bluuebunny/crossref_metadata_2025 huggingface dataset:
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: March 2006

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.
Cluster Compounds: Inorganometallic Compounds Containing Transition Metal & Main Group Elements
Authors: Fehlner Thomas P.
Published: March 2006

The relationships between p‐block element cages and d‐block element clusters, as evidenced by the structures and properties of mixed main group–transition metal compounds, are explored. After a review of the cluster‐bonding characteristics of main group and transition metal homonuclear clusters, including the role of hydrogen atoms and internal or external ‘ligand’ fragments, the related but more complex behavior of main group–transition metal clusters is illustrated. The emphasis is on how the competition between the fundamental bonding properties of p‐block and d‐block elements is expressed in geometric and electronic structure as well as reactivity. Thus, for a limited number of cluster shapes, examples of known compounds are compiled, thereby illustrating the convenience of the cluster versus metal–ligand view as well as outlining the numerous compounds yet to be synthesized and evaluated. Finally, the reactivity of these ‘inorganometallic’ clusters is illustrated. Not only are examples analogous to those found in the homonuclear systems given but so also are examples of unexpected reactivity generated by the synergistic main group–transition metal network.
Iridium: Inorganic & Coordination Chemistry
Authors: Housecroft Catherine E.
Published: March 2006

This article provides an overview of the inorganic and coordination chemistry of iridium, beginning with a summary of physical properties, accessible oxidation states, and common coordination geometries. Solid‐state compounds are summarized with pertinent examples. Mononuclear compounds and complexes are classified according to the oxidation state of the iridium center and the donor atom(s) of the ligand; dinuclear and multinuclear complexes are discussed in a separate section. References make use of review articles where appropriate, as well as the primary literature. It is not the aim of this article to provide a comprehensive survey of the chemistry of iridium.
Iron: Heme Proteins, Mono‐ & DioxygenasesBased in part on the article Iron: Heme Proteins, Mono‐ & Dioxygenases by Masanori Sono & John H. Dawson which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Wong Luet‐Lok , Bell Stephen G.
Published: March 2006

Heme (iron protoporphyrin IX) proteins and enzymes play crucial roles in all living organisms. Iron is very tightly bound to the porphyrin and does not dissociate under physiological conditions. Indeed the heme group is almost like a separate element altogether. It is redox‐active, and heme proteins such as cytochrome
c
have important electron‐transfer functions. The strong π donor nature of heme iron(II) also means that heme proteins are involved in the binding and activation of small molecules such as oxygen. The P450 heme monooxygenases use two electrons and two protons to activate O
2
, giving one molecule of water and a high‐valent iron‐oxo intermediate that is sufficiently reactive to attack aliphatic CH bonds in a diverse range of organic molecules. The heme dioxygenases, as the name implies, insert both oxygen atoms of the O
2
molecule into the substrate. The known heme dioxygenases are indoleamine ring cleavage enzymes but have not been studied in detail, and no crystal structure of the enzymes is available. It is known that the active form of the enzyme has Fe(II), and that both oxygen and superoxide could be the source of the two oxygen atoms inserted into substrates. The main focus of this article is on the cytochrome P450 monooxygenases.

The P450 (CYP) superfamily of enzymes is found in virtually all organisms where they catalyze the oxidation of endogenous and exogenous organic compounds. These reactions are vital steps in the biosynthesis of steroids and other hormones, and the detoxification and oxidative removal of xenobiotics. The primary activity of P450 enzymes is CH bond oxidation, but other activities such as alkene epoxidation, CC bond cleavage, heteroatom oxidation, and even reductive reactions are known. These reactions and their mechanisms are surveyed. The major sections review and summarize the steps in the P450 catalytic cycle, the nature of the rate‐limiting step, the chemical properties and electronic structure of the heme in the intermediates, and the mechanism of CH bond oxidation. The unique activity of P450 enzymes does not arise solely from the heme group; the pivotal roles of the protein environment and dynamics are discussed. The mechanism of aliphatic CH bond oxidation is one of the great challenges in chemistry. The nature of the active intermediate in P450 catalysis and the mechanism of action remain controversial. The results of mechanistic and theoretical approaches are brought together and reviewed.
Iron: Heme Proteins, Peroxidases, Catalases & Catalase‐PeroxidasesBased in part on the article Iron: Heme Proteins, Peroxidases, & Catalases by Ann M. English which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Bhaskar B. , Lad Latesh , Poulos Thomas L.
Published: March 2006

