bluuebunny/crossref_metadata_2025 huggingface dataset:
Cytochrome OxidaseBased in part on the article Cytochrome Oxidase by Gerald T. Babcock which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Wikström Mårten
Published: December 2011

Heme‐copper oxidase is the current generic name for the superfamily of respiratory enzymes that are responsible for cell respiration in the mitochondria of eukaryotic cells as well as in aerobic prokaryotes, and which shows substantial structural and functional homology among its members. This group of enzymes is responsible for more than 90% of the consumption of dioxygen by living organisms on this planet. Harnessing the high oxidizing power of the O2/H2O redox couple some 2.5–3 billion years ago made a revolutionary impact on evolution. The oxidation of foodstuffs by O2gave an approx. 15‐fold energetic advantage over previously predominant glycolytic oxidations, and was the prerequisite for the development and evolution of all higher multicellular organisms. Apart from mastering the O2reduction chemistry in a safe and economical fashion, these enzymes also function as remarkable energy transducers that convert much of the free energy released in the process into a proton gradient, which is subsequently used to drive virtually all energy‐requiring processes in the cell. This article reviews the major advances in the last 10 years of research on this group of enzymes. The major highlights are the elucidation of the crystal structures of several oxidases, major advances in the understanding of the mechanism of O2activation and reduction at the bimetallic heme iron‐copper active site, and recent progress in the ongoing research on the perhaps most intriguing but least known function, redox‐coupled proton translocation.
Decarbonylation Catalysis
Authors: Jardine Fred H.
Published: December 2011

Aldehydes or acid chlorides can be decarbonylated to alkanes or alkenes respectively by rhodium complexes. However, α,ω‐alkenals frequently isomerize to cyclic ketones. Palladium complexes decarbonylate aroyl cyanides. Stoichiometric reactions occur with many carbonyl compounds at low temperatures. Above 150°C, several of the reactions are catalytic. Aldehyde tautomers such as allylic alcohols or carbohydrates are also decarbonylated.
Defects in Solids
Authors: Tilley Richard J. D.
Published: December 2011

The chemical aspects of defects of relevance to inorganic crystalline solids are described, especially the close links with the composition of crystalline nonmetallic inorganic nonstoichiometric compounds. The discussion begins with a description of simple point defects in stoichiometric materials and the relationship between the population of these defects and the Gibbs energy of formation of the defects. This includes defects in monatomic structures and balanced populations of point defects, Frenkel and Schottky defects, in binary compounds. Superionic compounds that contain quasi‐liquid cation arrays are considered as extreme cases of these simpler defect populations. Nonstoichiometric compounds treated from the standpoint of small populations of point defect populations are considered, including color centers. Defect chemistry, the incorporation of point defects into chemical nomenclature, is described, including the Kroger–Vink notation and the construction of Brouwer diagrams. Grossly nonstoichiometric compounds, including those that can be considered to contain point defects or structurally organised defect clusters, such as the iron oxide wüstite, and the defect fluorite structures are illustrated. Extended defects portray a wider view of defect chemistry, in which planar faults and intergrowths are prominent structural features of many complex inorganic solids. To illustrate planar defects, examples are taken from crystallographic shear phases, twinned structures, intergrowth structures, and the niobium oxide block structures. Structures containing pentagonal columns linked by vertices provide an example of structures that contain columnar defects. Examples of inorganic solids that possess more than one defect type are taken from layered perovskite phases, tungsten bronzes, and high‐temperature ceramic superconductors. The final section is concerned with structures that were once described as containing defects but that have recently been found to be essentially defect free. These materials, in which defects are eliminated by the device of continuous structural modulation affecting one part of the structure, are widespread over many classes of inorganic solids. Examples drawn from oxides related to tantalum pentoxide, layer misfit phases, and anion‐excess fluorite‐related materials.
Diffraction Methods in Inorganic Chemistry
Authors: Howard Judith A. K. , Batsanov Andrei S.
Published: December 2011

X‐ray diffraction is by far the largest source of information on crystal and molecular structures and (at high precision) can reveal fine details of the electron density distribution. Neutron diffraction is indispensable for special tasks, for example, locating hydrogen atoms and studying crystal dynamics and magnetic properties. After a brief introduction on crystal symmetry and diffraction physics, the article surveys the currently available instrumentation and practical techniques of diffraction experiments, of interpreting the structural data and assessing their quality, with an emphasis on recent progress in technology (synchrotron radiation, area detectors, low‐ and high‐temperature, and high‐pressure experiments), software (direct phasing, dealing with twinning and incommensurate phases), and data storage (crystallographic databases).
Dioxygen & Related LigandsBased in part on the article Dioxygen & Related Ligands by Rebecca R. Conry & Kenneth D. Karlin which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Conry Rebecca R.
Published: December 2011

