Metal cluster isomers

Figure 1. Equilibrium geometries of the two lowest energy isomers, Xy and Xy, of anionic, neutral, and cationic noble metal clusters. The roman numerals identify each clusters in Table 1.

The cluster [CpRhFe2(CO)6(n3-Te)2] (213) exists in solution as an equilibrating mixture of two isomers, one in which the rhodium atom is in a basal site of the Fe2RhTe2 square pyramid, and another in which it occupies the apical site.131 In several other mixed-metal clusters that also exhibit a... 

The terminal acetylene derivatives Osg(CO)i6(RCCH) (R = Me, Et, Ph) have been shown to react with CO to give Osg(CO)i7(RCCH), (89), in which the acetylene ligand is still intact and sits on the base of the capped pyramidal osmium polyhedron. This compound was found to convert by the action of heat to an isomer in which the proton from the acetylene has been transferred to the metal array. The process is accompanied by an opening up of the metal cluster (Scheme 34). 

Even alkali metal clusters are relatively simple species in terms of their electronic structure, as discussed byothersin this volume ( ). However, the nuclear motion (among many isomers for large clusters) becomes much less simple, as shown, for example, in the work of Lindsay in this volume (35). 

Of particular interest are the predicted magnetic moments and the question of whether or not isomers of transition metal clusters can be separated using inhomogeneous magnetic fields. To date, cluster isomers have only been detected via their different chemical reactivity (70-73). One would expect abnormally large magnetic moments for the Ih clusters if they had unusually high density of states at the Fermi level, as has been postulated for aluminum (74)-... 

If the octahedral and icosahedral 13-atom nickel clusters are representative, an inhomogeneous magnetic field is not a good tool for separating isomers of transition metal clusters. This result is reminiscent of the fact that geometry plays no strong role in the ionization potential variation of alkali clusters (80). However, many calculations must be made before we can be definitive here. 

Solution of the Kohn-Sham equations as outlined above are done within the static limit, i.e. use of the Born-Oppenheimer approximation, which implies that the motions of the nuclei and electrons are solved separately. It should however in many cases be of interest to include the dynamics of, for example, the reaction of molecules with clusters or surfaces. A combined ab initio method for solving both the geometric and electronic problem simultaneously is the Car-Parrinello method, which is a DFT dynamics method [52]. This method uses a plane wave expansion for the density, and the inner ions are replaced by pseudo-potentials [53]. Today this method has been extensively used for studies of dynamic problems in solids, clusters, fullerenes etc [54-61]. We have recently in a co-operation project with Andreoni at IBM used this technique for studying the existence of different isomers of transition metal clusters [62,63]. 

The interaction of TM metal clusters with the surface of MgO has been studied with both cluster [32-34] and slab [146] models, but only very small clusters have been considered, containing up to 4 to 5 atoms. The metals considered are Co, Ni, Cu, Pd, and Ag, and the various isomers studied are shown in Figure 3. They can be classified into three main groups planar or nearly planar structures with the cluster plane parallel to the MgO surface (Fig. 3 a-c), planar structures with the cluster plane normal to the surface (Fig. 3 d-f), and tetrahedral or distorted tetrahedral structures (Fig. 3 g-i). The results show that microclusters adsorbed on the regular MgO(OOl) surface do not necessarily tend to adhere to the surface with the largest... 

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