Mechanistic and Energetic Details of Adduct Formation and σ-Bond Activation in Zr⁺(H₂)n Clusters

The formation of Zr + (H 2 ) n clusters ( n = 1−7) has been studied both experimentally and theoretically. Binding energies were determined via temperature-dependent equilibrium measurements, giving −Δ H 0 ° = 14.5 ± 0.3, 10.7 ± 0.2, 10.1 ± 0.3, 9.1 ± 0.5, 9.2 ± 0.5, 8.9 ± 0.6, and 8.5 ± 0.8 kcal/mol for n = 1−7, respectively. Both DFT(B3LYP) and MP2 calculations gave binding energies in excellent agreement with experiment. Zr + appears to insert into the first H 2 to form the dihydride as predicted by theory. The rate of insertion was observed to have a negative temperature dependence and a positive pressure dependence, suggesting a cluster-assisted insertion mechanism. Interestingly, both DFT and MP2 calculations suggest that the dihydride may “uninsert” to form a pure dihydrogen cluster with n between 5 and 7.