Complexes containing four metal ions

In investigating systems of metal complexes, the latter method was also used by Gerischer [5-7]. With a sufficient excess of cyanide, Zn(II) complexes containing four cyanide ions prevail in solutions therefore, the net reaction taking place on the amalgam electrode can be written as follows ... 

The structures of these materials are as yet unknown. Interestingly, a tetranuclear complex containing four cw-(NH3)2Ptn units and two /t4-car-bonato ligands, [Pt4(NH3)8(C03)2](N03)4, has been isolated from a preparation of a Pt deoxyuridine blue and characterized by X-ray crystal structure analysis [85]. The four Pt ions in this complex form roughly a square of sides of ca. 3.18-3.28 Aand consequently do not form metal-metal bonds. 

Fig. 3a-d. A diagrammatic representation of double-stranded helicates (23, 34] containing (a) two, (b) three, (c) four, and (d) five 2,2 -bipyridine subunits [23], The Cu(I) ions function as a template around which the helicates can assemble. Furthermore, this assembly process exhibits (i) self-self recognition in the preferential pairing to the same ligand in the presence of others in the reaction mixture, and (ii) positive cooperativity in which the complexation of one metal ion facilitates the binding of the next... 

In recent years. X-ray crystallography has led to the discovery of several novel metalloclusters of complex architecture that contain at least four metal ions (4, 5). They represent the active site of several redox enzymes that contain molybdenum, nickel, and manganese, as well as the most commonly encountered iron and copper (Fig. 6). These enzymes are extremely specialized in the oxidation or reduction reactions of the smallest molecules and anions (which include N2, CO, and H2). A common feature of such clusters is that they are present in enzymes as part of a more extensive electron transfer chain that involves a series of... 

Optical isomerism is concerned with chirality, and some important terms relating to chiral complexes are defined in Box 19.2. The simplest case of optical isomerism among fi -block complexes involves a metal ion surrounded by three didentate ligands, for example [Cr(acac)3] or [Co(en)3] (Figures 3.16b and 19.12). These are examples of tris-chelate complexes. Pairs of enantiomers such as A-and A-[Cr(acac)3] or A- and A-[Co(en)3]Cl3 differ only in their action on polarized hght. However, for ionic complexes such as [Co(en)3], there is the opportunity to form salts with a chiral counter-ion A. These salts now contain two different types of chirality the A- or A-chirality at the metal centre and the (-I-) or (—) chirality of the anion. Four combinations are possible of which the pair (A-(- -) and A-(—) is enantiomeric as is the pair A-(—) and A-(- -). However, with a given anion chirality, the pair of salts A-(—) and A-(—) are diastereomers (see Box 19.2) and may differ in the packing of the ions in the solid state, and separation by fractional crystallization is often possible. 

Structure and Mechanism. Cytochrome-c oxidase catalyzes the four-electron reduction of molecular oxygen to water and couples these redox processes to proton transfer across the mitochondrial membrane.As depicted in Fig. 24, the enzyme is structurally complex and contains four metal centers which are redox active, two copper ions and two heme a groups. One copper ion, Cug, and one of the heme a groups, cytochrome <13, (cyt a ), form a binuclear center which binds dioxygen. Electrons are ... 

Surface complexation involves all four of the planes depicted in Fig. 5.4. Complexed protons and hydroxide ions reside in the s plane, inner-sphere complexes containing trace metal cations or oxyanions are assigned to the a plane, outer-sphere complexes with the ions of a background electrolyte are assigned to the j3 plane, and the d plane marks the beginning of the aqueous solution phase, where the diffuse ion swarm is found. 

Yam and Lo (Chap. 2 ) focus on the photophysics of a broad family of supramolecular metal complex clusters derived from the group Ib transition metal ions (Cu, Ag, and Au). This chapter discusses basic types of excited states found in clusters that contain two, three, and four metal ions held together by ligands such as halide, phosphine, and acetylide. Continuing on this theme are S. W. Jones and cowoikers (Chap. 4), who outline the photophysical and electrochemi-... 

The strength of this bonding depends on the kind of ether Simple ethers form relatively weak complexes with metal ions but Charles J Pedersen of Du Pont discovered that cer tain polyethers form much more stable complexes with metal ions than do simple ethers Pedersen prepared a series of macrocyclic polyethers cyclic compounds contain mg four or more oxygens m a ring of 12 or more atoms He called these compounds crown ethers, because their molecular models resemble crowns Systematic nomencla ture of crown ethers is somewhat cumbersome and so Pedersen devised a shorthand description whereby the word crown is preceded by the total number of atoms m the ring and is followed by the number of oxygen atoms... 

EthylenediaminetetraaceticAcid. Ethylenediaminetetraacetic acid (EDTAH has six potential donor groups two nitrogen atoms and four carboxylate groups. If EDTA 4— acts as a hexadentate ligand to a metal, the resulting complex contains five five-membered chelate rings and has a charge that is four less than that of the metal ion. 

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