Copper complexes four-coordinate

Figure 11. Diagrammatic representation of active site which includes a four-coordinate copper complex, sugar-substrate bound outer-sphere to the Cu(II) atom, imidazole and indole rings, and nonpolar side chains (X). The distance between the Cu(II) and indole is estimated by fluorescence energy-...

Coordination number The number of bonds from the central metal to the ligands in a complex ion, 409,412t four-coordinate metal complex, 413 six-coordinate metal complex, 413-414 Copper, 412 blister, 539... 

As discussed above, the unstable four-coordinate copper(II) complex 10(4)2 + rearranges within less than 1 s. By contrast, after oxidation of 11(4)+ to 11(4)2+, the thermodynamically unstable form of the complex seems to be stable for several hours, also showing that the axis of 10(4)+ and 11(4>+ with its rigid purely aromatic connector between the phen and terpy fragments is much less favorable to fast gliding than the flexible axis originally used to prepare 7(4) +. ... 

The most important azo compounds employed in the manufacture of dyes of this type are those containing the < ,o -dihydroxyazo-, the o-hydroxy-o -carboxyazo- and the o-hydroxy-o -amino-diarylazo systems. It is well established3 33-0 that these form four-coordinate copper and nickel complexes (35) in which the coordination sphere of the metal can be completed by a variety of neutral ligands. In both cases the light-fastness of the parent azo compound is improved as a result of complex formation but the nickel complexes are insufficiently stable towards acid to be of commercial interest as dyestuffs. The history of copper complexes has already been discussed (Section 58.1) and will not be considered further here, although it is worthy of mention that currently the most important copper complex dyestuffs are those containing fibre-reactive systems, e.g. (36), for application on cellulosic fibres. 

Fig. 10 Principle behind electrochemically induced molecular motion in a copper(l) complex pseudorotaxane. The stable, four-coordinate, monovalent complex is oxidized to an intermediate tetrahedral, divalent species. This compound undergoes a rearram gementto afford the stable, five-coordinate...

A number of studies have shown that the assumptions discussed above are valid for certain classes of redox couples, and reasonably accurate predictions for reduction potentials have been obtained for redox couples with large structural differences between the oxidized and reduced forms of the complex (see Section 11.1). This is the case for hexaaminecobalt(III/II)[2311 and tetraaminecopper(I I/I)f3411 couples, where the metal-donor bond length differences are approximately 0.2 A each. Note that additional assumptions are required for copper(II/I) couples, where the stoichiometries for the oxidized and reduced forms (five- or six-coordinate cop-per(II) versus four-coordinate copper(I)) are different. These are also discussed in Section 11.1, and it appears that they do not lead to undue inaccuracies. 

The formation of type C products for S-apip and S-ahaz is rather surprising, because these substances should be generated through the four-coordinated-copper(II) complexes. The four-coordinated species are considered to be present in only a small ratio compared with the five-coordinated species, as disscussed earlier. Therefore, the formation of C for these diamines (S-apip and S-ahaz) seems to suggest the low reactivity of acetone dimers towards their five-coordination complexes. 

Structural distortions and associated high lability often prevent kinetic data on substitution reactions of four-coordinate copper(II) compounds being obtained. Slightly distorted square-planar coordination is found for the copper complexes [CuXL] (L is AT-(2-(diethylamino)ethyl)salicylaldiminate, 6 X are monodentate anionic ligands or water), confirmed by X-ray analysis for X = NCS . The... 

The abundance of five-coordinate copper complexes corresponds overwhelmingly to the Cu(ll) ion, since Cu(l) has little tendency to go beyond fonr coordination. Hence, the tripod ligand tris(pyridy]methyl)amine forms complexes fliat are essentially four-coordinate tetrahedral with Cu(l), with distances to the amine nitrogen atom longer than 2.40 A, but trigonal bipyramidal pentacoordinate with Cu(ll), with distances to the apical N shorter than 2.20 A. 

Table 7. Bond lengths and angles in some four-coordinate copper(I) and gold(I) phosphine complexes...

Derived from the German word meaning devil s copper, nickel is found predominantly in two isotopic forms, Ni (68% natural abundance) and Ni (26%). Ni exists in four oxidation states, 0, I, II, III, and IV. Ni(II), which is the most common oxidation state, has an ionic radius of —65 pm in the four-coordinate state and —80 pm in the octahedral low-spin state. The Ni(II) aqua cation exhibits a pAa of 9.9. It forms tight complexes with histidine (log Af = 15.9) and, among the first-row transition metals, is second only to Cu(II) in its ability to complex with acidic amino acids (log K( = 6-7 (7). Although Ni(II) is most common, the paramagnetic Ni(I) and Ni(III) states are also attainable. Ni(I), a (P metal, can exist only in the S = state, whereas Ni(lll), a cT ion, can be either S = or S =. ... 

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