Metal clusters and cluster metals

Abstract

Abstract The desirable aim to create uniform naked metal clusters still fails. On the other hand, there exist many ligand stabilized clusters. If there was any possibility of eliminating the ligand sphere, a wide field of study, naked clusters of the same size, could be opened. Naked clusters with a magic number of metal atoms (full shell clusters) should have a special stability because of their complete outer geometry. Two-shell clusters of the type M55L12Clx (M = Au, Rh, L = PPh3, x = 6; M Rh, Ru, L = P(t-BU)3, x = 20, M = Pt, L = As(t-Bu)3, x = 20; M = Co, L = PMe3, x = 20) seem to be suited for studying those problems. As can be seen from HRTEM images and from other investigations, they have a cubic close packed structure (ccp) with the outer geometry of a cuboctahedron. The 12 vertices are coordinated by the 12 ligands L, and the chlorine atoms partially fill the gaps between the phosphine or arsine molecules. The nature of the figands makes the cluster molecules soluble in organic solvents. The transformation into water soluble compounds succeeds if PPh3 is exchanged by the hydrophilic derivative Ph2PC6H4SO3Na·2H2O. So, the cluster Au55(Ph2PC6H4SO3Na·2H2O)12Cl6 can be obtained as a very stable (in contrast to Au55(PPh3)12Cl16) water soluble compound which is completely dissociated in aqueous solution. Using a 100 kV electron beam, small crystalline areas of probably intact cluster molecules can be imaged by TEM. The giant cluster Pd56l(Phen)360190–200 (phen = phenanthroline) is obtained by reduction of palladium acetate with H2 in the presence of appropriate amounts of phen with subsequent oxidation by oxygen. The generation of naked clusters indeed succeeds if the M55L12Clx species are degraded in solution by 20 V d.c., using Pt electrodes. The ligand sphere together with the outer shell of metal atoms is eliminated, generating the inner naked M13 nuclei. These full-shell particles are metastable and build up novel structures. The structural principle can be described as an aggregation of 13 M13 particles, combined in a kind of close packed arrangement. The (M13)13 superclusters again act as building blocks and form particles [(M13)13]n. This construction principle is realized for all metals in this investigation, i.e. Au, Rh, Ru, Pt and Co.