Metallic atom clusters

Rearrangements of clusters, i.e. changes of cluster shape and increase and decrease of the number of cluster metal atoms, have already been mentioned with pyrolysis reactions and heterometallic cluster synthesis in chapter 2.4. Furthermore, cluster rearrangements can occur under conditions which are similar to those used to form simple clusters, e.g. simple redox reactions interconvert four to fifteen atom rhodium clusters (12,14, 280). Hard-base-induced disproportionation reactions lead to many atom clusters of rhenium (17), ruthenium and osmium (233), iron (108), rhodium (22, 88, 277), and iridium (28). And the interaction of metal carbonyl anions and clusters produces bigger clusters of iron (102, 367), ruthenium, and osmium (249). 

Nine-metal arrays in conventional coordination clusters are the confacial bioctahedron and two M4 cubanes linked through a non-cluster metal atom among naked metal clusters the M9 polyhedron is the tricapped trigonal prism or the monocapped square antiprism, and in gold clusters the metal array is the centred cube, the centred crown, or a fragment of the centred icosahedron. 

The favorable bonding situation between hydrogen and cluster metal atoms also provides some driving force for the many observed HX oxidative additions to clusters ranging from C- H cleavage to cluster expansion with... 

The structure of the adsorbed clusters has been partially optimized. " """ In two of the adsorption geometries, denoted A and B (Fig. 22), the interaction is primarily between the cluster metal atoms and the surface oxygen atoms. In geometry B each metal atom is placed on-top of the surface oxygen centers this corre-... 

The total number of electrons associated with the valence shell of cluster metal atoms corresponds to the sum of the metal valence electrons, v, plus those provided by the ligands, L. 

By considering the oxidation states of metal atoms, reactions may be roughly classified into two groups Reactions involving changes in the oxidation states of cluster metal atoms and the reactions in which metal oxidation states remain unchanged. 

The same size effects appear to prevent the addition of other electrophiles to cluster metal atoms. The attack of other nucleophiles normally leads to their attachment to the ligand sphere at the high electron density point. Thus for instance the reaction of FeCo2(/i3-S)(CO)9 with the electrophile Cr (CO)5 which attacks at the /13-sulfur ligand may be explained. 

Complexes containing several interacting metal atoms are named clusters. Metal atoms in clusters bound by chemical forces are localized at rather short distances for efficient overlap of their valent orbitals. This overlap results in the system of molecular delocalized states (Fig. 17.2). 

PPha, pyridine) organic groups (olefines, aromatic derivatives) and also form other derivatives, e.g. halides, hydrides, sulphides, metal cluster compounds Compounds containing clusters of metal atoms linked together by covalent (or co-ordinate) bands, metaldehyde, (C2H40) ( = 4 or 6). A solid crystalline substance, sublimes without melting at I12 1I5" C stable when pure it is readily formed when elhanal is left in the presence of a catalyst at low temperatures, but has unpredictable stability and will revert to the monomer, ft is used for slug control and as a fuel. 

Herrmann A, Leutwyler S, Schumacher E and Woste L 1978 On metal-atom clusters IV. Photoionization thresholds and multiphoton ionization spectra of alkali-metal molecules Hel. Chim. Acta 61 453... 

Morse M D 1986 Clusters of transition-metal atoms Chem. Rev. 86 1049... 

There are only a few weU-documented examples of catalysis by metal clusters, and not many are to be expected as most metal clusters are fragile and fragment to give metal complexes or aggregate to give metal under reaction conditions (39). However, the metal carbonyl clusters are conceptually important because they form a bridge between catalysts commonly used in solution, ie, transition-metal complexes with single metal atoms, and catalysts commonly used on surfaces, ie, small metal particles or clusters. 

Note that we always speak of C q and C70 as a molecule and not a cluster. We reserve the word cluster to refer to units composed of several fullerenes and metal atoms. 

The structures observed in the mass spectra of fullerene molecules covered with alkaline earth metals, as described in the previous section, all seem to have a geometric origin, resulting in particularly stable cluster configurations every time a highly symmetrical layer of metal atoms around a central fullerene molecule was completed. When replacing the alkaline earth metals by an alkali metal (i.e., Li, Na, K, Rb, or Cs), a quite different situation arises. 

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