Density functional study of Ni6 clusters containing impurity atoms

For the reference system, bare Ni6, we calculated two stable octahedron-like isomers in D3d and Cs symmetry, each with eight unpaired electrons. The latter structure features a broken Ni-Ni bond. We determined only few stable locations of the impurity atoms H, C, or O at the cluster Ni6. The binding energies of these impurities to Ni6 increase from 305 kJ/mol for H in a bridge position, to 419 kJ/mol for O in a three-fold position, and 682 kJ/mol for C in a four-fold position. Calculated detachment energies of Ni atoms from Ni6 and Ni6+ agree well with experiment. Ligand detachment energies of XNi6 suggest that X = H binds weaker to the cluster than a Ni atom, X = C or O bind stronger. The clusters XNi6 show some similarities to their smaller congeners XNi4 and XNi5, but also notable differences which emphasize the molecule-like individual character of these cluster compounds.