Metal–dioxygen interactions are important in the activation of dioxygen by aerobic organisms and in other processes that utilize dioxygen, for instance, in catalytic oxygenation reactions. Most d‐block metals have been found to interact with dioxygen and many dioxygen‐containing complexes have been isolated. A number of binding modes have been found in these complexes, including the simple binding of one O2molecule to one metal center and systems where the O2moiety bridges two or more metal ions. Some reduction of dioxygen occurs upon binding; thus, these complexes are generally regarded as containing superoxide (one‐electron reduced) or peroxide (two‐electron reduced) ligands, which are typically spectroscopically distinguished. In general, superoxo complexes exhibit υ(OO) in their infrared or Raman spectra between about 960 and 1200 cm−1and usually contain planar metal2–O2units for dinuclear complexes, whereas peroxide complexes result in υ(OO) between about 650 and 950 cm−1and the dinuclear complexes typically have nonplanar metal2–O2units. Examples of superoxo and peroxo complexes for the different binding modes are discussed, including the discovery that some dinuclear metal–dioxygen complexes [M(O2)M; M = metal] are in equilibrium with bis‐μ‐oxo complexes [M(O)2M; i.e. the OO bond is no longer present].
Electronic Structure of Solids
Authors: Canadell Enric
Published: December 2011

The basic language needed to understand the electronic structure of solids is introduced. The approach is chemically oriented and the notion of orbital interaction, so useful to understand the electronic structure of molecules, is used as much as possible. A step‐by‐step approach to the electronic structure of several solids is discussed. Notions like Density of states (DOS), Fermi surface (and nesting), Peierls distortions, charge and spin density waves (CDW/SDW), and COOP (crystal orbital overlap population) curves are presented and analyzed with reference to selected examples. The role of electron repulsions is also considered. The relationship between the electronic structure and the structural and transport properties of solids is discussed. A brief account of the more commonly used computational approaches for solids is also presented.
Fluorides: Solid‐State Chemistry
Authors: Massa Werner
Published: December 2011

Inorganic solid fluorides and fluorometalates play a unique role in inorganic chemistry. The preparation, mostly using aqueous or anhydrous HF or even elemental F2, requires special conditions: no glass or silicate containing vessels can be used. Their structural chemistry is governed by the outsider role of the fluoride anion. It is the smallest and least polarizable anion and its mainly ionic compounds follow the rules of hard spheres packing better than any other class of substances. Nearly all metal ions except those with the largest ionic radii form fluoride structures with octahedral [Mn+F6](6−n)–units. If the metal:fluorine ratio is less than 6, these octahedra share common vertices or, in rare cases, edges or even faces to account for electroneutrality, the more the lower the oxidation state of the metal. In this way, a variety of 1D chain, 2D net, and 3D framework structures is formed that can be classified into a few large structure families. In binary fluorides, for example, VF5has an octahedral cis‐chain structure, VF4a layer structure based on a square net of corner‐connected octahedra, VF3has a ReO3‐related 3D structure with all corners of octahedra connected, and VF2has the rutile‐type structure with partial edge‐sharing of octahedra. In ternary or quaternary compounds, for example, alkali or alkaline earth fluorometalates, the same principles are valid for the anionic substructure while the counter cations fill interstices and stabilize at the same time the structure. The main structure families are grouped according to the basic principles of octahedral nets. These are the ReO3‐related trifluorides, the fluoride perovskites AMF3, their relatives elpasolites A2A′MF6, and cryolites A3MF6, as well as some layer structures with square nets. A second large family is based on mixed octahedral nets including triple units: it includes for example, the pyrochlores, the weberites, the hexagonal and tetragonal tungsten bronze structures, and some derivable layer structures. Recent developments deal with ‘tailoring’ of low‐dimensional or framework structures using ‘soft‐chemistry’ methods. The relatively simple structural chemistry of fluorides invoked their use as model systems, for instance, for the investigation of electronic effects like Jahn–Teller ordering, or for magnetic investigations. Besides the cryolite Na3AlF6used in Al production, some fluorides are of technical interest owing to their special optical properties (e.g. laser optics, fluoride glasses), their ability for fluoride ion conductivity, or their role as catalysts for fluorinations or halogene exchange in organic systems.
Fluorine: Inorganic ChemistryBased in part on the article Fluorine: Inorganic Chemistry by Darryl D. DesMarteau, Charles W. Bauknight, Jr., & Todd E. Mlsna which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Atwood David A.
Published: December 2011

This article provides an overview of the extensive and diverse inorganic chemistry of fluorine. Key topics are covered, such as the methods used for the formation of element–fluorine bonds with hydrogen fluoride and elemental fluorine. The article includes a general description of the structure and bonding of common fluorine compounds and the various spectroscopic techniques for their characterization. More specific details are provided for the fluoride compounds of boron, and the groups 14–17 elements. New topics such as the means by which carbon–fluorine bonds may be broken and the fluorination of carbon‐based materials such as fullerenes and nanotubes are included in the appropriate sections.
Gold: Organometallic Chemistry
Authors: Schier Annette , Schmidbaur Hubert
Published: December 2011

