Stabilization of unusual oxidation states

In summary, it is now agreed that coordination concepts are valuable in explaining a wide variety of inorganic phenomena of theoretical and practical nature, such as the stabilization of unusual oxidation states, analytical implications of metal complexes, and the industrial use of com-plexing agents. Aside from the ammines and the hydrates, discussed mostly by Werner, there are many important types of coordination compounds, such as complex cyanides of heavy metals, metal carbonyls (formed in the catalysis of petroleum products and used to produce metals), and others. 

Equipment using high pressures with reactive gases has been developed mainly for fundamental scientific research. The stabilization of unusual oxidation states of transition elements is one area of research involving such equipment. Specific reaction vessels with external heating have been developed to synthesize oxides which require high temperatures (Fig. 7.11). Due to the large decrease of the mechanical properties of alloys with temperature, only a limited pressure-temperature domain is available (Fig. 7.12). 

Complexation prior to the electron transfer step is particularly attractive, since species may be trapped in a chemical environment which is conducive to rapid electron transfer and facilitates more unusual electron transfer mechanisms (e.g. stabilization of unusual oxidation states). Conducting polymers have been employed for determination of electro-inactive species on the basis of counter-ion replacement (86). 

Another characteristic feature of the hydrogen-reduced transition metal zeolites is their acidic properties, as demonstrated by their catalytic behavior (7). Naccache and Ben Taarit (8) gave IR evidence of the subsequent formation of protons on hydrogen-reduced Cu(II)-Y zeolite. Furthermore, transition metal ions have various oxidation states. Owing to the shielding effect caused by the zeolite network and the electric fields, the transition metal ions may be stabilized in unusual oxidation states—i.e. Ni(I) (9). 

By applying ESR spectroscopy, Kirmse and co-workers proved the existence of unusual oxidation states, as Au(ll) and Ag(ll) were stabilized by different thiacrowns <2002ZFA34, 2004ZFA2669>. 

The [X"+M6024](12 ") ion, its protonated form [X, +M6024H6](6 ") , and the [Xn+M9O32](10 ", ion are among those in which the heteroatom finds itself in an octahedron of oxygen atoms. The latter is known for only two cases, namely, those in which Xn+ = Mn4+ or Ni4+ and M = Mo, but these are of interest because the hetero ions are examples of unusual oxidation states stabilized by the unusual ligand. ... 

The ample diversity of properties that these compounds exhibit, is derived from the fact that over 90% of the natural metallic elements of the periodic table are known to be stable in a perovskite oxide structure and also from the possibility of synthesis of multicomponent perovskites by partial substitution of cations in positions A and B giving rise to compounds of formula (AjfA i- )(ByB i-J,)03. This accounts for the variety of reactions in which they have been used as catalysts. Other interesting characteristics of perovskites are related to the stability of mixed oxidation states or unusual oxidation states in the structure. In this respect, the studies of Michel et al. (12) on a new metallic Cu2+-Cu3+ mixed-valence Ba-La-Cu oxide greatly favored the development of perovskites exhibiting superconductivity above liquid N2 temperature (13). In addition, these isomorphic compounds, because of their controllable physical and chemical properties, were used as model systems for basic research (14). 

Stability of the Cr is unusually high, and may be due to complex formation and ligand stabilization of this oxidation state. 

Seppelt, K., "Stabilization of Unusual Oxidation and Coordination States by the Ligands OSF5, OSeFs, and OTeFs," Angew. Chem., Int. Ed. Engl. 1982, 27, 877. 

The di- and triborolyl ligands tend to i -coordination in sandwichforming reactions. There is a clear-cut tendency for stacking processes followed by the formation of multidecker species and often stabilization of the unusual oxidation states of the transition metals. The route to the linked sandwich and multidecker complexes is attractive for materials chemistry. Thia- and azaborolyl organome-tallic chemistry follows the same trends, although in the azaborolyl complexes the i -rather than i -coordination is sometimes realized. Moreover, coordination via the boron atom is known. In the B, N, Si-heterocycles, the heteroring is j " -coordinated. 

The investigation of plutonium chemistry in aqueous solutions provides unique challenges due in large part to the fact that plutonium exhibits an unusually broad range of oxidation states -from 3 to 7-and in many systems several of these oxidation states can coexist in equilibrium. Following the normal pattern for polyvalent cations, lower oxidation states of plutonium are stabilized by more acidic conditions while higher oxidation states become more stable as the basicity increases. 

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