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Within the last five years significant advances have been made in elucidating the chemistry of technetium. While this element is continually produced in some abundance during the operation of fission nuclear reactors, its practical application has been limited to nuclear medicine. Owing to the development of the "technetium generator" almost two decades ago, 99mTc is now the preferred isotope for many diagnostic studies involving the imaging of internal organs by radioscintigraphic techniques; however, many of the agents employed have yet to be well characterized from a chemical standpoint. Owing to its position in the middle of the periodic table, the chemistry of this element has already proven rich, even when limited to aqueous solution. Subtle chemical differences often control the relative stabilities of the various oxidation states and each oxidation state exhibits more than one geometry. Coordination complexes involving Tc(III), Tc(IV) and Tc(V) tend to predominate in preparations used for diagnosis. Recent synthetic advances now pave the way for new series of technetium compounds with well-described chemistry. Of these, methods for coordinating chelating nitrogen ligands to Tc(V) offer substantial versatility. (This review covers the literature through August, 1981.) [ABSTRACT FROM AUTHOR]

Electron nuclear double resonance (ENDOR) of transition metal complexes with organic ligands is considered from an experimental and theoretical point of view. In the experimental sections, instrumentations used in conventional ENDOR and multiple resonance spectroscopy and in some new double resonance methods are discussed. Particular emphasis is placed on various experimental techniques which supplement the conventional ENDOR experiment. The different methods are illustrated by a number of applications on transition metal complexes1. In the theoretical sections, the analysis of ENDOR spectra of complex spin systems is described including second order effects, transitions probabilities and determination of signs of the magnetic parameters. Some of the approaches used in ENDOR to interpret ligand hyperfine and quadrupole data are outlined. Finally, a review of the ENDOR literature on transition metal complexes with organic ligands is presented. [ABSTRACT FROM AUTHOR]

This article is thought to be a supplement to an earlier one "Mechanism of Hydrogen Transfer in Redox Enzymes"33). Most of the basic concepts and hypotheses put forward then, are still valid. Substantial new evidence supporting our views has been advanced since then. The new data presented here concern modified or as we call them "mutilated flavins", namely 5-deaza- and 5-thiaflavin, which upon incorporation into apoflavoprotein retain some, but not all of the activities encountered with the native flavoproteins. [ABSTRACT FROM AUTHOR]

Although the process of light energy conversion to chemical form by specialized plant cell organelles, the chloroplasts, is well understood, many contemporary investigations have been concerned with the catalytic role played by manganese in oxygen evolution. Current views of the photosynthetic electron transport chain or the so-called "Z-scheme" intermediates identify the cytochromes b6, b-559 and f, the chlorophyll linked primary electron acceptor and donors of photosystems -I and -II and a number of e.p.r. distinct iron-sulfur centres involved in non cyclic and possibly cyclic electron transfer pathways. The accumulation of four redox equivalents leads to the oxidation of water to yield molecular oxygen by a water splitting metalloenzyme (designated -Y) which involves at least two manganese atoms per molecule of enzyme. During the four electron oxidation of water, e.p.r. and proton relaxation evidence suggest the two manganese atoms undergo redox changes between manganese(II) and manganese(III) states. A number of proposed manganese model systems exhibiting the features required by the well known S-state mechanism of oxygen evolution are discussed. The modelling of both the charge separation function of the photosystems and the properties of the catalytically active manganese is of current interest from an energy production point of view. [ABSTRACT FROM AUTHOR]

The major solid state principles involved in mineralization in biological systems are discussed. Three major biological control factors of mineralization are described; structural, spatial, and chemical control. Factors determining nucleation on organic surfaces, mineral growth, mineral structure, and morphology are reported. Oriented growth of minerals on organic matrices can occur by three processes; (i) lattice matching (epitaxis) between the organic matrix and depositing crystal faces, (ii) surface structural relationships between the organic matrix and depositing crystal faces, (iii) ordered aggregation of preformed mineral particles. Several a priori reasons indicate that epitaxis is unlikely to be a major process of oriented growth in biomineralization. The presence and importance of biogenic amorphous minerals is also described. [ABSTRACT FROM AUTHOR]

The redox chemistry of chromium(VI) is discussed with respect to the cellular metabolism of the carcinogen chromate in vivo. Possible sites for cellular reduction of chromium(VI) to chromium(III) are considered. The reactions of amino acids, ascorbic acid, carboxylic acids, thiol-containing mole-cules and other small molecules with chromate under physiological conditions are presented. In general only ascorbate and those molecules containing sulfhydryl groups are capable of easily reducing chromate at pH 7.4. Thus, in the cytoplasm, glutathione, cysteine and ascorbate are likely candidates to react with chromate. While most proteins are unreactive toward chromate, certain redox proteins are active in reducing chromate. The heme proteins hemoglobin and cytochrome P-450 possess chromate-reductase activity, whereas cytochrome c and myoglobin are inactive. The NADPH-dependent flavoenzymes glutathione reductase and NADPH-cytochrome P-450 reductase also possess chromate-reductase activity. However, the NAD(P)H enzymes, isocitrate dehydrogenase, glutamate dehyrogenase and malate dehydrogenase do not reduce chromate. Both microsomes and mitochondria possess chromate-reductase activity. The microsomal activity is accounted for by the NADPH-cytochrome P-450 reductase/cytochrome P-450 system. The enzyme(s) responsible for the mitochondrial reduction of chromate have not been identified. Chromium(VI) and its metabolite chromium(III) inhibit the normal activities of enzymes which bind chromium(III) or reduce chromate. The metabolism of chromate involves the generation of reactive intermediates which ultimately bind to cellular constituents and damage their function in the cell. [ABSTRACT FROM AUTHOR]