Organogold chemistry reaches back to the beginning of the twentieth century, when Grignard compounds were first reacted with chloroauric acid. Since then, the number of organogold compounds has grown rapidly and now includes species with gold in various oxidation states (0, +1, +2, and +3) and with virtually all classes of organic substituents and ligands (alkyl, alkenyl, alkynyl, ylide, heteroaryl, ylide, carbonyl, isocyanide, carbenes, alkenes, alkynes, etc.). This development was possible owing to the discovery of many new synthetic pathways that reflect the general upsurge of organometallic chemistry in the last century and to the introduction of new physical and analytical methods as routine tools for the determination of composition and structure. Incentive and motivation for organogold research were also triggered by the emerging applications of gold and its compounds in medicine and pharmacology, as well as in many areas of modern technology, with material science and homogeneous and heterogeneous catalysis as the most prominent examples. The discovery of a novel type of bonding interactions, most common between low‐valent gold atoms with a closed‐shell electronic configuration (aurophilicity), has led to a rapid growth of the supramolecular chemistry of gold. The aggregation of mononuclear compounds to give dimers, oligomers, polymers, and clusters was found to induce novel photophysical properties (luminescence etc.), which attract widespread interest. In polynuclear compounds, aurophilic bonding is responsible for unique configurations and conformations of molecules, including clustering of gold atoms around hypercoordinate carbon atoms as one of the most curious phenomena.
Heterogeneous Catalysis by Metals
Authors: Ma Zhen , Zaera Francisco
Published: December 2011

Approximately 80% of the processes in the chemical industries require the use of catalysts. Because of that, catalysis generates annual global sales of about $1.5 trillion, and contributes directly or indirectly to approximately 35% of the world's GDP. Most catalytic processes are heterogeneous in nature, and often rely on the use of transition metal particles dispersed on high surface area supports. In this chapter, the basic chemical concepts behind heterogeneous metal catalysis are introduced. Examples are then provided for typical practical catalytic systems used in the generation of energy, the production of chemicals, the preparation of materials, and the cleaning of the environment. Finally, several promising new research directions related to the future development of heterogeneous metal catalysis are highlighted, in particular those associated with the designing and characterization of novel catalytic materials, the surface science of catalysis, high throughput catalyst testing, improved selectivities, and green chemistry.
Hydride Complexes of the Transition Metals
Authors: Crabtree Robert H.
Published: December 2011

The topic Hydride Complexes of the Transition Metals is reviewed with emphasis on structure and bonding, spectroscopy, energetics, synthesis, and reactions. Terminal, bridging, and cluster hydrides are covered, together with dihydrogen complexes and nonclassical hydrides.
Hydrides: Solid State Transition Metal Complexes
Authors: Yvon Klaus , Renaudin Guillaume
Published: December 2011

At present, some 50 different homoleptic transition (T) metal hydride complexes are known to occur in over 127 compounds that cover 47 structure types. The complexes form with T elements from group seven (Mn) to closed d‐shell elements of group twelve (Zn). While the great majority are centred by one T‐metal atom and display up to nine terminal hydrogen ligands, some are centred by two (Ru, Pt), four (Rh, Ni), or more (Ru, Rh) and display both terminal and bridging hydrogen ligands, and/or TT metal bonds. The structural and electronic configurations are consistent with those usually found in coordination compounds. The complexes are stabilized by charge transfer from the surrounding cation matrix, are often 18‐electron, and less often 16‐ or 14‐electron. Some hydride structures show evidence for metal–metal interactions as in typically interstitial metal hydrides while others contain additional hydride anions (H−) bonded to electropositive metals only, such as in saline hydrides. No HH bond formation is observed. Most hydrides are nonmetallic and many are colored. Some, however, show hydrogenation induced complex formation and metal‐to‐insulator transitions. The systems generally display very high volume efficiencies for hydrogen storage but are thermally too stable and/or too expensive for practical applications. Thermodynamic data suggest that their enthalpy of formation scales with the thermal stability of the binary hydrides of the electropositive metal constituents.
Hydrocyanation by Homogeneous Catalysis
Authors: Casalnuovo Albert L. , McKinney Ronald J. , Tolman Chadwick A.
Published: December 2011

The homogeneous catalysis of hydrocyanation is discussed with particular reference to the hydrocyanation of butadiene to adiponitrile, as employed in the commercial synthesis of nylon. Nickel complexes with bulky phosphite ligands are most effective and the mechanism is discussed. Asymmetric versions of the reaction are also discussed.
Hydrosilation CatalysisBased in part on the article Hydrosilation Catalysis by John F. Harrod which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Marschner Christoph
Published: December 2011

The addition of a hydrosilane across an unsaturated molecule is called hydrosilation. This can be catalyzed by a number of reagents among which transition metal compounds are most important. The reaction of a variety of different silanes with alkyl, alkoxy, and halogen substituents is well established. The unsaturated molecules involve alkenes, alkynes, and a huge number of different carbonyl compounds as well as combinations thereof. While hydrosilation of alkenes and alkynes can mainly be seen as a silylation method, the reaction with carbonyl compounds constitutes a formal reduction. Among the used catalyst systems hexachloroplatinic acid in isopropanol (Speier's catalyst) is most frequently used.

Mechanistically the transition metal catalyzed reaction of an alkene can be seen as an oxidative addition of the silicon hydrogen bond, followed by the migratory insertion of the coordinated alkene into the metal hydrogen or silicon bond and a final reductive elimination of the alkylsilane. The formation of vinylsilanes through the dehydrogenating silation reaction is a frequently occurring side reaction. Such by‐products are thought to be formed by β‐hydride elimination of an intermediate β‐silylalkyl metal complex.

