bluuebunny/crossref_metadata_2025 huggingface dataset:
Copper Proteins: Oxidases
Authors: Stoj Christopher S. , Kosman Daniel J.
Published: March 2006

Oxidases are enzymes that use dioxygen as the electron acceptor in oxido‐reduction reactions. Many members of this enzyme class (EC 1.) rely on the Cu1+/Cu2+redox cycle of copper prosthetic group(s) in their catalysis of the electron transfer from reducing substrate to O2. Those copper oxidases that catalyze the four‐electron reduction of O2to 2H2O are known as multicopper oxidases (MCO) because each member of this group contains at least four copper atoms of three distinct electronic and spectral types. These three types are: type 1 or ‘blue’ Cu(II), so‐called because of its distinguishing strong absorbance at ∼600 nm; type 2 Cu(II), comparable in electronic structure to that found in square‐planar copper complexes; and type 3 Cu(II), a Cu(II)Cu(II) pair bridged by O(H) and thus diamagnetic. With four 1e−metal redox sites, MCOs are the all‐copper equivalent of the copper‐heme respiratory terminal oxidases. However, MCOs are true oxidases in that all members can use as electron acceptors common (di)phenolic, and aromatic (di)amine substrates, for example, hydroquinone, ascorbic acid, and phenylenediamine. The laccases and ascorbate oxidase are examples of these MCOs (EC 1.10.3). Members of a separate MCO class (EC 1.16.3) also use lower valent metal ions as substrates, for example, Fe2+, Cu1+, and/or Mn2+. These are thereby designated metallo‐oxidases; the most widely studied activity of this class has been towards Fe2+giving rise to the common enzyme name, ferroxidase. The typical MCO structure consists of three ‘cuprodoxin’ domains, a Greek‐key beta‐fold that characterizes a family of mononuclear copper electron‐transfer factors. Among MCOs are both soluble and type 1 membrane proteins. Functionally, MCOs are known to play essential roles in biodegradation and transformation, cell redox balance, and metal metabolism. This article summarizes current knowledge of this class of copper oxidase enzymes.
Fluorides: Solid‐State Chemistry
Authors: Massa Werner
Published: March 2006

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.
Gold: Organometallic Chemistry
Authors: Schier Annette , Schmidbaur Hubert
Published: March 2006

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: March 2006

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.
Hydrogenation & Isomerization of Alkenes
Authors: Jardine Fred H.
Published: March 2006

Alkenes are isomerized by complexes of late, second or third series transition metals. This occurs either by successive β‐hydride abstraction reactions or, more rarely, by a π–allylic mechanism. Both decyclization and annulation reactions of strained, cyclic hydrocarbons are catalyzed by these complexes. Palladium complexes are active catalysts for the Cope‐ and Claisen rearrangements. Homogeneous, catalytic hydrogenation can occur via three possible routes. Ruthenium complexes catalyze transfer hydrogenation. Catalyst modifications can be made to minimize by‐product formation during hydrogenation or deuteriation.
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: March 2006

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.
Intercalation Chemistry
Authors: Jacobson Allan J. , Nazar Linda F.
Published: March 2006

The term ‘intercalation’ refers to a process whereby a guest molecule or ion is inserted into a host lattice. The structure of the guest–host or intercalation compound is only slightly perturbed from the host structure and the reaction used to form the compound is reversible.

In this chapter, the major classes of host lattice are classified by both dimensionality and electronic properties. In three‐dimensional systems, the sizes of the guest species are constrained by the dimensions of the host lattice. In lower dimensional systems, no such restriction exists and the strongly bonded layers or chains can adjust their separation freely to accommodate guest species of different sizes. With increasing guest size, the structural correlation between layers or chains is diminished and in the limiting case, complete separation of single layers or chains occurs in solution, resulting in the formation of colloidal dispersions.

Insulating host lattices undergo reactions that involve ion exchange, acid–base chemistry, solvent exchange, and sorption of neutral molecules from the gas phase. The insulating hosts have a fixed concentration of ionic guests that remains unchanged throughout subsequent reactions. The conducting host lattices have the feature that the concentration of ionic guest species can be altered by oxidation or reduction of the host. The chemistry of most compounds involves reduction and cation intercalation with the major exceptions of graphite and some layered oxides, which can be both reductively intercalated with cations and oxidized with anion insertion.

