Metal-oxide cluster compounds

Perspectives for fabrication of improved oxygen electrodes at a low cost have been offered by non-noble, transition metal catalysts, although their intrinsic catalytic activity and stability are lower in comparison with those of Pt and Pt-alloys. The vast majority of these materials comprise (1) macrocyclic metal transition complexes of the N4-type having Fe or Co as the central metal ion, i.e., porphyrins, phthalocyanines, and tetraazaannulenes [6-8] (2) transition metal carbides, nitrides, and oxides (e.g., FeCjc, TaOjcNy, MnOx) and (3) transition metal chalcogenide cluster compounds based on Chevrel phases, and Ru-based cluster/amorphous systems that contain chalcogen elements, mostly selenium. 

There is now not only a great number but also a great variety of metal atom cluster compounds. In this essay I should like to discuss the differences between those that have metal atoms in a relatively high mean oxidation state (+2 to +4, and even, in rare cases, a bit higher) and those with metal atoms in oxidation states in the range -1 to +1. To keep the discussion within reasonable limits I shall restrict it almost exclusively to clusters consisting of only two or three metal atoms. I eschew the pedantic assertion that two atoms do not a cluster make. 

Okuhara, Mizuno, and Misono report the catalytic properties of heteropoly compounds as exemplified by H,PWl3O40 and the anion [PW,2O40p. Some of these compounds are strongly acidic, and some have redox properties the large-scale applications involve acid-catalyzed reactions. The heteropoly compounds are metal oxide clusters, used as both soluble and solid catalysts. Their molecular character provides excellent opportunities for incisive structural characterization and for tailoring of the catalytic properties. Physical properties also affect catalytic performance. Catalysis sometimes occurs on the surface of the solid material, and sometimes it occurs in the swellable bulk. 

The last chapter by Michalkova et al. presents a review of the experimental and theoretical data on nerve agent interactions with different surfaces. Particular attention is given to molecular simulations of interaction and decomposition of phosphoroorganic compounds on various metal and metal oxide clusters. 

Both of these elements show a marked tendency to form metal atom cluster compounds in their lower oxidation states. The best known are oxo and halide cluster complexes. 

Niobium and tantalum, though metallic in many respects, have chemistries in the V oxidation state that are very similar to those of typical non-metals. They have virtually no cationic chemistry but form numerous anionic species. Their halides and oxide halides, which are their most important simple compounds, are mostly volatile and are readily hydrolyzed. In their lower oxidation states they form an extraordinarily large number of metal-atom cluster compounds. Only niobium forms lower states in aqueous solution. The oxidation states and stereochemistries (excluding those in the cluster compounds) are summarized in Table 26-B-l. 

Research on small transition-metal clusters was reviewed in a comprehensive way in 1986 [14] and again in 2002 (taking then in also lanthanide clusters) [15]. Moreover, small metal oxides and, less detailed, metal oxide clusters of the transition metals were recently reviewed [16]. Herein we restrict the discussion to some examples, most of them involving the matrix isolation technique this article is not a comprehensive overview of research in this field. Different from the three aforementioned review articles, our examples will also include main-group element compounds. 

A particularly significant part of rhenium chemistry involves cluster compounds in which there is metal—metal bonding. This chemistry centers largely around the +3 oxidation state. 

Many novel cluster compounds have now been prepared in this way, including mixed metal clusters. Further routes involve the oxidative fusion of dicarbon metallacarborane anions to give dimetal tetracarbon clusters such as (103) and (104) O (jjg insertion of isonitriles into inetallaborane clusters to give monocarbon meiallacarboranes such as (105) and the reaction of small ii/t/o-carboranes with alane adducts such as Et3NAlH3 to give the commo species (106) ... 

The known halides of vanadium, niobium and tantalum, are listed in Table 22.6. These are illustrative of the trends within this group which have already been alluded to. Vanadium(V) is only represented at present by the fluoride, and even vanadium(IV) does not form the iodide, though all the halides of vanadium(III) and vanadium(II) are known. Niobium and tantalum, on the other hand, form all the halides in the high oxidation state, and are in fact unique (apart only from protactinium) in forming pentaiodides. However in the -t-4 state, tantalum fails to form a fluoride and neither metal produces a trifluoride. In still lower oxidation states, niobium and tantalum give a number of (frequently nonstoichiometric) cluster compounds which can be considered to involve fragments of the metal lattice. 

---