Reductive elimination of the 2′-hydroxyl group from ribonucleotides to yield 2′-deoxyribonucleotides, the monomeric precursors of DNA, requires an uncommon type of enzyme catalysis in which the transition metals, manganese, iron, or cobalt, and free radical intermediates cooperate. In the group of deoxyadenosylcobalamin (coenzyme B 12)-dependent ribonucleotide reductases the coenzyme supplies a transient radical pair of deoxyadenosyl· and cob(II)alamin whereas in the nonheme-iron group of enzymes a protein subunit carries a stable tyrosyl radical coordinated to a binuclear iron(III) complex; the manganese-dependent enzymes are less precisely known. The radicals are thought to function in hydrogen transfer from cysteine SH to the ribonucleotide substrate, and the metal complexes are apparently needed to generate and stabilize the radicals. In aerobic organisms oxygen also plays a critical role in these processes and hence in DNA synthesis and cell proliferation. In addition to the transition metals, Mg2+ or Ca2+ are required by several ribonucleotide reductases for structural integrity. However the most potent inhibitors of deoxy-ribonucleotide biosynthesis (of potential interest in chemotherapy) are not metal chelators but radical scavengers. Cell cycle arrest and cell death produced by simple chemicals like N-hydroxyurea and hydroxamates can be traced back to their reaction with ribonucleotide reductase. Evidence is accumulating that independent enzymes of deoxyribonucleotide and DNA synthesis are functionally coupled in a novel type of supramolecular structure. [ABSTRACT FROM AUTHOR]

A brief discussion of the vast and increasingly important area of organoselenium chemistry is presented as it relates to the biochemistry of this element. Organo selenoamino acids are the basic building blocks of many selenium-containing macromolecules, which are the primary focus of this article. First to be discussed are synthetic selenium analogues of sulfur-containing natural products such as selenocoenzyme A and selenobiotin. Secondly, are those enzymes which will indiscriminately incorporate selenium and the most important of these are considered. Finally, the most important and interesting class of selenium-containing macromolecules are the naturally occurring selenoproteins, of which there are eight currently known and only three which have been extensively studied. Physical studies which relate to or have the potential to aid in investigations of selenium biochemistry constitute the final topic. [ABSTRACT FROM AUTHOR]

Both natural and polluting metal ions are taken up by marine organisms. For many metal, and metal-containing ions, mechanisms exist which allow these organisms to discriminate between essential and non-essential ions, and reject the latter. In this article we examine the accumulation mechanisms and effects of metal and metal-containing ions on bivalve molluscs and tunicates (sea squirts). Special emphasis is put on the internal or physiological environment of metal ions in marine organisms. A molecular mechanism based on experimental studies is presented to explain the extraordinary accumulation of vanadium in tunicate blood cells called vanadocytes. Vanadium in the form of vanadate anion is transported into the blood cells by passage through anionic channels. Inside the cell it is reduced to cationic V(III) and V(IV) ions which are trapped inside. [ABSTRACT FROM AUTHOR]

Vanadium is an essential element. It is found widely distributed in minor and trace amounts throughout the lithosphere and biosphere. Vanadium is present in all mammalian tissues at concentrations of about 10 µM or less. Numerous physiological effects of this element are known at both the organismic and cellular levels, and in some instances these effects can be explained in terms of its biochemistry. Vanadate(V) and/or vanadyl(IV) ions are very potent inhibitors (K1 ≲ 10−6 M) of certain phosphatases, ATPases, phosphotransferases, nucleases, and kinases, among others. Vanadate(V) activates adenylate kinase. These ions have been quite useful as kinetic and spectroscopic (EPR) probes of complex cellular processes. Vanadium is an insulin mimetic agent. The ability of vanadium at low concentrations to influence enzymes is probably important to its function in vivo. Much of the biochemistry of vanadium is intricately related to the redox and coordination chemistry of this element. In particular, the parallel between the chemistries of vanadates and phosphates can account for the observed inhibition of various phosphohydrolases. However, there is ample evidence that vanadium in the body exists primarily as the vanadyl(IV) ion, VO2+, complexed to proteins and other cellular components; several biological reducing agents can convert vanadium(V) to this form. Serum transferrin appears to be involved in the metabolism of vanadium, perhaps as the transport protein for VO2+. Despite recent major advances in our understanding of the biochemistry of vanadium, the delineation of the physiological function(s) of this important element has remained an elusive goal. [ABSTRACT FROM AUTHOR]