This article describes recent advances made in understanding structure–function relationships in representative peroxidases, catalases, and catalase‐peroxidases. Emphasis will be placed on those enzymes for which crystal structures are available. Of particular interest are recent advances in elucidating the structure of peroxidase compounds I and II, important intermediates that are formed from the reaction of the enzyme with peroxides. The repertoire of known structures in the catalase/peroxidase superfamily now include KatG catalases‐peroxidases and di‐heme peroxidases. These new structures coupled with a wealth of biochemical and biophysical data have provided important new insights into structure and function.
Iron: Models of Proteins with Dinuclear Active SitesBased in part on the article Iron: Models of Proteins with Dinuclear Active Sites by Will E. Lynch & Donald M. Kurtz, Jr. which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Chavez Ferman A. , Que Lawrence
Published: March 2006

The nonheme diiron enzymes are a class of metalloproteins that share a common function in the binding and activation of dioxygen. In recent years, interest in these enzymes has grown at a steady rate. The focus is both of a fundamental and applied nature. Understanding how these enzymes have evolved to perform synthetically challenging organic transformations under moderate conditions will provide insights into evolutionary processes and possibly help to develop techniques for engineering such active sites into proteins. These studies have led to the development of synthetic catalysts for these transformations through the ‘biomimetic’ approach. Through structural studies it has become clear that many of the metal binding sites in nonheme diiron enzymes possess carboxylate‐rich ligation, which is believed to be important in facilitating redox processes and structural rearrangements during catalysis. The use of model compounds to study these enzymes has many advantages that are not offered with other approaches. For example, a model for a proposed reactive intermediate may be stabilized at low temperature and studied. Synthetic compounds can also be selectively modified to examine the influence of a certain geometry, ligand set, or chemical environment on the properties and reactivity of the compound. In this article, several approaches are discussed toward the development of diiron model compounds. In one approach, multidentate ligands possessing two metal binding sites are examined. This approach has led to metal complexes with closely spaced iron centers, a key structural feature in these enzymes. In a second approach, mononuclear metal complexes supported by multidentate ligands are bridged by oxygen groups to generate the diiron core structure. A third approach focuses on dicarboxylate ligands that bridge the two iron centers. This approach allows for variation of the remaining terminal ligands. The fourth approach relies on steric interactions. Intramolecular ligand interactions are used to control the geometry of the diiron complex. To date, these four approaches have been very successful in generating many of the intermediates seen in the diiron enzyme catalytic cycles and a few have been shown to reproduce the catalytic activity to some extent. It is anticipated that many more insights will be garnered from model studies and that new chemistries will be discovered.
Iron–Sulfur Proteins
Authors: Johnson Michael K. , Smith Archer D.
Published: March 2006

Iron–sulfur proteins contain iron coordinated by at least one sulfur ligand and include proteins with mononuclear Fe centers with partial or complete cysteinyl sulfur ligation as well as proteins containing clusters of iron and inorganic sulfide. This review summarizes the structural, electronic, and redox properties of protein‐bound mononuclear FeS centers and FeS clusters containing [2Fe2S], [3Fe4S], and [4Fe4S] cores. In addition to the ubiquitous role in mediating biological electron transport, the roles of FeS centers have proliferated to include coupling of electron and proton transfer, substrate binding and activation, determining protein structure, regulation of gene expression and enzymatic activity, disulfide reduction, and iron, electron, or cluster storage. The diverse roles of FeS clusters are illustrated by discussion of specific systems: the mitochondrial and photosynthetic electron transport chains and a wide range of soluble redox enzymes and proteins; the active sites of nitrile hydratase, aconitase‐type (de)hydratases, superoxide reductase (SOR), radical‐SAM (S‐adenosylmethionine) enzymes, NiFe‐ and Fe‐hydrogenase, sulfite and nitrite reductase, nitrogenase, CO dehydrogenase, and acetyl‐CoA synthase; the structural FeS centers in DNA repair enzymes; the regulatory roles of FeS centers in the SoxR, fumarate–nitrate reduction (FNR), IscR/SufR, and iron‐regulatory protein (IRP) proteins and in selected enzymes; the two classes of FeS cluster containing disulfide reductases typified by ferredoxin–thioredoxin reductase (FTR) and heterodisulfide reductase (HDR); and the role of 8Fe ferredoxins and polyferredoxins in iron, electron, or cluster storage.
Iron Transport: Siderophores
Authors: Matzanke Berthold F.
Published: March 2006

