 } 2 ] − ( 4 ), respectively, formed by donor–acceptor interactions between the rhenium hydrides and the Ag + cation. 1 H and 109 Ag NMR spectroscopies showed that each Ag atom is surrounded by four hydrido ligands, bridging Re–Ag interactions. The Re 2 AgRe 2 cluster 3 therefore adopts the well known "bow-tie" geometry, constituted by two triangular clusters sharing the Ag vertex (spiro bis-cyclopropane). The Re 4 AgRe 4 anion 4 has instead an unprecedented spiro bis-cyclopentane-like structure, which is stable in solution only at temperatures lower than 220 K. The NMR investigation of the behavior in solution of these complexes revealed different intra- and inter-molecular dynamic processes, involving reversible fragmentation of the cyclic structures, with elimination of neutral or anionic hydrido-carbonyl fragments and interconversion among the different spiro derivatives. In particular, at temperature higher than 220 K the onset of a dissociative equilibrium was observed, involving the egress of a Re 2 (μ-H) 2 (CO) 8 fragment from 4 , with formation of the [Ag(μ-H) 4 {Re 2 (μ-H)(CO) 8 }{Re 4 (μ-H) 3 (CO) 16 }] − anion 5 , with a mixed spiro cyclopropane–cyclopentane-like structure, never previously observed. The fragmentation reaction was completely reversible on lowering the temperature. The mixed spiro derivative 5 was obtained (in mixture with the two symmetrical spiro derivatives 3 and 4 ) by either adding 1 equivalent of [Re 2 (μ-H) 2 (CO) 8 ] to 3 , or by reacting 1 equivalent of Ag + with a 1:1 mixture of the free hydrido complexes 1 and 2 . Intermolecular exchanges between the Re 4 H 5 fragments of 4 and 5 , and between the Re 2 H 3 fragments of 3 and 5 were revealed by 2D 1 H EXSY NMR experiments, which also showed exchange between the spiro bis-cyclopropane 3 and the free hydrido complex 1 . Furthermore, a higher binding ability towards Ag + of 1 with respect to 2 was proved, since 1 was found to be able to displace 2 from the spiro bis-cyclopentane adduct 4 and from the mixed spiro cyclopropane–cyclopentane 5 , leading to free hydrido complex 2 and the spiro bis-cyclopropane 3 .

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.

The reaction of the ansa-zirconocene meso-[C2H4(4,7-Me2Indenyl)2]ZrMe2 with B(C6F5)3 afforded, besides the expected ion pair with outward MeB ( C 6 F 5 ) 3 - anion, also a minor isomer (molar fraction 0.043) with the bulky methylborate anion in the inward site. 2D 1H EXSY experiments revealed exchange between the two isomers, at T > 320 K, through the migration of the MeB ( C 6 F 5 ) 3 - anion between outward and inward sites. The presence of free B(C6F5)3 fastened the exchange, suggesting B(C6F5)3 attack on the zirconium bound methyl ligand of the two ion pairs.