Studies on the enzyme nitrogenase have shown that it contains two novel metal-sulfur clusters: the iron-molybdenum cofactor (FeMo-co), which contains one Mo, 6-7 Fe, and ∼ 4 S atoms in a spincoupled paramagnetic (S=3/2) cluster; and the P clusters, variants of normal but highly reduced 4 Fe-4 S clusters in which three Fe atoms are somehow distinguished from the fourth and which, when oxidized, exhibit an usually high spin (S=5/2 or 7/2) ground state. No structures consistent with all available spectroscopic data have been proposed for either unit. Synthetic efforts aimed at developing models for FeMo-co have utilized tetrathiomolybdate as starting material, and have resulted in two well-characterized classes of Mo-Fe-S cluster: those containing the MoFe3S4 "cubane" core and those containing a "linear" MoS2Fe unit. The former apparently contain Mo in a relatively reduced valence state (ca. + 3) and exhibit Mo-S and Mo-Fe distances in good agreement with those found for the Mo site of nitrogenase; the latter contain Mo in a higher oxidation state (ca. + 6) and have significantly shorter Mo-S distances. Synthetic work related to the P clusters has centered on the properties of the [Fe4S4(SR)4]3− ions, for which two different structures with different magnetic properties are found, and on the properties of 4 Fe-4 S clusters with non-thiolate ligands. [ABSTRACT FROM AUTHOR]

A detailed picture of the unique electronic structure of active sites in copper proteins is essential to our understanding of their biological functions. This has been achieved through a combination of complementary spectroscopic techniques, chemical perturbations, and parallel studies on copper complexes which serve as spectral analogues. These studies provide a basis for the generation of a "spectroscopically effective" picture of the active site and allow a correlation of changes in geometric and electronic structure with variation in function. When combined with high-resolution crystallographic information, single crystal spectral studies allow a correlation of electronic features with specific geometric features and provide a great deal of insight into the bonding at the site. [ABSTRACT FROM AUTHOR]

Theoretical aspects of mineral flotation in particular the adsorption of flotation reagents at the mineral-water interface, were reviewed in 1959 for the last time. Several papers have been published since on this topic. The aim of this chapter is to survey recent publications that have contributed to the development of the flotation theory. [ABSTRACT FROM AUTHOR]

Studies on the solvent extraction of actinide ions by different combinations of extractants have been reviewed. Various equilibria involved in the extraction processes and the formation of the extractable complexes have been considered along with their equilibrium constant data. Various methods which are useful in establishing the composition and the nature of the extractable complexes are presented. The data on isolation and structural studies of some complexes, involved in synergic extraction, are also included. A brief description of the different areas in which synergic extraction is finding application is also given. Many combinations of extractants, where the studies conducted are very few but, which are likely to yield enhanced extractions are indicated. Areas of research, both from the academic and applied points of view, which require attention are suggested. [ABSTRACT FROM AUTHOR]

In a homogeneous chemical reaction maintained far from equilibrium where the reaction mechanism contains coupled feedback steps, oscillations in the concentrations of certain intermediates are possible. Until recently, the few known chemical oscillators were discovered accidentally, were variants of these reactions, or were extracted from biological systems. To provide new chemical oscillators for theoretical and experimental study, a program has been developed to design homogeneous oscillators based on reactions of inorganic compounds and ions. Mechanistic detail is presented for the chlorite-iodide-arsenite oscillator, and other newly designed oscillators are discussed more briefly. A classification scheme for grouping these and related oscillators is presented. [ABSTRACT FROM AUTHOR]

This review departs from a discussion of some basic features of light-induced electron transfer reactions in solutions. Particular emphasis is placed on charge separation and kinetic control of the events by organized molecular assemblies such as micelles and vesicles. Subsequently, we describe some fundamental aspects of redox catalysis with colloidal dispersions of noble metals and their oxides. The application of these concepts to the problem of hydrogen and oxygen production from water by visible light is illustrated. Detailed analysis is made of methods to obtain highly active and selective catalysts, and the performance of bifunctional redox catalysts in cyclic water cleavage systems by visible light is described. [ABSTRACT FROM AUTHOR]

Solar energy is the prominent renewable source of electricity if cheap methods will be designed for its concentration and conversion. The geographical, seasonal and spectral distribution of the terrestrial solar energy are described. The concept of non-tracking fluorescent concentrators and the mathematical background for collection and concentration of direct and diffuse light are discussed. The single plate, multiple stacks and thin films doped by one or more colorants are reviewed. The differences between inorganic and organic materials are analyzed. Suggestions for new hybrid systems are made. [ABSTRACT FROM AUTHOR]