The present understanding of the mechanism of intercalation with particular emphasis on layered structures is reviewed. Reactions involve adsorption of guest species on host crystals, exchange or insertion at the host surface, the formation of intermediate stages in layered compounds, and transport within the host lattice. Macroscopic effects such as variations in crystal size, dislocations, stacking faults, and pore mouth blockage can strongly influence the reaction kinetics.

The major classes of host lattices in each category and their chemical behavior are described. The insulating materials discussed include zeolites, clays, layered double hydroxides, acid phosphates, and layered oxides while graphite, C60, carbon nanotubes, transition metal chalcogenides, metal phosphorus trisulfides, metal oxyhalides, metal nitride halides, and metal oxides are the major examples of conducting hosts. In the case of metal oxides, examples of both reductive intercalation of lithium ions and oxidative intercalation of oxide ions are discussed. These two types of intercalation reactions are important for lithium batteries and high‐temperature superconductivity, respectively.
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: March 2006

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.
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: March 2006

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.
Mercury: Organometallic ChemistryBased in part on the article Mercury: Organometallic Chemistry by Richard C. Larock which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Rais Daniela , Vilar Ramón
Published: March 2006

Over the past 10 years, a wide range of new organomercury compounds have been prepared and their reactivity extensively studied, leading to interesting applications in organic synthesis. The solvomercuration of alkenes is still the most widely employed organometallic reaction of mercury, although there have been important advances in the use of the mercury‐mediated ring opening of cyclopropanes. Several structural investigations of organomercurials have been recently carried out providing detailed information on the connectivity of a wide range of mono‐ and polymercuric complexes. In this article, an overview of the most important advances on the organometallic chemistry of mercury between 1993 and 2003 will be provided.
Metal Ion Toxicity
Authors: Martin R. Bruce
Published: March 2006

General principles of metal ion toxicity in humans are discussed including comparative stabilities with a variety of ligands and interdependencies among metal ions. Short reviews are presented for cadmium, mercury, lead, and aluminum.
Metathesis Polymerization Processes by Homogeneous Catalysis
Authors: Wagener Kenneth B. , Boncella James M. , Smith Dennis W.
Published: March 2006

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 Valence Compounds
Authors: Fackler John P.
Published: March 2006

The concept of mixed valence has referred to compounds in which a given element exists for an appropriate time in what can be described as more that one formal oxidation number. Clearly it is a formalism that has limited meaning when electron delocalization occurs on a timescale at or near the frequency of vibrations. The formalism that historically has been valuable, the Robin and Day classification, classifies such substances into three types depending upon an ability of experiment to identify well defined oxidation states. When electron transfer occurs readily between these mixed valence species, the concept of mixed valence may lose its meaning. These types of compounds are of considerable interest today since the nature of the bonding connecting the components influences significantly the electronic properties of the material. Understanding these factors is needed to help us develop and understand organic and ionic high‐Tcsuperconductors.
Nickel Enzymes & Cofactors
Authors: Ragsdale Stephen W.
Published: March 2006

This review covers the structure and function of the eight known Ni enzymes, with a focus on the chemistry and biology of the metallocenter. For the Ni redox enzymes, the Ni ion exhibits versatility in redox properties in exploiting the +3/+2 (superoxide dismutase and hydrogenase), the +2/+1 (hydrogenase, CO dehydrogenase, acetyl‐CoA synthase, and methyl‐CoM reductase), and perhaps even reaching the 0 (acetyl‐CoA synthase) states for the redox enzymes. On the other hand, the nonredox Ni enzymes (urease, glyoxylase‐I, aci‐reductone dioxygenase) apparently remain in the +2 state during catalysis. The Ni site(s) in these enzymes adopt a wide variety of coordination environments; some form mononuclear square planar or octahedral complexes, while others are homo‐ or hetero‐ (with iron) dinuclear with oxo or thiolato bridges and still others join heteromultinuclear clusters. Acetyl‐CoA synthase contains the most complex Ni metallocenter yet described, which is a [4Fe–4S]
2+
cluster bridged by a cysteine thiolate to a binuclear NiNi center. The three‐dimensional structures of all these enzymes are known and some (acetyl‐CoA synthase, aci‐reductone dioxygenase, CO dehydrogenase, and superoxide dismutase) have been determined in the past two years leading to a significant reinterpretation of spectroscopic results. In many Ni enzymes, density functional theory has begun to play a major role in assigning active‐site structures and testing mechanistic hypotheses.
Nickel: Inorganic & Coordination Chemistry
Authors: Collinson Simon R. , Schröder Martin
Published: March 2006