The hydrosilation reaction can promote the transformation of a prochiral (alkene, ketone) to a potentially chiral molecule. It bears potential for asymmetric synthesis, which is currently heavily investigated. The reaction is equally popular for classical organic synthesis as well as for material science tasks such as the chemical bonding of a polymer to a surface.
Hypervalent CompoundsBased in part on the article Indium: Inorganic Chemistry by Dennis G. Tuck which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Sean McGrady G. , Steed Jonathan W.
Published: December 2011

The chemistry of compounds formally exhibiting more than eight valence electrons is introduced and surveyed. The review covers the definition and representation of hypervalent bonding and its orbital description, including recent developments in electron localization function calculations. The synthesis and reactivity of hypervalent compounds is examined including their use in organic synthesis, along with the factors contributing to their stability. Techniques for determining the structures of hypervalent species are described, including gas‐phase electron diffraction and vibrational and NMR spectroscopic methods. Novel hypervalent intermediates and transition states are also surveyed, including recently appreciated inter‐ligand hypervalent interactions. Particular emphasis is placed on the important topic of pentacoordination. The review includes chemistry of the halogens, chalcogens, pnictides, noble gases, tin, silicon, and a variety of transition metals.
Indium: Inorganic ChemistryBased in part on the article Indium: Inorganic Chemistry by Dennis G. Tuck which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Zhao Yanbao , Zhang Zhijun
Published: December 2011

This review highlights the development in inorganic chemistry of indium made in the last ten years. The first parts of this article describe the preparation of new low‐valence indium complexes and organoindium (III) compounds, and the use of these compounds as starting materials for further synthesis. A special highlight is the compounds R
4
In
4
and R
4
In
2
, which show the lively development and the current trends in this field. The following parts present the development of organoindium compounds with group 15 or 16 ligands as single‐source molecular precursors for the deposition of indium‐containing semiconductors by MOCVD; the preparation and characterization of nanoparticles, nanowires, and nanotubes of indium metal, alloys, oxides, and indium‐V/VI semiconductors, which play a sizable role in the new generation of nanoscale electronic and optical devices; and the preparation of new indium intermetallic compounds, which present large structural variety and peculiar magnetic and electrical properties. The last part advances the applications of indium and indium halides in organic synthesis with particular reference to regio‐, stereo‐, and chemoselectivity. These reactions include the regioselective allylation of unsaturated functional groups, the acylation of alcohols and amines, the rearrangement of epoxides, the stereoselective debromination, and other reactions.
Ionic Conductors
Authors: Greenblatt Martha
Published: December 2011

The fundamental structural and physical properties of ionic conductors in the solid state are briefly discussed. Recent developments in ionically conducting solids are reviewed. Materials discussed include: crystalline cation conductors with Na+, Li+, and H+ion motion; crystalline anion conductors, primarily with the fluorite‐type structure and with oxide‐ and fluoride ion mobilities; mixed ionic/electronic conductors; amorphous glass electrolytes and polymer solid electrolytes. Applications of some of these solid‐state ionic conductors in electrochemical devices such as sodium and lithium rechargeable batteries, sensors, fuel cells and electrochromic smart windows are presented.
Iron: Heme Proteins & Electron Transport
Authors: Durham Bill , Millett Francis S.
Published: December 2011

This chapter discusses the structure and function of cytochromes, which are a class of iron‐containing heme proteins primarily involved in biological electron‐transfer reactions. Subcellular organelles called mitochondria are responsible for carrying out oxidative phosphorylation, the major energy‐transduction process in eukaryotic cells. Enormous progress has been made in our understanding of the molecular mechanism of the mitochondrial electron‐transport chain over the past decade. X‐ray crystal structures have been determined for three of the four electron‐transfer complexes: succinate–ubiquinone reductase, cytochromebc1, and cytochromecoxidase (CcO). Cytochromec(Cc) is a small heme protein with a molecular weight of 12 500 Da that transports electrons from cytochromebc1to CcO. It is a very positively charged protein, and is known to bind to both cytochromebc1and CcO by means of electrostatic interactions. Extensive chemical modification studies have demonstrated that the binding domain on Cc for both proteins involves lysines immediately surrounding the heme crevice, and Cc functions as a mobile shuttle during electron transport. The reactions of Cc with its redox partners are too fast to resolve by conventional techniques such as stopped‐flow spectroscopy. A new method to study biological electron transfer has been introduced that utilizes a photoactive tris(bipyridine)ruthenium complex, Ru(II), which is covalently attached to a protein such as Cc. Photoexcitation of Ru(II) to the metal‐to‐ligand charge‐transfer state, Ru(II*), a strong reductant, leads to rapid electron transfer to the ferric heme group in Cc. Subsequent electron transfer from photoreduced heme c to redox center(s) in another protein can be measured on a timescale as short as 50 ns. This technique has been used to measure intracomplex electron transfer between Cc and its physiological partners, CcO, cytochromebc1, cytochromecperoxidase (CcP), and cytochromeb5. The ruthenium technique was used to characterize sequential electron transfer in cytochromebc1from ubiquinol to the Rieske iron–sulfur protein (2Fe2S), cytochromec1, and finally to Cc. Cytochrome oxidase contains four redox centers, CuA, heme a, heme a3, and CuB. A ruthenium Cc derivative was used to demonstrate that the initial site of electron entry into CcO is CuA, followed by electron transfer to heme a, and then heme a3.
Iron: Inorganic & Coordination ChemistryBased in part on the article Iron: Inorganic & Coordination Chemistry by Pelham N. Hawker & Martyn V. Twigg which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Burgess John , Twigg Martyn V.
Published: December 2011