In graminaceous plants such as barley, oats, and wheat, novel iron-chelating amino acids are secreted from the roots. A typical example is mugineic acid. A phytosiderophore, mugineic acid significantly stimulates iron-uptake and chlorophyll synthesis in rice plants. Most microbial siderophores have hydroxamate or phenolate groups as Fe(III)-coordination donors, while phytosiderophores consist of carboxyl, amine, and hydroxyl groups as the ligand functional groups. The mugineic acid-Fe(III) complex and its structurally analogous Co(III) complex have been characterized by some spectroscopic and X-ray diffraction methods. The coordination of mugineic acid to Co(III) and Fe(III) ions involves the azetidine nitrogen, secondary amine nitrogen, both terminal carboxylate oxygens as basal planar donors, and the hydroxyl oxygen and intermediate carboxylate oxygen as axial donors in nearly octahedral configuration. The Mössbauer (ΔEQ = 0.24 and δFe = + 0.39 mm/sec) and ESR (g = 9.4, 4.51, 4.44, and 4.31) parameters of the mugineic acid-Fe(III) complex are characteristic of high-spin (S = 5/2) ferric type. Of special interest is the apparent high reduction potential (E1/2 = − 102 mV vs. NHE) of the mugineic acid-Fe(III) complex, as compared to those of the microbial hydroxamates and ferric enterobactin. The mechanism of iron-absorption and -transport in gramineous plants probably includes Fe(III)-solubilization by mugineic acid and reduction from the thermodynamically stable ferric mugineic acid complex (log KMLM = 18.1) to the weakly bound ferrous complex (log KMLM = 8.1). [ABSTRACT FROM AUTHOR]

Various chemical syntheses of the lysine containing analogues of siderophores di-Na,Nω-(2,3-dihydroxybenzoyl)lysine (I) and di-Na,Nω-(2,3-dihydroxybenzoyl)lysyl-Nω-(2,3-dihydroxybenzoyl)lysine are reported. The most effective synthon for introduction of the catechol moiety proved to be the N-hydroxysuccinimide ester of 2,3-dibenzyloxybenzoic acid. [ABSTRACT FROM AUTHOR]

A brief presentation of iron chemistry is made with emphasis on those aspects relevant to siderophore biochemistry. Siderophore structure and biosynthesis is described. The underlying chemistry associated with, 1, the movement of iron(III) complexes across membranes and 2, the removal of iron from such complexes is discussed in detail. The ability of siderophores to interact with other metals is considered. Finally, the role of siderophores in infection and their clinical potential as iron scavenging molecules are reviewed. [ABSTRACT FROM AUTHOR]

Siderophores, defined as iron(III) specific transport compounds, are widely distributed in aerobic and facultative anaerobic microbial species. The list of microbes known to form siderophores includes various enteric bacteria; human, animal and plant pathogenic bacteria and fungi; soil microorganisms; Gram positive and negative species, blue green alga (Cyanobacteria) and higher algae; nitrogen fixing bacteria; and many types of fungi including all species of Penicillia, Neurospora, basidiomycetes and certain types of yeast. As natural products, siderophores are classed conveniently as hydroxamates, catechols and "miscellaneous", the latter possibly structurally related to the phytosiderophores of plants. A variety of standard methods has evolved for detection, enhanced production, isolation, characterization and synthesis of the siderophores. The cloning of the enterobactin and aerobactin systems of Escherichia coli has been reported. A very large number of siderophores remain to be characterized as either known compounds or new products. The opportunities for technical exloitation of the substantial reservoir of basic research knowledge of siderophores abound in fields as diverse as clinical medicine and plant pathology. [ABSTRACT FROM AUTHOR]

The current status of research on the aerobactin gene complex of Escherichia coli has been reviewed from the standpoint of its organization and regulation. The reasons for selection of the basidiomycetous yeast, Rhodotorula pilimanae, as the most suitable eukaryotic species to which to extend similar studies have been enumerated. A convenient bio-assay is described for detection of mutants of Rhodotorula pilimanae blocked in the biosynthesis of rhodotorulic acid. The application of the bio-assay to a mutagenized culture of Rhodotorula pilimanae enabled collection of a series of mutants, one of which forms an altered version of the constituent amino acid of rhodotorulic acid, Nδ-acetyl-Nδ-hydroxyornithine. [ABSTRACT FROM AUTHOR]

A new type of solid state laser, tunable in the visible range, has been developed recently. Incorporation of perylimide and pyrromethene dyes into glasses prepared by the sol-gel method enables the design of new types of visible, stable solid lasers. These can be prepared either in the form of slabs or rods or as waveguiding active media deposited on glass or polymer supports. The difference between the refractive indices of the film and its support determines whether waveguiding occurs in the film or a leaky waveguide laser is formed. The efficiency of the lasers obtained recently is up to 70% when appropriate optical pumping is designed. We discuss the chemical and physical aspects of the photostability of the dyes in glasses, phenomena responsible for non-radiative relaxation of their excited state, and energy efficiencies of laser. [ABSTRACT FROM AUTHOR]

Cerium(III) energy levels and photophysics represent an intermediate status between d-group behavior and the other lanthanides showing narrow absorption bands and luminescence bands reminiscent of 4fq configuration atomic energy levels. Once the energetic separations of 0.5 to 2 eV of 5d-like orbitals are recognized to provide huge Stokes shifts and rather wide emission bands, it is also obvious that Ce(III) does not have a standard symmetry of ligating atoms (e.g. coordination number N = 9 and D3h know from aqua ions and anhydrous LnCl3 and LnBr3) but N is often 12, 11, 8, 7 or 6 (and rarely octahedral, as frequent in the d groups), 8 in irregular symmetries perhaps most typical. Scintillators for detecting very high kinetic energies (approaching the relativistic regime of photons and particles with very low rest-mass) involve closed shells, such as 4f and 5d in lutetium(III); barium 5p, 5d and 4f in highly electrovalent barium(II) in transparent crystals; and perhaps 4f and 6s in hafnium(IV) and tantalum(V) fluorides. [ABSTRACT FROM AUTHOR]

