Tetrahedral copper ions

In terms of formal point defect terminology, it is possible to think of each silver or copper ion creating an instantaneous interstitial defect and a vacancy, Ag and VAg, or Cu and Vcu as it jumps between two tetrahedral sites. This is equivalent to a high and dynamic concentration of cation Frenkel defects that continuously form and are eliminated. For this to occur, the formation energy of these notional defects must be close to zero. 

The tetrahedral Cu04 group is rarely observed and no strong evidence has ever been presented for this highly symmetrical environment about the divalent copper ion. Coordinations higher than VI (or IV + II) are also very rare. Statistical data for Cu2+ coordination environments in 234 oxysalts and 75 minerals will be summarized. 

In the metal-binding site (Figure 12) the Cu(II) ion is coordinated by four histidines in a distorted square-planar coordination sphere and by a fifth axial water ligand, while the tetrahedral zinc ion is coordinated by three histidines, one of which is deprotonated and bridges to the copper, and by a carboxylate group °. 

The biological significance of these reactions is considered further in Chapters 18 and 24. The 132-kDa dimeric N20 reductase from Pseudomonas stiltzeri contains four copper atoms per subunit.546 One of its copper centers resembles the CuA centers of cytochrome c oxidase. A second copper center consists of four copper ions, held by seven histidine side chains in a roughly tetrahedral array around one sulfide (S2 ) ion. Rasmussen et al. speculate that this copper-sulfide cluster may be an acceptor of the oxygen atoms of N20 in the formation of N2.546a There is also a cytochrome cdj type of nitrite reductase.1433... 

Reaction of the Schiff base ligand A,Ar -bis(o-diphenylphosphinobenzylidene)(ethylene-diamine (49 en=P2) with AgBF4 produced a pale yellow salt. The IR spectrum of this complex showed strong bands due to the imino and BF4 group (v(C=N) 1653 cm-1, v(BFj) 1080 cm-1). The crystal structure of the Cu1 analogue was reported and the copper ion was found to adopt a severely distorted tetrahedral geometry. This strain was manifested in its reactivity since both the copper and silver complex reacted with f-butyl isocyanide. In the case of silver(I) a five-coordinate adduct was obtained, [Ag(en=P2)(Bu NC)]BF4.396... 

Cuprous ion complexes with four ligands are normally tetrahedral, involving spi hybrid orbitals (electronic distribution A). However, the cuprous hydrogen complex II, which is of the form (Cu X3H), is isoelec-tronie with four coordinate complexes of cupric ion, of the form (Cu11 ), which are known to be planar and to use dsp2 orbitals (distribution B). It seemed possible, therefore, that because of its unusual electronic structure, complex II was also planar. Construction of scale (Fischer-Taylor-Hirschfelder) models indicates that this is probably not the case. A planar model of I can be constructed but not of II insufficient space exists to accommodate the hydrogen atoms between the copper ions in II. If, however, tetrahedral coordination is permitted about the copper ions, no... 

The copper proteins containing the type 2 active site are also known as normal copper proteins, because their spectroscopic features are similar to those of common Cu coordination compounds. The copper ion in these proteins is surrounded by four N and/or O donor atoms in either square-planar or distorted tetrahedral geometry [3, 4]. Examples of proteins with this active site include... 

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). 

Circular helicates exhibit a particular aesthetic appeal as they comprise closed tori of chiral architectures. Hannon and coworkers [138] have reported the self-assembly of the chiral ball ([Cu3l73])42+ (19) assembled from four ho-mochiral, circular helicates [Qi3l73]3+ (18) that are held together via CH-7T interactions (Fig. 14). Each copper ion is tetrahedrally coordinated by two imine-ligands 17, resulting in the solution-stable trimeric helicate 18. The resulting chiral tetramer 19 is quite remarkable as it is self-assembled from a one-pot reaction of 48 simple and achiral building units. 

Type 3 centres include haemocyanin which is used by arthropods and molluscs to transport dioxygen in the haemolymph. Generally hamocyanin contains two copper ions each coordinated to three histidines. In the colourless, Cu+, deoxy form these are separated by 3.6 A. Once dioxygen binds between the two coppers they both adopt a tetrahedral geometry and are oxidized to Cu2+ turning the complex blue. Binding has not been fully characterized and may be side-on or end-on . 

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