Iron, the most abundant transition element in the biosphere, is essential in higher forms of life and its compounds have numerous industrial applications. The element has catalytic properties. Several simple iron compounds are industrially important, for example, large amounts of iron oxides are produced for pigment and magnetic recording media applications. Oxidation states ranging from −2 to +6 are known, the most familiar are +2 (ferrous iron) and +3 (ferric iron). Most common complexes are octahedral, although examples of higher (e.g. seven coordinate) and lower (e.g. tetrahedral and square planar) geometries are well known, as are a variety of mixed oxidation state compounds. Depending on ligands involved high‐ and low‐spin complexes can be formed, and temperature‐dependent spin‐state crossover complexes are well documented. The thermodynamics and kinetics of reactions of low‐spin iron(II) complexes with ligands such as 1,4‐diimines and CN−have been extensively studied. While low‐spin iron(II) is present in biological systems such as oxygenated heme centered oxygen carriers and thiolate clusters, iron(III) is more common. Thiolate‐containing iron clusters form readily, and here mixed oxidation states are a prominent feature that is exploited in some important electron transfer proteins. Microorganism iron uptake is facilitated by tris‐catecholate and tris‐hydroxamate ligands like enterobactin that have an extraordinarily high affinity for iron(III) that results in a dramatic enhancement of the 3+ relative to the 2+ state in terms of redox potential.
Iron: Organometallic ChemistryBased in part on the article Iron: Organometallic Chemistry by James R. Green which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Paley Robert S.
Published: December 2011

Studies involving the preparation, reactivity, and properties of the organometallic complexes of iron (compounds possessing a FeC bond) have played a central role in the growth of organometallics as a major subdiscipline with chemistry. From the discovery of ferrocene in the 1950s to the present day, interest and research activity remain high. This entry provides a survey of this large and diverse area of organometallic chemistry, and is arranged by increasing hapticity of the organic ligand of interest that is bound to iron. Included are discussions of the chemistry of iron σ‐alkyl, acyl, alkynyl, and vinyl complexes, iron carbenes, η1‐ and η3‐allyl iron complexes, η2‐alkene and η4‐diene iron complexes, trimethylenemethane and cyclobutadiene complexes of iron, cationic iron pentadienyl, cyclohexadienyl, and arene complexes, and ferrocenes and heteroferrocenes.
Lead: Organometallic ChemistryBased in part on the article Lead: Organometallic Chemistry by R. Bruce King which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Sharma Hemant K. , Pannell Keith H.
Published: December 2011

An update on the organometallic chemistry of lead since 1995 is presented. In particular the exciting chemistry associated with the synthesis, structure and reactivity plumbylenes, diplumbynes and related species are presented. In addition an extensive section on organolead transition metal complexes of is presented, an area not previously covered.
Manganese Proteins with Mono‐ & Dinuclear Sites
Authors: Penner‐Hahn James E.
Published: December 2011

Although manganese is not as widely recognized as iron and copper, it nevertheless plays an essential role in several key biological processes. Consequently, manganese concentrations are tightly regulated in most cells. Several Mn‐specific transporters have been identified and a novel Mn‐regulatory protein has been characterized. Manganese‐activated enzymes are proteins that have relatively weak affinities for Mn; in many cases these are isolated without any metal bound. Manganese‐activated enzymes are especially important in hydrolytic reactions, and appear, in at least some cases, to have physiologically significant changes in activity that respond to changes in Mn concentration. The best characterized Mn proteins are redox‐active enzymes, where Mn typically cycles between the MnIIand MnIIIoxidation states, although some MnIV‐containing species are known.
Mercury: Inorganic & Coordination ChemistryBased in part on the article Mercury: Inorganic & Coordination Chemistry by Gregory J. Grant which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Bebout Deborah C.
Published: December 2011

Mercury is one of the oldest known elements. The element exhibits surprisingly different chemistry from its congeners zinc and cadmium, and it is the only group 12 member without a known beneficial biological role. Unique properties associated with the element and its compounds have led to diverse applications. However, many of these are being phased out to minimize further increases in the environmental and atmospheric burden with this toxic heavy metal. Motivations to study the compounds and complexes of mercury include potential for discovery of novel properties such as high‐temperature superconductivity and improved understanding of its interactions in physiological systems. Valuable characterization of mercury‐containing compounds is possible with
199
Hg NMR techniques.

Mercury is the only group 12 member to have a stable monovalent state. Observed primarily in the oldest known and most common of all metal polycations, Hg
2
2+
, this metal–metal bonded species readily disproportionates into Hg
2+
and Hg
0
. Mercury(I) compounds, typically, have a linear structure of type XHgHgX. Halides, oxy salts, oxides, and coordination compounds with oxygen and weakly basic nitrogen‐donor ligands are best known for Hg
2
2+
.