In the last few years the sol-gel process has turned into an interesting and promising method of synthesizing materials for obtaining thin or thick films with definite functions. The techniques of film preparation such as dip and spin coating are simple and allow us to prepare coatings with smooth optical surfaces with controlled stoichiometry, structure and texture. In this paper we give an up to date overview of what has been achieved in the field of chromogenic materials such as anodic or cathodic electrochromic coatings, counter or ion storage electrodes, transparent electron conductors and ionic conductors to be used in electrochromic (EC) devices. We also review the sol-gel research in the related areas of photochromic, thermochromic and electrooptic sol-gel materials whose properties are essentially used to modulate the luminous or solar energy as well as sol-gel materials prepared in the form of nanoparticles proposed recently for the development of a new type of solar cell. Finally we stress the future developments in these fast growing fields. [ABSTRACT FROM AUTHOR]

The field of nonlinear optics has been characterized by a remarkable increase in research activities during the past decade. We summarize the specific research into nonlinear optical materials prepared by the sol-gel technology. The chapter describes the preparation of glasses doped by semiconductor nanoparticles such as CdS, CuCl, CuBr, Bi2S3 and PbI2, metal nanoparticles and organic dyes. The third order susceptibilities of the various materials are tabulated. Glasses doped by organic dyes, their preparation, and properties of second and third order nonlinearity are discussed. The potential of these materials for industrial and technical applications is stressed. [ABSTRACT FROM AUTHOR]

We review the principles of optical and electrochemical sensors based on the use of the sol-gel technique, in particular their fabrication, working principles, and various configurations. We also report on potential applications, e.g. to environmental and clinical analysis, to gas sensing, and to bioprocess monitoring. Methods are critically reviewed for making such sensors, how to encapsulate organic, inorganic and biological matter, and how to control the properties of the resulting materials. Specifically, sensors for gases, ions, and organic as well as bioorganic molecules are discussed in some detail. [ABSTRACT FROM AUTHOR]

Over the last few years, considerable progress has been made with sol-gel coating films for optical and electronic applications. Nonlinear optical films and ferroelectric films are among those that have been studied most extensively. Optical and electronic coating films based on inorganic-organic hybrid or composite films have attracted much attention. These and other recent developments are stressed in this article which deals with a wide variety of optical and electronic coating films. [ABSTRACT FROM AUTHOR]

The present article reviews the photochemical deactivation modes and properties of electronically excited metallotetrapyrroles. Of the wide variety of complexes possessing a tetrapyrrole ligand and their highly structured systems, the subject of this survey is mainly synthetic complexes of porphyrins, chlorins, corrins, phthalocyanines, and naphthalocyanines. All known types of photochemical reactions of excited metallotetrapyrroles are classified. As criteria for the classification, both the nature of the primary photochemical step and the net overall chemical change, are taken. Each of the classes is exemplified by several recent results, and discussed. The data on exciplex and excimer formation processes involving excited metallotetrapyrroles are included. Various branches of practical utilization of the photochemical and photophysical properties of tetrapyrrole complexes are shown. Motives for further development and perspectives in photochemistry of metallotetrapyrroles are evaluated. [ABSTRACT FROM AUTHOR]

Several tetrapyrroles are employed by Nature to carry out fundamental biological functions and synthetic models compounds have been extensively used in order to understand the mechanisms of action of natural systems. One of the structures present in many enzymes is the corrinoid structure. It consists of a tetrapyrrolic macrocycle, where a direct link between two pyrrole rings exists. Such a direct link and the different degrees of unsaturation that can be introduced into the macrocycle modulate its chelating properties towards metal ions and its reactivity. A most interesting example of the corrinoid structure is corrole, a macrocycle where an 18 electron aromatic π system analogous to that of a porphyrin is maintained. Corrole has been shown to be a versatile ligand capable of coordinating transition and main group metals without significant distortion of the macrocycle plane. The present article reviews the developments of the chemistry of corrole and its metal complexes considering the synthetic procedures that can be followed in order to prepare such compounds, their spectroscopic characterization and redox reactivity and demonstrates the peculiar ligand field effect of this macrocycle. The chemistry of other synthetic corrinoids is also reviewed with special attention to their preparation procedures and axial ligand binding reactions. [ABSTRACT FROM AUTHOR]

