The Oxidation States of Copper

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In the periodic table of the elements, copper is listed in group 11, together with silver and gold. Copper, as a late transition element, occurs in a range of oxidation states (Cu(0), Cu(I), Cu(II), Cu(III), and Cu(IV)), and the ions readily form complexes yielding a variety of coordination compounds. Oxidation states I, II, and III 

In inorganic and coordination chemistry, the Cu(II) state is the most abundant one, and is regarded as more stable than the Cu(II) state under normal conditions [32]. Although numerous examples of Cu(I) coordination complexes are known, their chemistry is rather limited and they are readily oxidized to Cu(II) species [32]. Of the common oxidation states, compounds derived from copper(III) are rare, with only 30-40 reported examples [32]. Despite the small number of isolated Cu(III) compounds, however, organocopper] 111) species have been proposed as important intermediates in copper-mediated organic reactions (Chapts. 4 and 10). 

As a consequence of its electronic configuration, a variety of coordination numbers and geometries have been observed for copper(I) compounds, especially for inorganic representatives (see Fig. 1.3) [32]. In the organometallic chemistry of copper, the linear and trigonal coordination geometries in particular, though distorted towards T-shaped, are frequently encountered. 

there is ample evidence that organocopper compounds are usually highly 

Trigonal pyramidal See-saw Square planar Square pyramidal Trigonal bipyramidal Fig. 1.3. Various coordination geometries of copper(l) compounds. 

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When the radicals have p hydrogens, alkenes are formed by a process in which carbocations are probably bypassed. Instead, the oxidation and the elimination of a proton probably occur in a single step through an alkylcopper species. The oxidation state of copper in such an intermediate is Cu(III). 

Cu,Zn superoxide dismutase. Essentially, these observations support a stepwise one-electron model again. Interestingly, the oxidation state of copper does not change during the catalytic reaction, i.e. the sole kinetic role of the histidine coordinated metal center is to alter the electronic structures of the substrate and 02 in order to facilitate the electron transfer process between them. 

The copper to oxygen distance will vary according to the oxidation state of copper, the degree of covalency in the bond, and the geometry about the copper atom. The data given below are the averages of reported (and theoretical "ionic") radii obtained from... 

Neither the calcium nor the strontium compound is a superconductor. A possible reason may be that the oxidation state of copper is equal, or very close, to 2+. 

Redox titrations proved to be the most reliable way to measure the oxidation state of copper and thereby deduce the oxygen content of YBa2Cu30A.25 An iodometric method includes two experiments. In Experiment A, YBa2Cu30A is dissolved in dilute acid, in which Cu3+ is converted into Cu2+. For simplicity, we write the equations for the formula YBa2Cu307, but you could balance these equations for x =A 7.26... 

The d-d absorption of the copper complex differs in each step of the catalysis because of the change in the coordination structure of the copper complex and in the oxidation state of copper. The change in the visible spectrum when phenol was added to the solution of the copper catalyst was observed by means of rapid-scanning spectroscopy [68], The absorbance at the d-d transition changes from that change the rate constants for each elementary step have been determined [69], From the comparison of the rate constants, the electron transfer process has been determined to be the rate-determining step in the catalytic cycle. 

Reactions (4.22)-(4.24) are examples of oxidation reactions that will lead to copper oxidation. Oxidation reactions are reactions that increase the oxidation state of a species such as a metal. For example, in reaction (4.23), the oxidation state of copper is raised from 0 to 2+. Consequently, electrons are a product of... 

The oxidation states of copper, sih er, and gold represented in their important compounds are shown in tlie diagram below. 

The Cu+ ion, for instance, has a Sd structure. It is diamagnetic in all its compounds. The Cu + ion, however, has a 3d structure and its paramagnetic moment corresponds to the presence of one free electron. Thus the oxidation state of copper in one of its compounds can be found by measuring its paramagnetic susceptibility. Indeed the magnetic criterion of unpaired spin is so well established that, unless a compound is paramagnetic, the existence of an unpaired electron can be discounted. 

Laccase contains four copper atoms and catalyzes the four-electron reduction of dioxygen to water. X-Ray absorption edge spectroscopy has been used to determine the oxidation states of copper in Rhus vernicifera laccase, following the reaction of the reduced enzyme with dioxygen (202). This study included the incorporation of mercury(II) in the Type 1 copper site (see Section IV,B). The results demonstrate that the Type 2/Type 3 trinuclear copper site, as found in ascorbate oxidase (103), represents the minimal active site required for the multielectron reduction of dioxygen. 