The most prevalent ionization state of mercury, Hg
2+
, is a d
10
metal ion, which lacks strong coordination number and geometry preferences. As a result, structural characterization of its compounds by X‐ray crystallography is particularly critical. A wide range of halides, pseudohalides, and chalcogenides of Hg
2+
are known. The softness of Hg
2+
has led to an investigation of coordination, with a wide variety of sulfur ligands and physiological sulfur donors being assumed to be the most important. Mercury(II) typically has a stronger affinity for S‐, N‐, and P‐donors than any other divalent metal ion. However, Hg
2+
also has a tendency to undergo incredibly rapid ligand exchange. Recent studies have shown that sterically demanding multidentate ligand systems can suppress ligand exchange processes.
Metal Vapor Synthesis of Transition Metal Compounds
Authors: Schmidt Eckhardt , Klabunde Kenneth J. , Ponce Aldo , Smetana Alexander , Heroux David
Published: December 2011

The title topic is reviewed with special reference to the reactivity of metal atoms, their energetics of vaporization, vaporization techniques, and some detailed examples of reaction products. Recent advances in nanoparticle work are also discussed.
MetalloregulationBased in part on the article Metalloregulation by Robert P. Hausinger which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Rouault Tracey A. , Philpott Caroline C.
Published: December 2011

Metalloregulation refers to the concept that living organisms from bacteria to mammals must orchestrate the processes of metal import, utilization, sequestration, and export to optimize availability of metals for use in metabolism. Metals are required for the function of many proteins, but metals can also be toxic because free metal ions may react with protein, nucleic acid, and lipid contents of cells. In general, metals are transported across membranes by specific unidirectional transporters, and cells and organisms regulate the abundance of specific transporters and metal sequestration proteins to meet their needs. Changes in abundance result mainly from changes in either the rate of transcription, the stability of mRNA, the translation rate, or the stability of the metal protein. Complex regulatory frameworks exist to govern metal homeostasis in virtually all cells and organisms. Usually a central component of a successful regulatory network is a protein that registers metal availability by directly binding the metal ion that requires regulation. Metal binding then drives a change in function of the central regulatory protein, which usually has multiple regulatory targets that include transporters, chaperone proteins, sequestration molecules, and others. In this chapter, we focus on the main regulatory proteins of iron homeostasis in bacteria, yeast, plants, and mammals, and those involved with regulation of zinc homeostasis in bacteria and yeast. We describe the sensing mechanism and targets of the bacterial transcription factor, fur, and the yeast transcription factors known as Aft 1 and 2. In mammals, we focus on the sensing and regulatory mechanisms utilized by iron regulatory proteins (IRPs) and on the role of the peptide hormone hepcidin in physiologic regulation of iron metabolism. In zinc homeostasis, we focus on the bacterial transcription factors Zur and Znt 1, and the yeast transcription factor, Zap1. References for regulation of copper, manganese, and nickel metabolism are cited for further reading. Nature has produced a wide variety of solutions to the important question of how cells and organisms can ‘sense’ their metal status and modify gene expression to ensure that metals are appropriately available for use in metabolism.
Metathesis Polymerization Processes by Homogeneous Catalysis
Authors: Wagener Kenneth B. , Boncella James M. , Smith Dennis W.
Published: December 2011

Alkene polymerization reactions proceeding by homogeneous metathesis catalysis are reviewed. Fundamental mechanistic aspects of these reactions are discussed with special emphasis on ring opening metathesis polymerization and acyclic diene metathesis polymerization chemistry.
Mixed Donor LigandsBased in part on the article Mixed Donor Ligands by Nikolay N. Gerasimchuk & Kristin Bowman‐James which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Lawrance Geoffrey A.
Published: December 2011

Mixed donor ligands are extremely common and are linear, branched, or cyclic molecules that contain at least two different donor groups capable of chelation to metal ions by those groups as polydentate ligands. The most common donor atoms met are N, O, S, and P, although other donors such as group 14 elements (including C) are known. Whereas mixed donor ligands are considered classically to involve groups with different donor atoms (such as N from an amine and O from a carboxylate), different classes of donor groups of one type of donor atom (such as pyridine and amine groups) also effectively produce mixed donor ligands. The coordination chemistry of mixed donor ligands is very extensive; all metal ions form mixed donor complexes, and the number of isomers possible for complexes with mixed donors is usually greater than with symmetrical ligands of comparable denticity.
Molybdenum: Inorganic & Coordination Chemistry
Authors: McCleverty Jon A.
Published: December 2011

The inorganic and coordination chemistry of molybdenum is described, being presented according to ligands and/or donor atom set (halides, cyanides, ligands containing O, S, Se, Te, N, P, or As‐donor atoms) and are discussed in relation to oxidation state. Special mention is made of metal–metal bonded dinuclear species, a brief description of polyoxoanionic species is given, and tri‐ and tetranuclear cluster compounds and some species of higher nuclearity containing chalcogenide atoms are described.
Molybdenum:MPT‐Containing Enzymes
Authors: Young Charles G.
Published: December 2011

