Palladium clusters geometry

Monodentate bridging ligands snch as SO2, PPh2, and CO usually lead to nuclearities higher than two. Several examples of homo- and heteronuclear, three or four-member palladium clusters that adopt a variety of geometries are known. 

Trimerization of acetylene into benzene is known to proceed on a single crystal of palladium and on fine particles of palladium dispersed on a substrate. Among them, Pd (111) surface is the most active for the trimerization because the surface has a site with three fold symmetry at which three acetylene molecules are adequately adsorbed for the trimerization into benzene geometry-controlling reaction. In the trimerization involving a palladium cluster, it is expected that the catalytic activity of the trimerization begins to appear at a critical size as the cluster size increases because a small cluster does not have such an active site with three-fold symmetry but a larger cluster should have. 

Balch et al. have described the preparation of (18 equation 10).107 This unusual cluster contains a linear array of palladium atoms, each palladium atom having a square planar geometry and Pd—Pd 2.592 A. 

In the present paper we have presented some studies concerning the adsorption of acetylene on copper (100) surfaces, and the adsorption of ethylene on the (100) surfaces of nickel, palladium and platinum. In all these studies we have used a cluster with the same shape and size. Despite the limited size of the clusters used, some very interesting features of the systems adsorbate - metal surfaces were determined, namely adsorbate geometries, adsorption energies and vibrational frequencies. 

The tetrahedral —> butterfly rearrangement is quite common in cluster chemistry but few examples are reversible. One example of a reversible process involves the interconversion of Pd4(CO)5(PBu 3)4 to Pd4(CO)6(PBu 3)4 (Scheme What is perhaps surprising about this reaction is that Pd4(CO)5(PBu"3)4 has a butterfly geometry whereas the cluster with the additional CO ligand has a tetrahedral palladium skeleton. This is clearly in contrast with the usual observation that tetrahedral clusters have two fewer valence electrons than butterfly clusters i.e. 60 and 62 cluster valence electrons (CVE), respectively). This anomaly is a consequence of the capacity of the palladium atom to form stable compounds with 14 and 16 valence electrons and as such Pd4(CO)s(PBu 3)4 and Pd4(CO)e(PBu"3)4 do not conform to the usual total electron counts for compounds that obey the EAN rule but have 54 and 56 electrons, respectively. 

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