The noble metals ruthenium, osmium, rhodium, iridium, palladium, platinum, silver, and gold form a variety of complexes M(P) LL′ with tetrapyrrole ligands which are studied in many laboratories because their coordination chemistry and catalytic power is similar to that of the corresponding iron and cobalt tetrapyrroles forming the prosthetic groups in heme proteins and vitamin B12. Apart from the corrins and corroles which are treated in the second article of this volume, the main interest is devoted to noble metal complexes of porphyrins, but complexes of hydroporphyrins, porphyrin analogs, homologs, porphyrinoids, and phthalocyanines are also mentioned where appropriate. The treatise is organized into four main topics: metal insertion ("equatorial coordination chemistry", Sect. 2, formation of the M(P) entity), substitution and redox reactions ("axial coordination chemistry", Sect. 3, exchange of axial ligands L and L′), organometallic chemistry (a special case of axial coordination chemistry, Sect. 4), and notes on current special chemical topics like electrochemistry, catalysis, supramolecular chemistry, and biological applications. Where possible, the material is compressed in schemes and tables giving a synopsis of consecutive reactions of Ru/Os and Rh/Ir dyads. The two other dyads, Pd/Pt and Ag/Au, are so strongly governed by the square planar d8 configuration that they are practically devoid of axial coordination chemistry. The redox reactions of the Ru/Os dyad live on the switches between d6-d5 and d6-d4-d2 configurations mostly with octahedral geometry about the metal, those of the Rh/Ir dyad on the switches between the d6-d7-d8 configurations, giving rise to octahedral, square-pyramidal, and square planar geometries. The occurrence of the dinuclear or mononuclear Rh d7 systems give a special touch of radical chemistry to the organometallic chemistry of rhodium porphyrins which is expected to be fruitful to organometallic chemistry as a whole. Although a broad overview was intended, many entries are just given in keywords or tables in order to give the reader a chance to get more information. Nevertheless, out of nearly 500 papers scrutinized, only nearly 400 references appear in order to keep the volume and the writing time within certain limits, and any research group missing a paper in this review is asked for patience with the authors. [ABSTRACT FROM AUTHOR]

This article gives a review of complexes in which a dioxygen ligand is bonded to a transition metal. Three aspects of these complexes are discussed in detail: the structure, the electronic structure, and the reactivity. The structural section summarises the recent X-ray crystal structure determinations, and the structural data obtained by other methods. The electronic structure is first considered in qualitative terms which allow the rationalisation of the different structures observed, and this qualitative model is compared with the results of calculations and with spectroscopic data. The reactivity of the complexes is discussed separately for each structural class in terms of the electronic structure. An attempt is made to compare the results obtained in historically different areas of research. Our objective is to give a clear summary of current knowledge of these compounds for workers interested in their application to catalysis and in their rôle in biochemical systems. [ABSTRACT FROM AUTHOR]

The aim of the present article is to review several biomolecules, the spectroscopic and structural properties of which have been so far interpreted in the light of the Jahn-Teller effect. Indeed the importance of the vibronic coupling in the interpretation of physical properties of molecules and crystals is widely recognized now, but only a desultory attention has been paid to the role played by such a coupling in biological systems. After a brief outline of the Jahn-Teller effect, the whole problem is critically re-examined separating the systems for which the experimental evidence of the Jahn-Teller effect is clear, from those where its effectiveness is still debated. Furthermore some suggestions are given for a proper inclusion of the vibronic coupling in future analysis on biomolecules. [ABSTRACT FROM AUTHOR]

This review shows that the growing interest aroused by phospholes in coordination chemistry can be attributed to three main reasons: 1. The influence of the partial delocalization of the phosphorus lone pair upon the complexing ability of phospholes. 2. The discovery of phosphametallocenes. 3. The tremendous variety of complexing modes found for the phosphole nucleus. For a given phosphole, no less than ten different types of complexes can be obtained. [ABSTRACT FROM AUTHOR]

The ability of coordination polyhedra of different central atoms in the solid state to undergo distortions has often been discussed. These distortions are also correlated with the nature of the central atom. This paper deals with the conception of crystal structure non-rigidity for central atoms of some selected complexes by the using of statistical distributions of interatomic distances central atom-nearest ligand atom (M-L; M=Mn(II), Fe(II), Fe(III), Co(II), Co(III), Ni(II), Cu(II) and Zn(II)). Based on the vector equilibrium principle the dispersion of these empirical distributions has been introduced as a measure of non-rigidity of central atoms, decreasing in the order: Cu(II) > Fe(II)HS > Mn(II)HS > Ni(II)LS > Zn(II) > Fe(III)HS ≈ Co(II)HS > Ni(II)HS > Co(III)LS, where LS and HS denote low-spin and high-spin states, respectively. Starting from the distribution of M-L distances for certain types of central and ligand atoms, the contribution of atoms of the inner coordination sphere to the vector equilibrium of crystal structures is discussed. The structures of complexes with all M-L bond lengths of at least one symmetrically independent central atom in the interval of maximum frequency exhibit in the appropriate set the most stable inner coordination sphere. These compounds are classified both with respect to the geometry of their inner coordination sphere fulfilling the above condition and to their crystal structure aspect. The square-planar geometry is found to be the most stable for Cu(II) and Ni(II)LS complexes, tetragonal-pyramidal for Zn(II) complexes, while for Fe(II)HS, Mn(II)HS, Fe(III)HS, Co(II)HS, Ni(II)HS and Co(III)LS complexes it is the regular octahedral geometry. Except for one Mn(II)HS complex, in the discussed structures chromophores with the most stable geometry of the coordination polyhedron form part of the final relatively isolated structural unit (island) with expressively chemical bonds between atoms. These facts are illustrated by the stabilization effected by the crystal field and by the properties of the ligands. [ABSTRACT FROM AUTHOR]