Molybdenum, an essential trace element for all forms of life, is found in a wide range of Mo‐MPT enzymes important in the metabolism of small molecules and anions of nitrogen, sulfur, carbon, chlorine, selenium, and arsenic, and in the global biogeochemical cycles of these elements. These enzymes feature active sites composed of a single (mononuclear) Mo atom coordinated by one or two MPT‐based ligands, L (MPT = ‘molybdopterin′ orMetal‐bindingPterin ene‐1,2‐diThiolate). These ligands contain a pyranopterin or simple pterin nucleus, an ene‐1,2‐dithiolate (dithiolene) unit, and a pendant phosphate group; in enzymes from prokaryotes, the phosphate is generally attached to a nucleotide, giving ‘dinucleotide’ forms of MPT. The dithiolene moiety functions as a bidentate S2‐donor ligand to Mo. The Mo‐MPT enzymes are classified into two broad functional types: oxotransferases and hydroxylases. The oxotransferases generally catalyze oxygen atom transfer reactions and fall into two families: the sulfite oxidase and dimethyl sulfoxide (DMSO) reductase families. Enzymes from these families contain square‐pyramidalcis‐[MoVIO2L(cysteinate)]−and distorted trigonal prismatic [MoVIO(AA)L2]−(AA = serinate, cysteinate, selenocysteinate, etc.) oxidized active sites, respectively. The hydroxylases all belong to the xanthine oxidase family and generally catalyze net insertion of oxygen into the CH bonds of heterocyclic aromatic compounds and aldehydes. They contain biologically unique, square‐pyramidal [MoVIOS(OH)L]−oxidized active sites featuring a catalytically essential terminal sulfido ligand. The sulfido ligand can act in a bridging capacity to Cu, as recently demonstrated for CO dehydrogenase. In most reactions catalyzed by Mo‐MPT enzymes, an oxygen atom is exchanged between substrate and water with concomitant consumption (for reductases) or production (for oxidases) of reducing equivalents (2e−, 2H+). Synthetic analogue (model) studies have produced complexes replicating or informing the composition, structures, spectroscopy, and chemistry of the enzymes. This account focuses on the Mo bioinorganic chemistry of the Mo‐MPT enzymes and selected synthetic analogues. It is divided into two main parts. Part 1 provides an overview of the enzyme systems, their distribution, roles, and essential chemistry, as well as a discussion of consensus structures and likely mechanisms. The genes and proteins involved in Mo trafficking and Mo‐cofactor biosynthesis are also briefly described. Part 2 summarizes important developments in the production of synthetic analogues, with a focus on new dithiolene, scorpionate, and other complexes sharing the structural, spectroscopic, or chemical attributes of the enzymes.
Nickel: Models of Protein Active SitesBased in part on the article Nickel: Models of Protein Active Sites by Michael J. Maroney which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Liaw Wen‐Feng
Published: December 2011

During the past decade, structures and functions of the active sites have been established for the nickel‐based metalloenzymes present in the hydrogenases, ureases, carbon monoxide dehydrogenases/acetyl coenzyme A synthase, and methyl coenzyme M reductase. Roles of nickel in these Ni‐containing enzymes serve as catalytic or redox centers. Model compounds and various biophysical techniques have been lent to elucidate aspects of the structure and function (catalytic mechanism) of the Ni sites in these enzymes. Structures/functions of these Ni‐containing enzymes, biomimetic model compound studies, and the proposed catalytic mechanisms are presented.
Nickel: Organometallic ChemistryBased in part on the article Nickel: Organometallic Chemistry by Clifford P. Kubiak which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Zargarian Davit
Published: December 2011

The organometallic chemistry of nickel has experienced rapid growth over the past decade. A number of exciting new areas have emerged, such as the reactivities of Ni(0)‐carbene complexes and the polymerization of olefins catalyzed by nickel complexes of nitrogen‐based ligands. In addition, the traditionally rich area of Ni‐catalyzed organic transformations has undergone major developments that have expanded the scope of their applications. These are the subjects of the present chapter.

Among the topics that will be discussed are the preparation, characterization, and reactivity studies of nickel complexes of allyl, cyclopentadienly, indenyl, and tris(pyrazolyl)borate ligands. The main structural and bonding properties of these complexes are presented and their major reactivities are outlined. Another topic of importance is the chemistry of the nickel complexes of hydride, alkyl, alkenyl, and silyl ligands. Hydrido and alkyl complexes of nickel are often involved in many catalytic processes of commercial importance, and so an in‐depth understanding of the fundamental reactivities of these complexes is crucial to expanding their practical applications. Silyl complexes are involved in the transformations of organosilicon compounds, and for this reason their basic reactivities and structural properties are of interest.

Zerovalent nickel complexes are also very important in the catalytic organic transformations. One of the most exciting developments in this area is the recent introduction of N‐heterocyclic carbene ligands. Ni complexes of these ligands have been shown to be important catalysts and reagents for a number of important organic transformations. Some of the practical methods for preparing Ni‐carbene complexes will be outlined, and the recently reported catalytic reactions promoted by these complexes will be described.