The general features and coordination chemistry of Ni0to NiIVand complexation to cyanide, N‐, P‐, O‐, S‐, and halide‐donor ligands are detailed. The reactivity of NiIIcomplexes in the template synthesis of macrocycles and for the oligomerization and polymerization of alkenes are highlighted. The stabilization of radical NiIand NiIIIcenters is related to the associated ligand types and metal‐ion stereochemistries. The supramolecular chemistry of NiIIincludes the formation of aggregates, framework polymers, and receptors. The structures of a number of Ni‐containing enzymes have been elucidated and the formation of low‐molecular weight complex mimics is discussed.
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: March 2006

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
.
Nitrogen Monoxide (Nitric Oxide): Bioinorganic Chemistry
Authors: Lancaster Jack R. , Giles Gregory I.
Published: March 2006

In only the last 20 years, Nitric oxide (NO, nitrogen monoxide), one of the smallest and simplest molecules in Nature, has been found to be involved in an astounding number of mammalian physiological and pathophysiological phenomena. The importance of NO as a messenger molecule in the cardiovascular system was recognized in 1998 by the awarding of the Nobel Prize in Physiology or Medicine and over 70000 papers have appeared involving NO in biology. The simplicity of its chemical structure stands in stark contrast to the complexity of its biological actions, including its “Janus‐faced” character as both beneficial and detrimental. This chapter is an overview of the history of this discovery, the fundamental chemistry of NO and its derivatives in the biological environment, and the multiple biological actions of NO.
Nitrogenase: Metal Cluster Models
Authors: Barrière Frédéric
Published: March 2006

This article reviews the synthesis and properties of inorganic clusters that model the iron–sulfur centers found in the nitrogenase enzyme. These sites are of the Fe
4
S
4
, Fe
8
S
7
, and MoFe
7
S
9
stoichiometry. The enzyme [Fe
4
S
4
]
2+/1+
center is well modeled by synthetic clusters bearing thiolate ligands at iron. However, the [Fe
4
S
4
]
0
state is stabilized in the enzyme and has not been reproduced in the laboratory so far. Recent progress in synthetic models of the Fe
8
S
7
P‐cluster is reported. One of its known structures can be obtained by self‐assembly from basic constituents, or synthesized in a stepwise fashion from smaller clusters yielding topological analogues. The most challenging cluster to model is the FeMo‐cofactor. This MoFe
7
S
9
center is the site of dinitrogen reduction to ammonia. The cluster also exists with vanadium or iron in place of molybdenum in alternative nitrogenases. It was shown only recently that the FeMo‐cofactor is centered on a light atom that is probably a nitrogen atom. Structural modeling of this center is limited. Only its molybdenum or heterometal end has been accurately modeled. Functional chemistry of good structural models, those with sulfur ligands and metal–metal bonds, only concerns the reduction of already partially reduced diazo substrates such as diazene or hydrazine.
Nitrogenase Catalysis & AssemblyBased in part on the article Nitrogenases & the Iron–Molybdenum Cofactor by Paul W. Ludden which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Hu Yilin , Schmid Benedikt , Ribbe Markus W.
Published: March 2006

Nitrogenases are metalloenzymes that catalyze biological N2fixation, in which dinitrogen is reduced to ammonia. The best‐characterized molybdenum nitrogenase is composed of the iron (Fe) protein that contains a [4Fe–4S] cluster and the molybdenum–iron (MoFe) protein that contains an [8Fe–7S] cluster (P cluster) and a [Mo–7Fe–9S–homocitrate] cluster (FeMo cofactor).

