Effect of coordination geometry

Figure 2. Effect of coordination geometry on V XANES spectrum V naphthenate (----), (---). ...

The current chapter focuses on the electrochemistry of the ionic forms of copper in solution, starting with the potentials of various copper species. This includes the effect of coordination geometry, donor atoms, and solvent upon the electrochemical potentials of copper redox couples, specifically Cu(II/I). This is followed by a discussion of the various types of coupled chemical reactions that may contribute to the observed Cu(II/I) electrochemical behavior and the characteristics that may be used to distinguish the presence of each of these mechanisms. The chapter concludes with brief discussions of the electrochemical properties of copper proteins, unidentate and binuclear complexes. 

Very often, the tetrad effect is not clearly discernible in the energies of processes in which 4f electrons are conserved. It may, for example, be obscured by irregularities caused by structural variations in either reactants or products. This is especially likely given the willingness of lanthanide ions to adopt a variety of coordination geometries. There is, however, no doubt that tetrad-like patterns are often observed. But does Table 1.2 provide a convincing explanation of what is seen ... 

The interesting observation, unexpected on the basis of limited published data of a tetradentate derivative of L8 249 250 but in full agreement with the MM analysis is that the stabilities do not follow the usual Irving-Williams behavior (Co2+ < Ni2+ < Cu2+ > Zn2+)251 252 but that L4 is zinc(II)-selective. This was interpreted as a combination of the steric effects discussed above, electronic effects (distorted coordination geometries, misdirected valences), which are unimportant for the d10 ion zinc(II), in contrast to copper(II), especially nickel(II) and smaller metal ions such as cobalt(III).78... 

Sodium and potassium cations are often encountered in the same biological environment and the transmembrane movements of both are required as part of an enzymatic pathway as in Na+, K+-ATPase. Under these circumstances it is essential that cation-specific channels are formed. What features of the channels contribute to the selectivity Earlier the preferred geometries of Na+and K+, sixfold octahedral and eightfold cubic respectively, were proposed as the main discriminatory factors. A computational analysis by Dudev and Lim [35] has considered the effect of coordinated water, number of available coordination sites in the channel walls, and the dipoles of the coordinating groups. The researchers investigated cation complexes with valinomycin and the protein KcsA, both K+-selective, and compared these with a non-selective NaK channel. 

Factors that influence ionic size include the coordination number of the ion, the covalent character of the bonding, distortions of regular crystal geometries, and delocalization of electrons (metallic or semiconducting character, described in Chapter 7). The radius of the anion is also influenced by the size and charge of the cation (the anion exerts a smaller influence on the radius of the cation). The table in Appendix B-1 shows the effect of coordination number. 

Investigations of the effects of coordinating anions on the kinetics of the redox reactions of Cu(II)/(I) superoxide dismutase with the [Fe(CN)6] couple indicate that the Cu(II) geometry of the anion adduct [unreactive square-planar (with CN and Ns") vs. reactive five-coordinate (with OCN and SCN )] affects its ability to accept an electron from [Fe(CN)6] . ... 

Copper(I) tends towards a tetrahedral coordination geometry in complexes. With 2,2 -bipyr-idine as a chelate ligand a distorted tetrahedral coordination with almost orthogonal ligands results. 2,2 -Bipyridine oligomers with flexible 6,6 -links therefore form double helices with two 2,2 -bipyridine units per copper(I) ion (J. M. Lehn, 1987,1988). J. M. Lehn (1990 U. Koert, 1990) has also prepared such helicates with nucleosides, e.g., thymidine, covalently attached to suitable spacers to obtain water-soluble double helix complexes, so-called inverted DNA , with internal positive charges and external nucleic bases. Cooperative effects lead preferentially to two identical strands in these helicates when copper(I) ions are added to a mixture of two different homooligomers. 

In our last example we return to the issue of the possible damaging effects of the standard geometry constraints. Two long trajectories have been computed for a partially hydrated dodecamer DNA duplex of the previous example, first by using ICMD and second with Cartesian coordinate molecular dynamics without constraints [54]. Both trajectories started from the same initial conformation with RMSD of 2.6 A from the canonical B-DNA form. Figure 5 shows the time evolution of RMSD from the canonical A and B conformations. Each point in the figure corresponds to a 15 ps interval and shows an average RMSD value. We see that both trajectories approach the canonical B-DNA, while the RMSD... 

FIGURE 10.83 The geometry and coordinate system used to modd the efFect of the exhaust inlet size. 

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