Electron high nuclearity clusters

Syntheses, crystallization, structural identification, and chemical characterization of high nuclearity clusters can be exceedingly difficult. Usually, several different clusters are formed in any given synthetic procedure, and each compound must be extracted and identified. The problem may be compounded by the instabiUty of a particular molecule. In 1962 the stmcture of the first high nuclearity carbide complex formulated as Fe (CO) C [11087-47-1] was characterized (40,41) see stmcture (12). This complex was originally prepared in an extremely low yield of 0.5%. This molecule was the first carbide complex isolated and became the foremnner of a whole family of carbide complexes of square pyramidal stmcture and a total of 74-valence electrons (see also Carbides, survey). 

High nuclearity clusters [Au(AuPR3) ]JC+ with central gold atoms adopt two types of structure those with 12n + 18 electrons have essentially spherical structures, whereas those with 12n + 16 electrons have the peripheral atoms on the surface of a toroid. 

This high nuclearity cluster displays an extensive redox activity, Figure 55,25 in that it undergoes four successive one-electron reductions with characteristics of chemical reversibility (E%Lp- — -0.77 V 7-/8- = 1-06 V E L/9- =-1.33 V 9-/10- = -1.61 V) and a one-electron oxidation complicated by slow degradation of the corresponding pentaanion ( 6-/5- = —1.33 V). 

More elaborate electron-counting procedures have been established, and these are usually more appropriate for the rationalisation of high-nuclearity clusters. An approach which was originally applied to complex boranes has been extended to carbonyl clusters and a number of topological theories have been described. Although there is some way to... 

A simple, but nevertheless very interesting electron counting rule for close-packed high nuclearity clusters has been described by Teo127a. The formula N = (mb + rms)n establishes a connection between the total number of electrons (N), the bulk atoms mb, and the surface atoms ms. The rule is based on an extended Hume-Rothery rule and works quite well for all kinds of close-packed clusters up to the Pt38 cluster of Chini61. ... 

Mingos has recently developed an electron counting procedure (which may be described as Polyhedral Inclusion ) which is based on the formal division of the high nuclearity cluster into an internal (encapsulated) polyhedron and an external (surface) polyhedron128. Three sub-classes of close-packed cluster have been identified ... 

Table 6. Characteristic electron counts (A) for interstitial moieties in high nuclearity clusters...

The samples were quite stable under the electron beam and their images are consistent with a cubic array of high nuclearity clusters occupying most of the sodalite cages, separated by nearly empty a-cages. However, Cd clusters located not only in the sodalite cages but also in the 8-rings of zeolites result in different properties. [43]... 

A third electron counting scheme, developed by Mingos from P.S.E.P.T., which can be used to rationalize metal framework geometries when the structure is known, is based on the premise that the total electron count for a condensed polyhedron is equal to the sum of the characteristic electron counts for the parent polyhedra (A) and (B) minus the electron count of the atom, pair of atoms, or face of atoms common to both polyhedra . This method is particularly useful for high nuclearity clusters, which are the topic of this discourse, and some examples of its use are shown in Fig. 2. However, as the nuclearity of the cluster core increases further and the band structure develops, ambiguities in this scheme become apparent. 

The electronic, optical, and magnetic properties of metal clusters are of great current interest, but these properties have been little studied with very mixed -metal clusters. This is to some extent a reflection of the difficulty of preparing high-nuclearity examples many of these interesting properties become important upon increasing cluster size. The limited magnetic studies to date are... 

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