The catalysis of molybdenum nitrogenase occurs in a series of steps. First, the reduced Fe protein binds 2MgATP, undergoes a conformational change, and forms a complex with the MoFe protein. Then, coupled with the hydrolysis of 2MgATP, one electron is transferred from the Fe protein to the MoFe protein within the complex. This is followed by complex dissociation, which allows the enzyme to start the next cycle of electron transfer. Once a sufficient amount of electrons is accumulated on the MoFe protein, substrate reduction takes place.

The assembly of the molybdenum nitrogenase Fe protein involves thenifHand at least thenifS, nifU, andnifMgene products that are implicated in the [4Fe–4S] cluster assembly, while the assembly of the MoFe protein requires at least 15nifgene products and involves the biosynthesis and insertion of the FeMo cofactor into the MoFe protein.
Organic Synthesis using Transition Metal Carbonyl Complexes
Authors: Söderberg Björn C. G.
Published: March 2006

Synthetic transformations using transition metal carbonyl carbene, arene, diene, and alkyne complexes are discussed. Reaction of synthetic importance and applications in organic total synthesis are included.
Oxide Catalysts in Solid‐State ChemistryBased in part on the article Oxide Catalysts in Solid State Chemistry by Toshio Okuhara & Makoto Misono which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Okuhara Toshio , Kamiya Yuichi
Published: March 2006

This chapter deals with catalysis of metal oxides containing single elements and more than two metallic elements. The metal oxides catalyze typically selective oxidation and acid‐base reactions. The first section outlines factors controlling catalyst performance (activity, selectivity, and durability), which are categorized into geometric (crystal plane and surface defects) and material‐intrinsic factors. Catalysis of the metal oxides is illustrated in detail by examples such as BiMoO, MoVTeNbO, VPO, Perovskite‐type oxide, and heteropoly compounds. Catalyst performances of these metal oxides are discussed from the viewpoint of geometric aspects. For the heteropoly compounds, their synthetic methods, chemical properties, and porosities are also described. Solid acid and base catalysts are finally summarized.
Periodic Table: Trends in the Properties of the Elements
Authors: Wulfsberg G. P.
Published: March 2006

The article gives examples of trends in properties of the elements, their ions, and their compounds, which can usefully be examined from the perspective of the periodic table as a whole, rather than by looking at one group of elements (or even individual elements) at a time. The properties discussed are those of great interest not only to inorganic chemists but also to research workers in allied fields, such as the other branches of chemistry, medicinal chemistry, environmental science, agricultural and geochemistry, industrial chemistry and safety, metallurgy, materials science, and surface chemistry. These trends can be organized, explained, and predicted on the basis of a small set of parameters of the elements and their ions, most notably the sizes of the ions, their charges (or oxidation numbers), and the Pauling electronegativities of the elements. Trends are emphasized in acid–base reactivity, precipitation and solubility trends, coordination chemistry, oxidation–reduction chemistry, and physical properties of the elements and their compounds. Much acid–base and precipitation chemistry can be predicted after calculating the p
K
a
values of cations and the p
K
b
values of anions, or simply predicting to which of the six acidity and basicity categories an ion belongs. Generally very useful in coordination chemistry are the chelate and macrocyclic effects and the Hard–Soft Acid–Base (HSAB) Principle. Oxidation–reduction chemistry of the elements can be summarized graphically in the form of Pourbaix diagrams or by redox predominance diagrams where the pH is set at the standard pH value of zero. Among physical properties, perhaps the most important for applications in materials science is whether a compound that might be synthesized is a solid with an extended structure; methods are given for predicting this on the basis of the radius ratios of the cations and anions (at least formally) contained in the compound. One physical property, metallic conduction, is discussed in further detail. As an example of chemical properties, trends in the acidity and basicity of oxides of the elements are discussed.
Polyphosphazenes
Authors: Allcock Harry R.
Published: March 2006

Polyphosphazenes comprise the largest class of inorganic polymers. During the past 10 years, new synthetic developments have arisen that allow carefully tailored macromolecules to be assembled with precise chain lengths and with access to star geometries and a range of hybrid phosphazene‐organic polymer systems. This has stimulated research in the use of polyphosphazenes in biomedicine, optoelectronics (photonics), lithium battery technology, and fuel cell membranes, as well as in new elastomers and hydrogels.
Quasicrystals
Authors: King R. Bruce
Published: March 2006