Proton transfer and hydrogen bonding are two aspects of the chemistry of hydrogen that respectively govern the behaviour and structure of many molecules, both simple and complex, from water to DNA. The β-dicarbonyls exhibit both of these features, and in ways which have singled them out for detailed study for many years. They provide the best known examples of keto ⇆ enol tautomerism, with the advantage of slow proton transfer and high concentrations of the enol tautomers in most cases. These enols are stabilized by intramolecular OHO hydrogen bonds which at various times have been thought of as being centred, linear hydrogen bonds that are somehow incorporated into the delocalised π system to give "aromatic" systems. Research involving structural, spectroscopic and computational techniques has deepened our understanding of these compounds and changed our picture of them. The hydrogen bonding is surprisingly strong, surprising since it is neither centred, nor linear, nor involved in the ring's delocalized bonding, although it is certainly coupled to it. This review deals with controversies that have surrounded the β-dicarbonyls and discusses the current view that the enol tautomers are a double-minimum potential well with a low energy barrier. The review ends with a brief look at the β-thioxoketones, which provide an analogous system based on a heteronuclear SHO hydrogen bond in the enol or enethiol tautomer. [ABSTRACT FROM AUTHOR]

The Angular Overlap Model is applied to the calculation of the Jahn-Teller coupling constants for all possible ground states of ML6dx systems in Oh symmetry. In the weak field scheme the LSMLMS〉 basis is treated by an operator equivalent technique and results are also derived for the LSJMJ〉 basis to include the effects of spin-orbit coupling. In the strong field basis both high- and low-spin ground states are considered where these arise and in all cases the extent of quenching of Jahn-Teller activity due to spin-orbit coupling is evaluated. The derivation of the required angular overlap parameters and the general utility of the method are briefly considered. [ABSTRACT FROM AUTHOR]

As a consequence of the non-spherical symmetry of the copper(II) ion, d9 configuration, and of the influence of the Jahn-Teller, and pseudo Jahn-Teller effect on six-coordinate geometries, the stereochemistries of the copper(II) ion are characterized by non-rigid geometries (fluxional behaviour), and ranges of distorted geometries (Plasticity Effect). The latter may be connected by a series of Structural Pathways, which may be characterised by an ‘Electronic Criterion of Stereochemistry' for a related series of complexes, e.g. the [Cu(bipy)2X] [Y] complexes. [ABSTRACT FROM AUTHOR]

Metal Complexes of the allyl (C3H5) and cyclopentadienyl (C5H5) ligands have played a major role in the development of organometallic chemistry. Metal allyl complexes are well known for the many intricate organic transformations which they may mediate, yet often possess limited stability. Metal cyclopentadienyl complexes on the other hand possess very high stabilities but only very limited catalytic chemistry. The pentadienyl ligand (C5H7 as well as various methylated derivatives) has similarities to both the allyl and cyclopentadienyl ligands, and it has now been demonstrated that metal pentadienyl complexes possess not only thermal stability but also chemical and catalytic reactivities. This favorable combination then will allow for detailed correlations to be made between chemical reactivities and the physical natures (structural, spectroscopic, etc.) of these compounds. In this article the presently available information on metal-pentadienyl compounds is discussed in order to gain some understanding of their nature compared to the related allyl and cyclopentadienyl systems, and to anticipate future applications of metal-pentadienyls. Particular emphasis is given to the bis(pentadienyl)metal complexes, which have been referred to as the "open metallocenes". [ABSTRACT FROM AUTHOR]

A great number of active centers in proteins and enzymes contain transition metals. Each metal imposes specific catalytic properties on the protein which could not be achieved when employing a different metal. The special role and characteristic reactivity of icoordinated copper is illustrated by looking at the evolutionary aspects of copper proteins. The evolution of copper-binding sites is closely linked to that of copper proteins. Copper centers have evolved according to various principles, similar to the evolutionary development of proteins and enzymes. Three basic principles may be observed: the copper-binding centers of metallothioneins and type 1 copper centers developed in non-metal proteins; the transformation of the metal-binding centers of iron or manganese proteins led to the development of the type 2 copper centers; and the trinuclear copper-binding centers of the blue oxidases were formed by alterations and recombinations of type 1 copper centers. Simple, although as yet unknown, mononuclear copper centers gave rise to the type 3 copper-binding sites of the hemocyanins and tyrosinases, as well as to the CuA- and CuB-centers of the cytochrome oxidases and N2O-reductase. According to this assignment, copper proteins containing either type 1 copper, type 3 copper, CuA-centers, or CuB-centers share a common ancestor. They developed at least in part by divergent evolution. Type 2 copper proteins are the products of convergent evolution and, consequently, show little or no phylogenetic homology. [ABSTRACT FROM AUTHOR]

Hemeproteins are widespread in all groups of living organisms, both as components of key biological processes as well as in very specific metabolic pathways. By variation of either the heme porphyrin structure or the heme protein environment, the function and behavior of the hemeproteins may vary drastically, allowing its role in the most diverse processes. In this short review, selected examples of hemeproteins from anaerobes will be presented, aiming to show the basic, common features of these family of proteins, as well as their diversity in terms of function and composition. [ABSTRACT FROM AUTHOR]

