Formal oxidation state copper

In addition to the above oxides M2O, M2O2, M4O6, MO2 and MO3 in which the alkali metal has the constant oxidation state 4-1, rubidium and caesium also form suboxides in which the formal oxidation state of the metal is considerably lower. Some of these intriguing compounds have been known since the turn of the century but only recently have their structures been elucidated by single crystal X-ray analysis. Partial oxidation of Rb at low temperatures gives RbeO which decomposes above —7.3°C to give copper-coloured metallic crystals of Rb902 ... 

Oxidation state is a frequently used (and indeed misused) concept which apportions charges and electrons within complex molecules and ions. We stress that oxidation state is a formal concept, rather than an accurate statement of the charge distributions within compounds. The oxidation state of a metal is defined as the formal charge which would be placed upon that metal in a purely ionic description. For example, the metals in the gas phase ions Mn + and Cu are assigned oxidation states of +3 and +1 respectively. These are usually denoted by placing the formal oxidation state in Roman numerals in parentheses after the element name the ions Mn- " and Cu+ are examples of manganese(iii) and copper(i). 

CuO, the mineral tenorite, and AgO are well known but their structures are quite different. More importantly the valence states in these two compounds are quite different. In CuO, the copper is formally in the divalent state, whereas in AgO, there exist two types of silver atoms, one in formal oxidation state 1+, the other in 3+. These two silver ions also possess strong covalent character. PdO and CuO, however, have similar crystal structures based on chains of opposite edged-shared, square-planar M04 groups. 

The incorporation of Cu ions in the perovskite structure is known for only a few examples since this particular structure is normally stabilized by or requires a B atom in a high formal oxidation state such as Ti4+ in BaTiOs, or Rhs+ in LaRhOs. Further, since Cu can not be readily stabilized in its Cu(m) state, and is unknown in the tetravalent state, the simple formation of ternary compounds such as LaCuOg or BaCuOs is not expected. Even in the K2NiF4 structure, the stabilization of Cu4+ as in Ba2Cu04 is not expected, but the formation of a stable Cu(II) state is a distinct possibility, as in La2Cu04. Copper(II), however, has been introduced in the doubled-or tripled-perovskite structure. Examples of these, which include structural distortions from cubic symmetry, are listed ... 

Another closely related family of superconductors is represented by the formula TlBa2Can 1Cun02n+3 (n = 1, 2, 3, 4, 5). They contain single layers of T1 and O atoms that separate the perovskite-like Ba-Cu-Ca-O slabs (24)(25)(26)(27) (Figure 6). Distortions in the Tl-O sheets are also found in these compounds (26)(27). Note that if these phases were stoichiometric, copper would always have a formal oxidation state of greater than two. Therefore, the chemical composition of this homologous series allows the existence of holes in the copper-oxygen sheet. 

A new series of mixed-valence-state complexes of the general formula [Cu3(rt-Bu2Dtc)6] [MBr3] 2 (M = Zn, Cd, Hg) with copper in the formal oxidation state of +2 (M = Zn, Cd, Hg) have been described (172). 

Such J-mctals as Cu(I) [but not Cu(II)], form a variety of compounds with ethenes, for example [Cu(C2H4)(H20)2]C104 (from Cu, Cu2+, and C2H4) or Cu(C2H4)(bipy)+. It is necessary to mention that, of all the metals involved in biological systems, only copper reacts with ethylene [74b]. Such homoleptic alkene complexes can be useful intermediates for the synthesis of other complexes. The olefin complexes of the metals in high formal oxidation states are electron deficient and therefore inert toward electrophilic reagents. By contrast, the olefin complexes of the metals in low formal oxidation states are attacked by electrophiles such as protons at the electron-rich metal-carbon a-bonds [74c]. 

This type of active site is also known as a mixed-valence copper site. Similarly to the type 3 site, it contains a dinuclear copper core, but both copper ions have a formal oxidation state of +1.5 in the oxidized form. This site exhibits a characteristic seven-line pattern in the EPR spectra and is purple colored. Both copper ions have a tetrahedral geometry and are bridged by two sulfur atoms of two cysteinyl residues. Each copper ion is also coordinated by a nitrogen atom from a histidine residue. The function of this site is long-range electron transfer, and it can be found, for example, in cytochrome c oxidase [12-14], and nitrous oxide reductase (Figure 5.1 e). 

Electron superconductors, such as Nd2 Ce CuO, do not have oxygen vacancies and the formal oxidation state of copper in these compounds is less than 2. Conduction is by the electron current. 

It is no surprise when number crunching operations in a computer show that the conduction bands in Ba(Pb,Bi)03 and the copper oxide superconductors are very high in oxygen 2p character. This is entirely expected on the chemical grounds of very high covalency. There is no reason to pervert the system of formal oxidation states by suggesting that O 1 is present in these systems. 

Fig. 1. Copper K-edge XAS spectra of Cu20, CuO and NaCu02 whose formal oxidation states are Cu(I), Cu(II) and Cu(III). The differences between the monovalent spectrum and the others are plotted, showing that the A peak (Is2 3d10 4p° - Is1 3d10 4p ) is very well separated from the other features in the spectra...

The copper atoms in the vast majority of the clusters can be assigned a formal charge of +1, while the chalcogen ligands are formally viewed as E or RE groups. Some of the selenium-bridged species, however - and nearly all copper telluride clusters - form nonstoichiometric compounds that display mixed valence metal centers in the formal oxidation states 0 and +I or +I and +11. These observations correlate with those made for the binary phases CU2S, Cu2 xSe, and Cu2- Te [38-40]. 

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