Quasicrystals are solid materials exhibiting diffraction patterns with apparently sharp spots containing symmetry axes such as fivefold or eightfold axes, which are incompatible with the three‐dimensional periodicity associated with crystal lattices. Many such materials are aluminum alloys that exhibit diffraction patterns with fivefold symmetry axes; such materials are called icosahedral quasicrystals. The location of atoms in quasicrystals requires the use of six‐dimensional crystallography. However, a polyhedral shell structure using a number of polyhedra of icosahedral symmetry provides a three‐dimensional model for the structure of many icosahedral quasicrystals.
Semiconductors
Authors: Stanton Colby E. , Nguyen SonBinh T. , Kesselman Janet M. , Laibinis Paul E. , Lewis Nathan S.
Published: March 2006

The structural, electronic, optical, and electrical properties of semiconductors are treated within a common framework. The bonding in the lattice determines the band structure of the solid, which is used to describe the chemical, optical, and electrical properties of the semiconductor. These concepts enable this process to be understood from a qualitative, chemically based viewpoint.
Technetium: Organometallic ChemistryBased in part on the article Technetium: Organometallic Chemistry by Ronald J. Clark which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Rochon Fernande D.
Published: March 2006

The chemistry of organometallic technetium compounds is discussed along with its application in the field of nuclear medicine. Several
99m
Tc radiopharmaceuticals are presently used in hospitals using a
99
Mo−
99m
Tc generator to prepare the starting material [
99m
TcO
4
]
−
. The isotope
99m
Tc is a γ‐ray emitter. The most common organometallic radiopharmaceutical is [Tc(MIBI)
6
]
+
, where MIBI is an isonitrile ligand.
99m
Tc can be produced only at the tracer level and it disintegrates to the longer life isotope
99
Tc, which has a half‐life of 2.12 × 10
5
years. The chemistry of technetium has been studied with the latter isotope, which is commercially available. But it emits weak β
−
particles and special precautions must be taken in the laboratories in order to avoid health problems.

Most of the publications in the literature on organometallic Tc compounds contain the ligands CO, isonitriles or the cyclopentadienyl ion. These π‐accepting ligands usually bind to Tc in a low oxidation states. Tc‐CO compounds are the ones who have been most studied. Binary compounds and a great variety of mixed‐ligands complexes are discussed. The new complex ion [Tc(CO)
3
(H
2
O)
3
]
+
is particularly important, since a kit was recently developed for its preparation. It should open a new field for the development of
99m
Tc radiopharmaceuticals. It can bind to biomolecules like proteins, peptides, and antibodies which have very specific targets.

Several Tc‐cyclopentadienyl complexes have the sandwich or the piano stool structures. The arene compounds should have similar structures, but the latters have not been much studied yet. No Tc‐arene compound seem to have been characterized by crystallographic methods yet. But more recently, interest in the arene complexes has increased because of its potential use in nuclear medicine. Isonitriles or isocyanides and cyanide ligands are bonded to Tc through the C atom. A few crystal structures on mixed‐ligand isonitrile Tc complexes were determined and the segment TcCNC is linear. Fewer cyanide Tc compounds were studied. Finally a few Tc‐alkyl compounds have been reported.
Water &
O
‐Donor Ligands
Authors: Ogden Mark I. , Beer Paul D.
Published: March 2006

The chemistry of a range of O‐donor ligands is described, with an emphasis on the various ligand coordination modes and different structural types of the complexes formed. Water as a ligand is discussed along with the related hydroxide and oxide species. The structure, preparation, and reactions of alkoxide and aryloxide complexes are described. Similar coverage is provided for the diketone ligands. The rich variety of coordination modes of oxoanions, carboxylates, and related ligands, is emphasized. Complexation and structural chemistry of macrocyclic O‐donor ligands are discussed, particularly focusing on crown ether and calixarene metal complexes. The final section provides an overview of the chemistry of other commonly encountered η
1
O‐donor ligands.
Alkaline Earth Metals: Organometallic Chemistry
Authors: Alexander Jacob S. , Guino‐o Marites , Zuniga Maria Felisa , Hahn Roger C. , Ruhlandt‐Senge Karin
Published: March 2006

