Polyhedra of metal

A peculiarity of the three-dimensional clusters, containing more-or-less regular polyhedra of metal atoms, is the existence of a central cavity whose dimensions, as shown in Table III, are a function of the particular geometry of the polyhedron. The existence of such a hole is confirmed by the formation of a large number of carbide derivatives. 

Carbonyl hydrides and carbonylate anions are obtained by reducing neutral carbonyls, as mentioned above, and in addition to mononuclear metal anions, anionic species of very high nuclearity have been obtained, often by thermolysis. These are especially numerous for Rh and in certain Rh, Rh and Rhi5 anions have structures conveniently visualized either as polyhedra encapsulating further metal atoms, or alternatively as arrays of metal atoms forming portions of hexagonal close packed or body... 

The resonating-valence-bond theory of metals discussed in this paper differs from the older theory in making use of all nine stable outer orbitals of the transition metals, for occupancy by unshared electrons and for use in bond formation the number of valency electrons is consequently considered to be much larger for these metals than has been hitherto accepted. The metallic orbital, an extra orbital necessary for unsynchronized resonance of valence bonds, is considered to be the characteristic structural feature of a metal. It has been found possible to develop a system of metallic radii that permits a detailed discussion to be given of the observed interatomic distances of a metal in terms of its electronic structure. Some peculiar metallic structures can be understood by use of the postulate that the most simple fractional bond orders correspond to the most stable modes of resonance of bonds. The existence of Brillouin zones is compatible with the resonating-valence-bond theory, and the new metallic valencies for metals and alloys with filled-zone properties can be correlated with the electron numbers for important Brillouin polyhedra. 

The stabilizing influence of small amounts of B (M/B > 0.25) in the voids of the metal host lattice varies with the mode of filling (partial or complete) of the interstitial, mostly O, sites and whether the compounds develop from the binary-intermetallic host lattice. The structures of B-rich compounds (M/B < 4) are mainly determined by the formation of regular, covalent B polyhedra (O, icosahedron) and the connections between them (B frame structures). Typical metal (M) borides therefore are found within a characteristic ratio of metal to boron 0.125 < M/B < 4. 

The same atom-centered polyhedra can be used to describe interstitial diffusion in all the many metal structures derived from both face-centered cubic and hexagonal closest packing of atoms. In these cases the polyhedra are centered upon a metal atom and all the tetrahedral and octahedral interstitial sites are empty. The hardening of metals by incorporation of nitrogen or carbon into the surface layers of the material via interstitial diffusion will use these pathways. 

Electronic absorption spectra of complexes formed between cyanine dyes 27 with K3[Fe(CN)g] and K2[Ni(CN)4] in acetonitrile were reported <2002MI557>. The overlap of bands produced by electronic transitions of the metals with the bands due to transition with the dyes did not allow for an unambiguous conclusion regarding the coordination polyhedra of iron and nickel in these complexes. 

Even though qualitative bonding descriptions of metal atom clusters up to six or seven atoms can be derived and in some cases correlated with structural detail, it is clear that most structures observed for higher clusters cannot be treated thus. Nor do the structures observed correlate with those observed for borane derivatives with the same number of vertices. Much of borane chemistry is dominated by the tendency to form structures derived from the icosahedron found in elemental boron. However, elemental transition metals possess either a close-packed or body-centered cubic arrangement. In this connection, one can find the vast majority of metal polyhedra in carbonyl cluster compounds within close-packed geometries, particularly hexagonal close-packing. 

The already voluminous review literature on clusters will be considered as a basis for this review. The topics treated so far are clusters in general (109, 241) and in connection with metal-metal bonding (30, 338, 380), special types of clusters like those with TT-acceptor ligands (231), hydrides (233), carbonyls (85, 86) or methinyl tricobalt enneacarbonyls (313, 317) properties of clusters like structures (56, 316), fluxionality (110), mass spectra (226), vibrational spectra (365), and redox behavior (292). Clusters have been treated in the context of metal carbonyls (3, 4), metal sulfur complexes (2, 381), and in relation to coordination polyhedra (297). Reviews... 

In all these cases, hydride formation corresponds to partial occupation of the available holes, reminiscent of multihole polyhedra behavior. Occupation of all available holes would require a limiting stoichiometry MH3, and would correspond to occupation of the unique hole in isolated polyhedra. This situation is known for some rare earth hydrides (see Table III). Significantly, transformation of the metallic dihydride to the trihydride occurs with a decrease in apparent metal-metal distances and with a large increase in resistivity. These observations indicate a salt-like character and the disappearance of metal-metal bonds (27). 

We have shown that A) interstitial hydride formation is observed only with partial occupation of the available holes, B) occupation of the interstitial position in isolated polyhedra is not observed, and C) occupation of all the holes in a close-packed lattice cancels metal-metal interactions. Therefore, it seems that interstitial hydrogen can be tolerated only in a fraction of the total number of holes, and with the weakening of metal-metal interactions. This behavior indicates strong competition between metal-metal and metal-hydrogen bonds, which is unique for hydrogen because interstitial carbon can stabilize some unusual arrangements in carbonyl carbide clusters (29, 30). 

As with the BWH2 polyhedra, it can be shown that the optimum number of bonding electron pairs should be n + 1 for other deltahedral clusters (i.e., those having entirely triangular faces) of metal atoms M (n 5). For example, [Os5(CO),2]2" should have 12 electrons available for cluster bonding and its metal atoms are arranged in the form of a trigonal bipyramid. 

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