Siderophores (from the Greek: iron carrier) are low molecular mass (500–1500 Da) iron chelators that are synthesized in bacteria and fungi under conditions of iron deficiency. Siderophores exhibit extraordinarily high complex formation constants for ferric iron with β‐values ranging from 1020to approximately 1050. Fe2+‐siderophores are some 20 orders of magnitude less stable than their Fe3+counterparts. The d5 electronic configuration of Fe3+rules out any crystal field stabilization energy and makes ferric iron complexes relatively labile with respect to isomerization and ligand exchange. Siderophores display a selectivity for iron that is reflected in the corresponding complex stability constants that are higher with Fe3+than with Al3+, or with bivalent cations like Ca2+, Cu2+or Zn2+. Microorganisms excrete desferrisiderophores in order to scavenge iron from the environment. The competition for iron by siderophores, the mechanisms of siderophore uptake through microbial membranes, and the intracellular pathways of siderophore‐iron utilization strongly depend on thermodynamic, kinetic, and structural features of the ferric iron siderophore complexes.

The structural features of siderophores are diverse. The ligating groups contain oxygen atoms of hydroxamate, catecholate, α‐hydroxy carboxylic and salicylic acids, or oxazoline and thiazoline nitrogen. Reduction potentials of ferric siderophore complexes vary between −700 and −150 mV. In particular at the low potential end below −450 mV, special biological strategies of reductive iron removal are required because these potentials are too negative for typical cellular reductases.

The coordination and redox chemistry of siderophores is also reflected in the mechanisms of siderophore‐mediated iron uptake in microorganisms. A classic example is the intracellular removal of iron from enterobactin. The permeation of cell walls or bacterial membranes by siderophores is in most microbes a highly specific process requiring an array of up to eight proteins. The advent of modern molecular biology delivered a cornucopia of methods enabling high‐yield production of specific gene products relevant to siderophore synthesis and transport, analyses of structure‐function relationships (employing site directed mutagenesis), and detailed insights into the regulation of the corresponding processes.

One siderophore, ferrioxamine B, serves as a detoxifier in iron overload diseases and in the treatment of β‐thalassemia. Siderophores and siderophore analogs also play a role in MRI and are employed as basic models for actinide chelators in order to remove these metals from the environment or from a contaminated body.
Lead: Inorganic ChemistryBased in part on the article Lead: Inorganic Chemistry by Philip G. Harrison which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Bharara Mohan S. , Atwood David A.
Published: March 2006

This article covers basic chemistry of lead(II) including coordination geometries of halides, pseudohalides, oxides, carboxylates, thiolates, and nitrogen and/or phosphorus‐containing ligands. Coordination number around lead(II) ranges from 1 to 12 but is by no means comprehensive. Tetrahedral four‐coordination is observed only for lead(IV) compounds with bulky ligands. Lead forms halide complexes with both oxidation states but lead(IV) halides are much less stable then monomeric lead(II) halides. Like lead(II) halides, lead pseudohalides also form complexes with donor molecules and these complexes are irregular with high coordination number. Binary and ternary oxides of lead have found importance in electronic and technological applications, while polymeric lead(II) alkoxides [Pb(OR2)] are insoluble and poorly characterized. Ternary and mixed oxides of lead with perovskite structure are electroactive and are characterized by large dielectric constants. Lead(II) chalcogenides especially PbS, are photoconductors and widely used in photoelectric cells and IR detectors. Lead derivatives of oxyacids and organic acid, containing bivalent lead are extensively used in organic synthesis as oxidizing agents. The variable structure of highly coordinate lead(II) carboxylates and their complexes are characterized by intermolecular association.207Pb NMR and potentiometry have been widely employed for study of lead compounds in solution. Reduction of lead in liquid ammonia or direct fusion of various metals at 700°C results in the formation of anionic clusters. Such species have been identified with207Pb NMR in solution or recently by laser ablation in gaseous state.
Ligand Field Theory & Spectra
Authors: Smith Derek W.
Published: March 2006