The organometallic chemistry of the alkaline earth metals has undergone a renaissance during the last decade with exciting novel uses of the organomagnesium reagents in synthetic chemistry to significantly extend the utility of these well‐developed reagents. With the development of the organometallic chemistry of the heavier metals significantly lagging behind, and past work mainly focusing on and establishing π‐bonded metal–ligand systems, intense research efforts during the last decade have contributed significantly to expand the understanding of the organometallic compounds of these metals, establishing σ‐bonded organometallic compounds for the heavy alkaline earth metals. This improved insight did not only provide the basis for a more detailed understanding of the metal–ligand bonding characteristics, but also paved the way towards the successful applications of the compounds as polymerization initiators and in organic synthetic chemistry. This article summarizes some of the recent, exciting developments in regards to alkaline earth organometallics.
Luminescence Behavior & Photochemistry of Organotransition Metal Compounds
Authors: Yam Vivian Wing‐Wah , Lo Kenneth Kam‐Wing
Published: March 2006

In this article, the luminescence and photochemical properties of a selection of organotransition metal compounds are reviewed and discussed. The examples are classified according to the identity of the carbon‐containing ligands, which include the carbonyl, isocyanide, cyanide, alkynyl, alkenyl, arene, aryl, alkylidene, alkylidyne, cyclometalating, and alkyl ligands. The luminescence and photochemistry of selected tungsten, molybdenum, and rhenium carbonyl compounds are described in this article, followed by some examples with the analogous isocyanide and cyanide ligands. The luminescence properties and potential applications of these systems are discussed. Transition metal complexes containing alkynyl ligands display a very wide structural diversity and catalytic properties, and many systems also show intriguing photoluminescence properties, especially those of rhenium(I), platinum(II), palladium(II), and the coinage metals. Studies focused on the photophysical studies and the excited‐state chemistry of selected alkynyl systems and related alkenyl, arene, and aryl complexes are outlined and discussed. Luminescent metal–carbon multiple bonded systems including the alkylidene and alkylidyne compounds have received relatively little attention, and examples have been confined to the complexes of molybdenum and tungsten. However, much effort has recently been devoted to exploring the interesting emission properties of alkylidene and alkylidyne systems with other metal centers such as rhenium, ruthenium, osmium, platinum, and coinage metals, which are outlined in this article. Some recent examples in the areas of cyclometalated complexes, especially those of rhodium(III), iridium(III), platinum(II), and gold(III) centers are highlighted in this article as many examples display impressive photophysical and photochemical behavior. Metal alkyl systems represent a special class of luminescent organometallic system due to their strong σ‐donating properties of the alkyl ligands, and the photophysical and excited‐state properties of some selected examples are described. Finally, luminescent organotransition metal compounds with application potentials in the development of new materials, photodevices, and sensory systems are summarized in various parts of this article.
Metal‐Based Imaging Agents
Authors: Francesconi Lynn C. , Cantorias Melchor V. , Howell Robertha C.
Published: March 2006

Radiopharmaceuticals are drugs that contain a radionuclide and are routinely used for diagnosis and treatment of disease. This review discusses imaging agents that are useful for diagnosis using Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET). Moreover, this entry focuses on the growing class of metal‐based imaging agents, wherein the radionuclide is a transition metal or posttransition metal. The most widely used radionuclide for radiopharmaceuticals is technetium‐99m,
99m
Tc, owing its widespread availability from the parent–daughter generator and its excellent physical properties for imaging. The radionuclides
67
Ga,
111
In,
210
Tl,
57
Co, and
51
Cr are used in the clinic as well. There are a number of radioisotopes, including
64
Cu,
67
Cu,
68
Ga, that show promise for diagnostic purposes. An increased understanding of the coordination chemistry of
99m
Tc and other radiometals resulted in first‐generation metal‐essential imaging agents. This understanding of coordination chemistry is responsible for the introduction and development of targeted receptor‐specific agents, many of which are in clinical trials and some of which are on the market.
Dihydrogen Complexes & Related Sigma Complexes
Authors: Crabtree Robert H.
Published: March 2006