The mechanism of this eflFect is not known. Hill and Starcher (49) postulated that reduction of copper from its divalent (cupric) state to its monovalent (cuprous) state accounted for the impaired absorption of copper in the presence of ascorbic acid they produced the same effect with another reducing agent, dimercaptopropanol (BAL). This explanation has been accepted by others (56), although the oxidation state of copper for maximum intestinal absorption has not been established. An intramucosal competition of ascorbic acid for sulfhydryl sites on metallo-thioneins was demonstrated (57). If this ligand has any regulatory role in copper uptake, this alternative mechanism of ascorbic acid-copper interaction could explain the mechanism. Experimental confirmation of an ascorbic-acid-induced inhibition of copper absorption in the human intestine has not been presented. 

Because the silicon/oxygen (Si O) ratio is 4 10, or 1 l, this mineral should have an infinite sheet structure with the repeating unit Si205. The oxidation states of Si and O are -1-4 and —2, as usual, and that of calcium (Ca) is -1-2. For the total oxidation number per formula unit to sum to 0, the oxidation state of copper (Cu) must be +2. 

The oxidation state of copper in biological systems is -1-1 or +2. Copper(III) is found in inorganic systems and may occur as a reaction intermediate in galactose oxidase, laccase (a plant enzyme), and perhaps other enzymes. The coordination number of copper in these enzymes ranges from two to six and occasionally higher. 

Very little has appeared in the literature concerning the optimum conditions for the synthesis of Ba2YCu307 and only the above study has attempted to determine the degree and nature of the oxidation state of copper. The preparative conditions necessary to give pure Ba2YCu307 and the chemical instability of the pure phase towards moist air will be discussed in the second part of this paper. 

If the oxidation states of copper during its reaction with ozone are limited to Cu(II) and Cu(III), there are various ways in which ozone can bind to mono- or oligonuclear copper centers (Fig. 44). These structures are based on the assumption that copper is, initially, present in its Cu(II) oxidation state and is oxidized to Cu(III) following the binding of ozone. Ozone itself is reduced with one or two electrons, facilitating its further reaction to water and oxygen. 

Furthermore the oxidation state of copper is easily changed, Cu being reduced into Cu" by the CO probe molecule and Cu" being oxidized by the NO probe molecule even at 298 K. 

Copper, modified by oxygen, was shown to be an active catalyst for reducing NO in the presence of a hydrocarbon. It is a realistic alternative to noble metals. Oxygen is necessary during a preliminary step of the reaction. Subsequently, the reduction of NO is effective on an oxide surface without oxygen in the gas phase. Kinetics results i.e. the rate of NO consxunption and the N2/CO ratio in the products are strongly dependent upon the temperature of preoxidation of the catalyst. This result suggests that the reaction is sensitive to the oxidation state of copper in the superficial layers. 

In another approach using y-irradiation as O2" source and EPR as its detector a loose complex Enzyme-Cu(II) HO2 was postulated The O2 radical is relatively stable below 250 °K in aqueous solution or ice, respectively. Therefore, employing low temperature EPR (77 °K) superoxide can easily be detected after y-irradiation. Subsequent annealing to 200 °K results in a slow decay of the superoxide anion. The decay of the loose complex was postulated to proceed without changes in the oxidation state of copper. The observed delay in the changes of the oxidation state of copper was attributed to a reaction of copper with the reaction products of the dismutation. A reaction mechanism different from that obtained by pulse radiolysis was proposed A transient state reaction of two HOj with the native enzyme (eq. 22) and the reduced enzyme (eq. 23) was thought to be responsible for the dismutation of the superoxide anion ... 

What is the oxidation state of copper in your copper-ammine complex Justify your answer. 

Considering the complex [Cu(NH3)4], note from Table 8.2 that the ammonia ligand is also neutral. Hence the charge on Cu is (2 — 0) = 2, that is, the oxidation state of copper in the complex is Cu( + II). 

Bednorz and Muller showed that a cuprate, La2Cu04, becomes superconducting at Tc = 35 K if some lanthanum atoms are replaced by barium atoms. This doped compound may be written as La(2 x)BajCn04. The oxidation state for La is +3, for Ba +2, and for oxygen -2. The oxidation state of copper, y, is thus determined from... 

This copper(II) mononuclear complex is responsible for some remarkable properties, one of which is the high redox potentials of the copper(II) ions in these environments. These blue-copper proteins are engaged in electron transport, which involves a switching of the oxidation state of copper between +2 and + ... 

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