Five-atom clusters, reactivity

Other Clusters. The cobalt clusters with six and seven atoms are the ones which differ most dramatically from the corresponding nickel clusters. The eight and nine atom clusters are also much less reactive for cobalt than for nickel, while the Co g cluster is as strongly reactive as the Ni.g cluster. The experimental result for the four and five atom cobalt clusters is not completely clear to us. For the five atom cluster it is on the one hand claimed that it has reacted almost completely but on the other hand the relative reactivity is given in a figure as smaller than for the unreactive COg cluster (1). For all these clusters we have obtained some preliminary results, which are described below. 

Wc will first examplify the above principles on a six atom and a seven atom cluster. These two clusters are the most striking examples of the differences beween nickel and cobalt clusters. These cobalt clusters are the two least reactive of all the cobalt clusters, whereas for nickel these two clusters are as reactive as clusters of other sizes. In the final subsection we will discuss also the four, five, eight, nine and ten atom clusters of which in particular the eight and nine atom clusters have a markedly lower reactivity for cobalt than for nickel. All calculations described in this section used a one- or two- electron ECP level description of the metal atoms. No all-electron atoms were included. 

The Six Atom Cluster. For the six atom cluster two different structures are of interest. There is a nearly planar C structure, with one atom above a five-membered ring, and there is the octahedral (Oj ) structure. If all atoms are in the d s state the C structure is preferred and the wavefunction is a closed shell singlet. This does not mean that Ni is non-reactive to since the energy difference to the Oj structure is only 10 Real/mol, and the 0, structure has a triplet ground state with two open shells. Since the exothermicity is most certainly larger than 10 kcal/mol, this structural change is expected to occur during the interaction of with Nig. 

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