Mixed valence of copper

Moreover, the mixed valence of copper, Cu(II) - Cu(III), is absolutely necessary for the delocalization of holes in the copper oxygen framework, leading to semi-metallic or metallic properties. 

From these first observations it appeared that strong electron-phonon couplings might be at the origin of those superconducting properties, which could be related to the existence of the mixed valence of copper Cu(II)-Cu(III) and to its Jahn Teller... 

These two factors, two-dimensionality of the structure and mixed valency of copper appear to be absolutely necessary for the appearance of superconductivity. The fact that, to date, no tridimensional cuprate has been found to be a superconductor strongly supports... 

In 1996 Stack and co-workers reported an unusual 3 1 (copper 02 stoichiometry) reaction between a mononuclear copper(I) complex of a A-permethylated (lR,2R)-cyclohexanediamine ligand with dioxygen. The end product of this reaction, stable at only low temperatures (X-ray structure at —40 °C) is a discrete, mixed-valence trinuclear copper cluster (1), with two Cu11 and a Cu111 center (Cu-Cu 2.641 and 2.704 A).27 Its spectroscopic and magnetic behavior were also investigated in detail. The relevance of this synthetic complex to the reduction of 02 at the trinuclear active sites of multicopper oxidases4-8 was discussed. Once formed, it exhibits moderate thermal stability, decomposed by a non-first-order process in about 3h at —10 °C. In the presence of trace water, the major isolated product was the bis(/i-hydroxo)dicopper(II) dimer (2). 

In a serendipitous fashion, a novel mixed valence tetranuclear copper(II)/copper(III) dithiocarbamate [2]catenane was prepared in near quantitative yield by partial chemical oxidation of a preformed dinuclear copper(II) naphthyl dtc macrocycle (Scheme 6).49 X-ray structure, magnetic susceptibility, ESMS and electrochemical studies all support the tetranuclear catenane dication formulation. The combination of the lability of copper(II) dtc coordinate bonds and favourable copper(II) dtc-copper(III) dtc charge transfer stabilisation effects are responsible for the high yielding formation of the interlocked... 

Coucouvanis (Figure 1). The Cu ion is in a site with fourfold symmetry, a square-planar environment that is often found for metal ions with the d valence shell electronic stracture of Cu . Each Cu ion is found in a site wherein the metal ion is bonded to three sulfur atoms. The Cu sites are all equivalent and related to each other by the fourfold symmetry of the complex. Hence the two types of copper ions are in very different coordination environments. This is typical class 1 behavior since the copper ion sites are not interconvertible by symmetry. The intense blue coloration of the complex, the spectrum of which has not been completely assigned, presmnably arises because of the mixed valency of... 

Farrar JA, Neese F, Lappalainen P, Kroneck PMH, Saraste M, Zumft WG, Thomson AJ. 1996. The electronic structure of Cua a novel mixed-valence dinuclear copper electron transfer center. JAm Chem Soc 118 11501-11514. 

Rate and equilibrium constant data, including substituent and isotope effects, for the reaction of [Pt(bpy)2]2+ with hydroxide, are all consistent with, and interpreted in terms of, reversible addition of the hydroxide to the coordinated 2,2 -bipyridyl (397). Equilibrium constants for addition of hydroxide to a series of platinum(II)-diimine cations [Pt(diimine)2]2+, the diimines being 2,2 -bipyridyl, 2,2 -bipyrazine, 3,3 -bipyridazine, and 2,2 -bipyrimidine, suggest that hydroxide adds at the 6 position of the coordinated ligand (398). Support for this covalent hydration mechanism for hydroxide attack at coordinated diimines comes from crystal structure determinations of binuclear mixed valence copper(I)/copper(II) complexes of 2-hydroxylated 1,10-phenanthroline and 2,2 -bipyridyl (399). 

A ternary fluoride of copper with a mixed valency CsCuFs.e = = Csi,67Cu +o.07(Cu +F6), which crystallices cubically with a = 8.82 A (157), may also be mentioned in this context. 

The mixed valency situation may be equally well described in terms of bands. The band to be considered in the case of bismuth is the Bi6s-02p band. The band in the case of copper is the Cu3d y2 -02p band. Both of these are d bands. For the lowest oxidation state (Cu1 or Bira), these bands are filled for the highest oxidation state (Cu111 or Biv), these bands are empty. According to simple band structure considerations, we would expect metallic properties for any partial filling of these o bands in bismuth and copper oxides. While metallic properties are indeed observed for most of the intermediate... 

The other copper-only binuclear centre to be considered is the CuA or purple copper complex. It is part of the terminal oxidase in mitochondrial respiration, cytochrome c oxidase (COX). Its EPR signature, a seven-line spectrum, has since long been known to be different from the classes type 1 to 3 and arises from two copper ions in a 1.5 valence (or mixed valence) state, first proposed from EPR-analysis of a similar center in nitrous oxide (N20) reductase. There is a close correspondence between the blue and purple states of copper since each of the two copper ions in CuA can be considered as being structurally related to the mononuclear blue site coordination. 

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