Class metal ion

As a class, metal-ion derivatives of tetrapyrrole macrocyclic rings, such as the corrins or porphyrins (see Chapter 1 for the parent ring structures), are of major biological importance. 

Strong complexes are formed with (b) class metal ions, the stability constants for Hg11 (log Pi 29.9), Pd11 and Pt11 (log JS4, 35.0 and 43.7, respectively) being quite high.47 48 Alkaline thiosulfate... 

Stability constant determinations have been made on a range of monothio-jS-diketonato complexes in dioxane-water solution. The results indicate that (b) class metal ions form more stable complexes with monothio-/5-diketones than with /5-diketones. The stability sequence for monothio-/5-diketonato complexes is Cu > Ni > Zn > Cd > Pb.198 201... 

According to their preferences toward either class (n) or class (/ ) metal ions, ligands may be classified as type (a) or (h), respectively. Stability of these complexes may be summarized as h lows ... 

Thiourea, (H2N)2C=S (tu), acts as a unidentate ligand forming strong complexes with (b) class metal ions, in particular Cu , Ag, Au and Hg" it reduces Cu" to Cu, Au" to Au, Pt" to Pt and Te to Te", forming complexes with the metal in the lower oxidation state/ The only metal reported to be N-bonded is Ti, all others being S-bonded v(M—S) occurs at 300-200cm . ... 

Complexes of diphenyl sulfoxide exhibit S—O stretching frequencies ranging from 931 cm for Fe" to 1012cm for Mg". The IR spectra of the metal complexes of tetrahydrothiophene oxide (32) show that the complexes of Co", Ni" and Cu" are O-bonded, while those of the (b) class metal ions Pd" and Pt" are S-bonded. ... 

At potentials positive to the bulk metal deposition, a metal monolayer-or in some cases a bilayer-of one metal can be electrodeposited on another metal surface this phenomenon is referred to as underiDotential deposition (upd) in the literature. Many investigations of several different metal adsorbate/substrate systems have been published to date. In general, two different classes of surface stmetures can be classified (a) simple superstmetures with small packing densities and (b) close-packed (bulklike) or even compressed stmetures, which are observed for deposition of the heavy metal ions Tl, Hg and Pb on Ag, Au, Cu or Pt (see, e.g., [63, 64, 65, 66, 62, 68, 69 and 70]). In case (a), the metal adsorbate is very often stabilized by coadsorbed anions typical representatives of this type are Cu/Au (111) (e.g. [44, 45, 21, 22 and 25]) or Cu/Pt(l 11) (e.g. [46, 74, 75, and 26 ]) It has to be mentioned that the two dimensional ordering of the Cu adatoms is significantly affected by the presence of coadsorbed anions, for example, for the upd of Cu on Au(l 11), the onset of underiDotential deposition shifts to more positive potentials from 80"to Br and CE [72]. 

EDTA is one member of a class of aminocarboxylate ligands that form very stable 1 1 complexes with metal ions. The following table shows log Kf values for several ligands with Ca + and Mg +. Which ligand is the best choice for the direct titration of Ca + in the presence of Mg + ... 

Potentiometric electrodes are divided into two classes metallic electrodes and membrane electrodes. The smaller of these classes are the metallic electrodes. Electrodes of the first kind respond to the concentration of their cation in solution thus the potential of an Ag wire is determined by the concentration of Ag+ in solution. When another species is present in solution and in equilibrium with the metal ion, then the electrode s potential will respond to the concentration of that ion. Eor example, an Ag wire in contact with a solution of Ck will respond to the concentration of Ck since the relative concentrations of Ag+ and Ck are fixed by the solubility product for AgCl. Such electrodes are called electrodes of the second kind. 

An on-line concentration, isolation, and Hquid chromatographic separation method for the analysis of trace organics in natural waters has been described (63). Concentration and isolation are accompHshed with two precolumns connected in series the first acts as a filter for removal of interferences the second actually concentrates target solutes. The technique is appHcable even if no selective sorbent is available for the specific analyte of interest. Detection limits of less than 0.1 ppb were achieved for polar herbicides (qv) in the chlorotriazine and phenylurea classes. A novel method for deterrnination of tetracyclines in animal tissues and fluids was developed with sample extraction and cleanup based on tendency of tetracyclines to chelate with divalent metal ions (64). The metal chelate affinity precolumn was connected on-line to reversed-phase hplc column, and detection limits for several different tetracyclines in a variety of matrices were in the 10—50 ppb range. 

Attempts have been made to categorize the interactions between metal ions and ligands. Whereas all metal ions interact more strongly with fluoride than with chloride in the gas phase, in aqueous solution a number of exceptions occur. Metal ions that have the normal (class a) aqueous solution stabiUty order of F Cl > Br > I also have N P > As > Sb and 0 S > Se > Te donor stabiUty order (13). The inverse (class b) aqueous solution stabiUty... 

Greater selectivity in purification can often be achieved by making use of differences in chemical properties between the substance to be purified and the contaminants. Unwanted metal ions may be removed by precipitation in the presence of a collector (see p. 54). Sodium borohydride and other metal hydrides transform organic peroxides and carbonyl-containing impurities such as aldehydes and ketones in alcohols and ethers. Many classes of organic chemicals can be purified by conversion into suitable derivatives, followed by regeneration. This chapter describes relevant procedures. 

Two classes of aldolase enzymes are found in nature. Animal tissues produce a Class I aldolase, characterized by the formation of a covalent Schiff base intermediate between an active-site lysine and the carbonyl group of the substrate. Class I aldolases do not require a divalent metal ion (and thus are not inhibited by EDTA) but are inhibited by sodium borohydride, NaBH4, in the presence of substrate (see A Deeper Look, page 622). Class II aldolases are produced mainly in bacteria and fungi and are not inhibited by borohydride, but do contain an active-site metal (normally zinc, Zn ) and are inhibited by EDTA. Cyanobacteria and some other simple organisms possess both classes of aldolase. 

Metal-centered emission is found in several rare-earth complexes, for example europium diketone chelates [29]. These compounds emit relatively narrow spectra associated with the d-f transitions of the metal ion. Relatively few comprehensive studies of this class have been carried out to date, but the available evidence [30] seems to indicate that the photoluminescent efficiencies are somewhat lower than the ligand-centered emitters. 

The nature of the donor atoms in the chelating agent. Ligands which contain donor atoms of the soft-base type form their most stable complexes with the relatively small group of Class B metal ions (i.e. soft acids) and are thus more selective reagents. This is illustrated by the reagent diphenylthiocarbazone (dithizone) used for the solvent extraction of metal ions such as Pd2+, Ag+, Hg2+, Cu2+, Bi3+, Pb2+, and Zn2 +. ... 

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