One of the most important areas of growth for the chemistry of nickel has been the polymerization and copolymerization of olefins. Thus, this chapter will describe the discovery of a family of diimine complexes that catalyze the polymerization of aliphatic olefins. A series of new and highly efficient catalysts were introduced in rapid progression, a great deal of new chemistry has been discovered, and a number of industrial processes have been developed on the basis of these nickel complexes. We will also discuss the emergence of new strategies for the efficient cross‐coupling of alkyl halides with organozinc and Grignard reagents, as well as the coupling of dienes with CO
2
.
Niobium & Tantalum: Inorganic & Coordination Chemistry
Authors: Hubert‐Pfalzgraf Liliane G.
Published: December 2011

The chemistry of niobium and tantalum ranges from oxidation states +V to −III with no species of oxidation state −II known so far. The chemistry of those oxophilic metals is dominated by their highest oxidation states, V and IV. Pentahalides especially chlorides are the most usual starting material. They form numerous complexes especially with hard donors and are used for access not only to pentavalent metal alkoxides, amides, thiolates, and so on but also to most molecular species in lower oxidation states. Oxo species are formed quite easily by O‐donor abstraction reactions or adventitious hydrolysis. Pentavalent alkoxides are active in various catalytic processes and are useful precursors for access to oxide ceramics such as ferroelectrics via the formation of heterometallic species. The most relevant results in organometallic chemistry of Nb and Ta have been obtained with ancillary, bulky aryloxide or siloxide ligands. Niobium is more prone to reduction than tantalum. Chalcogenides and chalcogenohalides are often one–dimensional solids and display a rich structural and synthetic chemistry. Numerous trispyrazolylborate derivatives of Nb and Ta have been reported for oxidation states V to I. The development of the nonaqueous chemistry of those elements in intermediate oxidation states has become an active and fruitful area. Species based on MM or MM multiple bonds, the latter especially for Nb, have been characterized. The NbNb triple bond (2.20 Å) is much shorter than the MM distance in the metal. Tri‐ or divalent species are able to promote CE (E = O, S, N) cleavage and thus to generate novel ligands because of the trend of Nb and Ta to form multiple ME bonds and attain their highest oxidation states. CC coupling reactions are observed with unsaturated ligands (alkynes, nitriles, etc.). M
III
η
2
‐alkyne complexes have a metallocyclopropene character; they can react with unsaturated organic molecules via stoichiometric reactions. The reservoir of electrons of the MM bond is also of interest for activation of small molecules such as N
2
, H
2
, CO, and so on. Tri or tetradentate N,N; N,P; or N,O ligands have been used to stabilize dimeric dia or paramagnetic dinitrogen complexes. The bridging, highly activate N
2
ligand is amenable to functionalization in some cases, and cleavage into nitrides has been observed. The chemistry of reduced Nb and Ta halides is rich in clusters with nonintegral oxidation states (between III and I). Hexanuclear clusters dominate the aqueous chemistry for these oxidation states. One of their potential applications could be their use as contrasting agents. The species in the lowest oxidation states are essentially based on carbonyl ligands.
Nitrogen: Inorganic ChemistryBased in part on the article Nitrogen: Inorganic Chemistry by Harry H. Sisler which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Hammerl Anton , Klapötke Thomas M.
Published: December 2011

The chemistry of nitrogen and its compounds with hydrogen, oxygen, and halogens is discussed in this article. The high stability of the nitrogen–nitrogen triple bond in dinitrogen in comparison with nitrogen–nitrogen single or double bonds makes polynitrogen compounds promising, highly energetic materials. Recently, several new polynitrogen species have been detected experimentally and salts of the polynitrogen cation N5+have been isolated in bulk. Salts of the azide anion N3−are already used as highly energetic materials.

Binary compounds of nitrogen and hydrogen include ammonia, hydrazine, and several metastable species with nitrogen–nitrogen bonds. Ammonia, which is the starting material for all industrially produced nitrogen compounds, is probably the most important industrial chemical. Most of the ammonia produced is used as fertilizer, whereas hydrazine is used as an energetic material.

The monomeric nitrogen oxides are among the most simple odd‐electron molecules and play an important role in the chemistry of the atmosphere. In combustion engines, huge amounts of nitric oxide are formed, which influence the ozone cycle both in the troposphere and in the stratosphere. Nitric oxide is a chemical messenger in biological systems and it helps to regulate a diverse array of physiological processes, such as neurotransmission, blood pressure and blood clot prevention.

Oxoacids of nitrogen include hydroxylamine, nitrous acid, and nitric acid. Nitric acid is a major industrial chemical, which is primarily used for nitrate fertilizer production. Hydroxylamine is a starting material for the production of polyamide synthetics.

Chlorine, bromine, and iodine halogen nitrogen compounds include halogensubstituted derivatives of ammonia, hydrazine, and diimine. They are unstable species that decompose to molecules in excited states. Nitrogen fluorides are thermodynamically more stable than the other nitrogen halides.

Ternary compounds of nitrogen, oxygen, and halogens include nitrosyl‐ and nitrylhalides as well as the halogen nitrates. The stability of these compounds decreases from fluorine to iodine.