The terminal step in methane generation by several methanogenic organisms, of which the best studied is the archaeon Methanobacterium thermoautotrophicum, is catalyzed by the enzyme S-methyl coenzyme M reductase (methylreductase, EC 1.8.-.-). This enzyme contains a macrocyclic tetrapyrrole-derived cofactor, F430, at the active site coordinating Ni(II) in the resting state. A Ni(I) state (Ni1F430) has been proposed as the active form of the cofactor. Extensive mechanistic and spectroscopic studies have been performed on the holoenzyme, isolated cofactor, and various synthetic model compounds. These studies are summarized in the present review. [ABSTRACT FROM AUTHOR]

Nickel has so far been implicated as a cofactor in only a handful of biological reactions. These reactions are, however, central to many archae and bacteria as they provide reducing power from molecular hydrogen oxidation, readily available nitrogen from urease (also in plants and fungi) and acetyl-coenzyme A from CO and CO2. As new microbial proteins are studied, the role of nickel is likely to extent to several other metabolic processeses. One of the major issues concerning catalytic Ni is whether it can be redox active in the biological context. Recent results concerning hydrogenases and CODHs indicate that their catalyses are carried out by NiFe centers; thus the role of Ni in these enzymes may be essentially catalytic without necessarily involving redox chemistry. [ABSTRACT FROM AUTHOR]

Mixed valence has been considered with reference to three model systems: Fe3O4 (magnetite) is a representative of the ferrospinels and the iron oxides with spinel-related structures, Fe1−δO (wüstite) illustrates a system with random defects and clustering, CaFeO3 (a perovskite) illustrates the consequences of strong Fe4+-O-Fe4+ interactions. The influence of counter cations is particularly marked in systems like the spinels Fe2SnO4 and Fe2MoO4; more subtle effects are illustrated by a comparison of CaFeO3 with SrFeO3. The time τh for an electron transfer between cations of different valence has been considered relative to two times: the period ωR−1⋍10−12s of an optical-mode lattice vibration that traps a mobile electron and the time τn for a Mössbauer nuclear excited state to decay to its ground state. The full range of electron-transfer times τh<10−12s to τh>10−8s has been found for both Fe-Fe interactions across shared site edges or faces and Fe-O-Fe interactions across shared site corners. Perturbations of the periodic potential of an iron array by lattice defects or substitutional ions is shown to increase τh. Moreover, electron-lattice interactions were found to induce cooperative Jahn-Teller distortions in some compounds and charge-density waves (CDW) in others. In the mixed-valent compound Fe3O4, electrostatic interactions between mobile ions combine with electron-lattice interactions to produce, with increasing temperature, transitions from ionic ordering (or a static CDW) in the temperature interval 120

Sulphur containing amino acid residues are found among the structural and functional constituents of many proteins. The ease of oxidising sulphur in these biologically important polymers is well known. At present, there is no simple and fast technique available to monitor the respective oxidation states using chemical reactions. X-ray photoelectron spectroscopy has proven to be both convenient and efficient to follow these electron transport reactions. In the present survey a choice of some sulphur and selenium bearing proteins and metalloproteins will be examined upon excessive irradiation, treatment with hydrogen peroxide and cleavage of metal sulphur bonding. It will also be shown that the use of this technique in general will allow a quick decision on the rate of deterioration of a protein. The limitations of this spectrochemical approach will be debated. [ABSTRACT FROM AUTHOR]

The alternative approach to structures is an inversion of the traditional one: we concentrate on a packed array of cations, into the interstices of which the anions are inserted, rather than putting cations into a packed anion array. Even in conventional terms the logic of the new approach can be justified, since many structures have their own antitypes (and even more show a partial structure/antistructure relation). In this way we find that, in oxides for example, many cation arrangements are identical to the arrangements of the atoms in known (or plausible) alloy structures. (In some cases the cations and atoms are identical in the two cases.) This is particularly helpful in those oxide examples where it has not previously been possible to describe the structure in any simple terms, often because the anion array is not regular in any simple way, for instance in many metal sulphates. But it is also revealing in some cases that are describable in conventional terms, e.g. the humites. A long (but still incomplete) list of examples is given in Table 3. Sufficient specific cases are described and discussed in detail in the text in order to expose the principles and some of the advantages of this unfamiliar approach. One of these advantages is that, in favourable cases, it leads at once to complete quantitative descriptions of the structures (i.e. until cell dimensions and anion position parameters), e.g. for spinel, garnet, bixbyite etc. Another is that it throws a new light on possible martensitic mechanisms for phase transformations (e.g. β- → γ-Ca2SiO4, low- → high-temperature K2SO4), and on the possibility of anomalous substitution (e.g. of CO32− for PO43− in apatite). It also reveals an explanation for those phase transitions, due to high pressure, that are not attended by an increase in primary coordination number, e.g. olivine → spinel, zircon → scheelite. [ABSTRACT FROM AUTHOR]