The title topic is reviewed with emphasis on structure and bonding, dynamics, synthesis, and reactivity. Spectroscopic characterization is considered with special reference to diffraction, NMR, and IR methods.
Carbene Complexes
Authors: Crabtree Robert H.
Published: March 2006

The synthesis, structure, reactions, and applications of carbenes are discussed. Fischer and Schrock carbenes are distinguished. N‐heterocyclic carbenes (NHCs) are considered as a class of Fischer carbenes with promise as spectator ligands in homogeneous catalysis. Nucleophilic attack on carbonyls and alpha elimination are noted as leading pathways leading to Fischer and Schrock carbenes, respectively. Criteria are discussed for characterization of carbenes, notably nuclear magnetic resonance (NMR) spectroscopy. The Grubbs metathesis catalyst and alkene metathesis catalysis are also mentioned. Bridging carbenes and carbynes are briefly considered.
Alkane Carbon–Hydrogen Bond Activation
Authors: Crabtree Robert H.
Published: March 2006

The title topic is reviewed with attention to alkane complexes, Shilov chemistry, alkane functionalization, Fenton reagents, oxidative addition pathways, sigma bond metathesis pathways, and surface‐bound reagents. Methane to methanol chemistry is discussed and attempts to commercialize the reaction are described in connection with Periana systems. Some organic synthetic applications are also covered. Future challenges are posed.
Synthesis of 3‐Substituted Furans by Hydroformylation
Authors: Nanayakkara Perli , Alper Howard
Published: March 2006

A simple and novel method for the synthesis of 3‐substituted furans by the hydroformylation of substituted propargylic alcohols is described using rhodium acetate and triphenylphosphine in dichloromethane. The hydroformylation reaction proceeds in a regioselective manner under mild reaction conditions. This novel methodology is versatile and can be applied to the synthesis of a variety of 3‐aryl‐substituted furans.
Layered Double Hydroxides‐Supported Diisopropylamide: Synthesis, Characterization and Application in Organic Reactions
Authors: Kantam M. Lakshmi , Ravindra A. , Reddy Ch. Venkat , Sreedhar B. , Choudary B. M.
Published: March 2006

The layered double hydroxides‐supported diisopropylamide (LDH‐DA) catalyst is found to be an efficient and selective solid base for aldol, Knoevenagel, Henry, Michael, transesterification and epoxidation reactions under liquid phase conditions. LDH‐DA is synthesized by the interaction of lithium diisopropylamide with LDH‐NO3 (as‐synthesized) and calcined LDH‐NO3. The LDH‐DA (Mg/Al, 3/1) and their precursors are well characterized by using various instrumental techniques such as FT‐IR, TGA and DTA, powder XRD, solid state 27Al MAS NMR spectroscopy, SEM and XPS (ESCA).
Planar‐ and Central‐Chiral N,O‐[2.2]Paracyclophane Ligands: Non‐Linear‐Like Effects and Activity
Authors: Lauterwasser Frank , Vanderheiden Sylvia , Bräse Stefan
Published: March 2006

The non‐linear‐like effect (NLLE), activity, temperature dependence, and kinetics of hydroxy[2.2]paracyclophane ketimine ligands have been investigated with the 1,2‐addition reaction of diethylzinc to cyclohexanecarbaldehyde. A linear correlation between the enantiomeric excess of AHPC ketimine ligands bearing a phenylethyl side group and the product was observed with 0.5 mol % of catalyst loading. On increasing the catalyst loading to 4 mol %, a precipitate of the inactive heterochiral species was formed and resulted in a positive non‐linear‐like effect. The enantiomeric ratio was found to have linear temperature dependence.
A Bimetallic Ruthenium Complex as a Catalyst Precursor for the Atom Transfer Radical Polymerization of Methacrylates at Ambient Temperature
Authors: Haas Michel , Solari Euro , Nguyen Quoc T. , Gautier Sébastien , Scopelliti Rosario , Severin Kay
Published: March 2006

