Influence of single impurity atoms on the structure, electronic, and magnetic properties of Ni₅ clusters

With a gradient-corrected density functional method, we have studied computationally the influence of single impurity atoms on the structure, electronic, and magnetic properties of Ni₅ clusters. The square-pyramidal isomer of bare Ni₅ with six unpaired electrons was calculated 23 kJ/mol more stable than the trigonal bipyramid in its lowest-energy electronic configuration with four unpaired electrons. In a previous study on the cluster Ni₄, we had obtained only one stable isomer with an O or an H impurity, but we located six minima for ONi₅ and five minima for HNi₅. In the most stable structures of HNi ₅, the H atom bridges a Ni-Ni edge at the base or the side of the square pyramid, similarly to the coordination of an H atom at the tetrahedral cluster Ni₄. The most stable ONi₅ isomers exhibit a trigonal bipyramidal structure of the Ni₅ moiety, with the impurity coordinated at a facet, (μ₃-O)Ni₅, or at an apex edge, (μ-O)Ni₅. We located four stable structures for a C impurity at a Ni₅ cluster. As for CNi₄, the most stable structure of the corresponding Ni₅ complex comprises a four-coordinated C atom, (μ₄-C)Ni₅, and can be considered as insertion of the impurity into a Ni-Ni bond of the bare cluster. All structures with C and five with O impurity have four unpaired electrons, while the number of unpaired electrons in the clusters HNi₅ varies between 3 and 7. As a rough trend, the ionization potentials and electron affinities of the clusters with impurity atoms decrease with the coordination number of the impurity. However, the position of the impurity and the shape of the metal moiety also affect the results. Coordination of an impurity atom leads to a partial oxidation of the metal atoms.