Non-Integral Oxidation State

Niobium and Ta also form a number of polynuclear halides in which the metal has non-integral oxidation states (see text). 

Ruthenium provides more examples of dinuclear compounds in which the metal is present in a mixture of oxidation states (or in a non-integral oxidation state) than any other element. 

In this way we come to class III complexes, i.e. complexes in which the two sites are indistinguishable and the element has a non-integral oxidation state (delocalized valence). Usually one divides this class in two subclasses. In class IIIA the delocalization of the valence electrons takes place within a cluster of equivalent metal ions only. An example is the [NbgCli2] ion in which there are six equivalent metal ions with oxidation state + 2.33. In class IIIB the delocalization is over the whole lattice. Examples are the linear chain compound K2Pt(CN)4.Bro.3o. 3H2O with a final oxidation state for platinum of 2.30, and three-dimensional bronzes like Na WOg. 

In fact, one of the peculiar properties of the title class of compounds is the ability of the molecular entity to carry a charge which can vary considerably, also assuming fractional values in non-integral oxidation state (NIOS) salts. The different molecular oxidation states are reversibly accessible by chemical or electrochemical means. A good example is the case of fe(l,2-dithiolene) complexes of ds metal ions [such as Ni(II), Pd(II), Pt(II), Au(III)], whose charge can assume values typically ranging between —2 and 0 (see Scheme 4). 

It Is postulated that mixed-valence species or complex salts (12) formed as a result of this field Induced redox reaction control the semiconducting behavior of these films and these complex salts exist In a solid-state equilibrium with the simple 1 1 salt. Since non-integral oxidation states are common In solids, It Is difficult to predict exact stoichiometry In the equilibrium equation, but a likely equation for switching In Cu-TCNQ, for example, may Involve... 

The examples discussed in this chapter show that there are many different ways in which lattice-induced strain can be relaxed or aeeommodated, the partieular mode depending on the properties of the elements and the struetures involved. Many of these compounds have unusual properties resulting from non-integral stoichiometry, the presence of non-integral oxidation states, or the spontaneous breaking of symmetry, all of which are the direct consequence of lattice-induced strain. 

Table 43 Various Niobium and Tantalum Clusters with Non-integral Oxidation States...

Figure 1 Schematic representation of the band formation in a columnar stack structure. (A) Semiconductor based on integral oxidation state complex. (B) One-dimensional metal produced by partial oxidation and resulting in a non-integral oxidation state...

Table 3 Non-integral Oxidation State Metal Glyoximates and Benzoquinone Dioximates...

Segregated Stack Crystals, Non-Integral Oxidation State Metal Complexes... 

Table 9. Structures of metal benzoquinonedioximates in integral and non-integral oxidation states ... 

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