The bimetallic ruthenium complex [(1,3,5‐i‐Pr3C6H3)Ru(μ‐Cl)3RuCl(C2H4)(PCy3)] has been synthesized by reaction of [(1,3,5‐i‐Pr3C6H3)RuCl2)]2 with one equivalent of PCy3 in the presence of ethylene. It can be used as a catalyst precursor for the controlled atom transfer radical polymerization of methacrylates at 35 °C. The resulting polymers show low polydispersities.
Enantioselective Ring Opening of Epoxides with Cyanide Catalysed by Halohydrin Dehalogenases: A New Approach to Non‐Racemic β‐Hydroxy Nitriles
Authors: Elenkov Maja Majerić , Hauer Bernhard , Janssen Dick B.
Published: March 2006

Halohydrin dehalogenases (HheA, HheB and HheC) were found to efficiently catalyse a carbon‐carbon bond forming reaction between terminal aliphatic epoxides and cyanide, yielding β‐hydroxy nitriles. With all three enzymes nucleophilic ring opening of epoxides proceeds with high regioselectivity to the β‐carbon atom. Activity, enantioselectivity and enantiopreference depend on the type of enzyme and the substrate structure. HheC was found to be the most selective among the tested enzymes. The enantioselectivity toward monosubstituted epoxides varies from moderate to high (E=5–106), while resolution of 2,2‐disubstituted epoxides proceeds with very high enantioselectivity (E=141 and 200). The results show that halohydrin dehalogenases may become attractive catalysts for the facile preparation of enantiopure β‐hydroxy nitriles from racemic epoxides.
Promiscuous Acylases‐Catalyzed Markovnikov Addition of N‐Heterocycles to Vinyl Esters in Organic Media
Authors: Wu Wei‐Bo , Xu Jian‐Ming , Wu Qi , Lv De‐Shui , Lin Xian‐Fu
Published: March 2006

Three acylases, including D‐aminoacylase from Escherichia coli, acylase “Amano” from Aspergillus oryzae and immobilized penicillin G acylase from Escherichia coli have been found to possess novel activity to catalyze the Markovnikov addition reaction of N‐heterocycles to vinyl esters. The aza‐Markovnikov addition reactions of 4‐nitroimidazle to vinyl acetate catalyzed by D‐aminoacylase, acylase “Amano” and immobilized penicillin G acylase were up to 1260‐fold, 720‐fold and 320‐fold faster than the respective non‐enzymatic reaction. Some control experiments have been designed to demonstrate the catalytic specificity of acylases. Under the catalysis of these promiscuous acylases, a number of N‐heterocycles, including some pentacyclic N‐heterocycles, pyrimidines and purines, were successfully added to a series of vinyl esters in moderate to excellent yields to prepare N‐heterocycle derivatives. The acylase‐catalyzed Markovnikov addition reaction has provided a new strategy to perform the Markovnikov addition and expanded the application of biocatalysts.
The First Air‐Stable and Efficient Nucleophilic Trialkylphosphine Organocatalyst for the Baylis–Hillman Reaction
Authors: He Zhengrong , Tang Xiaofang , Chen Yaoming , He Zhengjie
Published: March 2006

1,3,5,‐Triaza‐7‐phosphaadamantane (PTA) is first reported to be a convenient and efficient nucleophilic trialkylphosphine organocatalyst for the Baylis–Hillman reaction. Thus, under the mediation of 15–20 mol % of PTA and practical conditions, both aromatic and aliphatic aldehydes react with the activated alkenes like acrylates and methyl vinyl ketone to afford the corresponding adducts in fair to excellent yields.
Alkylation of Arenes with Benzylic and Propargylic Alcohols – Classical versus Fancy Catalysts
Authors: Liu Jianhui , Muth Enrico , Flörke Ulrich , Henkel Gerald , Merz Klaus , Sauvageau Janelle , Schwake Erik , Dyker Gerald
Published: March 2006

Gold chloride and BF3⋅etherate were tested as Friedel–Crafts catalysts in the propargylation of electron‐rich arenes: differences in reactivity of the catalysts can be used to achieve high selectivity either for monoalkylation or for multiple alkylation products. In the case of a macrocyclization towards a heterocalixarene, gold catalysts exhibited a pronounced selectivity, whereas p‐toluenesulfonic acid as catalyst opened up a competing pathway to a structural isomer.