Theoretical and DFT Study of Atypical Pentanuclear [(Pr₃P)Ni]₅H (n = 4, 6, 8) Clusters: What are the Rules?

The structure, bonding, and properties of a series of atypical pentanuclear nickel hydride clusters supported by electron-rich Pr₃P of the type [(Pr₃P)Ni]₅H (n = 4, 6, 8; H, H, H) and their anionic models where Pr₃P are substituted by H^- (H', H', H') were investigated by density functional theory (DFT) calculations. All clusters were calculated to adopt a similar square pyramidal core geometry. Calculations indicate singlet ground states with small singlet-triplet gaps for H and H, similar to previously reported experimental values. Molecular orbital theory description clusters were investigated using the simplified model complexes [HNi]₅H^5- (n = 4, 6, 8; H', H', H'). The results show that there are three skeletal electron pairs (SEPs) in H'. The addition of two molecules of H₂ to form H' and H' results in the partial or full occupation of two degenerate MOs (e* set) that give two SEPs and one SEP, respectively. Indeed, the occupation of these low-lying weakly antibonding orbitals governs the multielectron chemistry available for these clusters and plays a role in their unique reactivity.