Theoretical and DFT Study of Atypical Pentanuclear [( i Pr 3 P)Ni] 5 H n ( 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 i Pr 3 P of the type [( i Pr 3 P)Ni] 5 H n ( n = 4, 6, 8; H 4 , H 6 , H 8 ) and their anionic models where i Pr 3 P are substituted by H - ( H 4 ' , H 6 ' , H 8 ' ) 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 4 and H 6 , similar to previously reported experimental values. Molecular orbital theory description clusters were investigated using the simplified model complexes [HNi] 5 H n 5- ( n = 4, 6, 8; H 4 ' , H 6 ' , H 8 ' ). The results show that there are three skeletal electron pairs (SEPs) in H 4 ' . The addition of two molecules of H 2 to form H 6 ' and H 8 ' 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.