In any chemical environment – traditionally represented as a crystal field (CF), in which the donor atoms are represented by point charges – annd subshell loses its fivefold degeneracy. The consequent splitting is described in detail for cubic (octahedral and tetrahedral) systems. The pure CF approach has serious shortcomings, but the effects of covalency can be accommodated without losing its simplicity altogether; this is often calledligand field theory(LFT). CF/LFT is important in the interpretation of the d–d spectra of complexes; the d–d states arising from cubic dnsystems and their relative energies are analyzed, along with a discussion of the nephelauxetic effect. As examples of the analysis of d‐orbital splittings in noncubic complexes, chlorocuprates(II) and tetragonal tetraamminechromium(III) complexes are examined in detail, using the angular overlap and cellular ligand field models. Among applications of LFT, ligand field stabilization energy (LFSE) is useful in the rationalization of thermodynamic and kinetic observations. The Jahn–Teller effect is important in the structural chemistry of copper(II) especially.
Luminescence
Authors: Fouassier C.
Published: March 2006

This chapter is devoted to the luminescence of inorganic compounds. Luminescence characteristics are strongly host‐dependent. This is illustrated by various examples of the influence of covalency, crystal field, phonon energies, and structural features. The chapter is organized as follows.

Section 2 describes the fundamentals of luminescence: probability of electronic transitions, shape of emission bands, quantum efficiency, energy transfer, and factors governing the position of the emitting level (nephelauxetic effect and crystal field splitting).

Section 3 reviews the main families of luminescent ions: transition metal ions, lanthanide ions, and s2ions. The various types of electronic transitions responsible for the luminescence of transition and lanthanide ions are presented: nd → nd, (n + 1)s → nd, and charge transfer (CT) transitions for the former, 4f → 4f, 5d → 4f, and CT transitions for the latter. Data are given on the luminescence wavelength range, excited‐state lifetimes, thermal stability, and concentration quenching.

Section 4 deals with donor‐acceptor transitions in ZnS.

Section 5 presents the luminescence mechanisms for excitation through the host lattice. Vacuum ultraviolet (VUV), electron beam, X‐ray, and γ‐ray excitation are considered. The most efficient luminescent materials are given for the various types of excitation.

Section 6 deals with electroluminescent materials.
Macrocyclic Ligands
Authors: Bowman‐James Kristin
Published: March 2006

The main classes of macrocyclic ligands are classified. Emphasis is placed on O and N donors but S, P and As‐donors are also included. The macrocyclic effect and anion coordination are both discussed.
Magnetic Oxides
Authors: Greedan John E.
Published: March 2006

An attempt has been made to survey the research activity in selected aspects of the field of magnetic oxides for the period ∼1993 to the present. Emphasis has been placed on oxides with the perovskite and related structures such as the Ruddlesden–Popper phases, A‐site and B‐site ordered perovskites, pillared perovskites, and materials with structures related to the hexagonal perovskites. As well, the field of geometrically frustrated magnetic materials is reviewed with emphasis on materials with the pyrochlore, spinel, and related structures. Phenomena exhibited by such compounds include colossal magneto‐resistance, charge and orbital ordering, half‐metallic ferromagnetism, low‐dimensional magnetism, spin glass, spin liquid, and spin ice ordering, among others.
Manganese: Organometallic Chemistry
Authors: Sweigart Dwight A. , Reingold Jeffrey A.
Published: March 2006

Ever since the characterization of Mn
2
(CO)
10
in 1954, manganese has played an important role in the development of organometallic chemistry. In some ways, however, manganese has not been as prominently utilized in synthesis and catalysis as some other transition metals such as iron, cobalt, chromium, and so on. The low cost of the metal, its accessible oxidation states, and the versatility of its many reaction pathways suggest that manganese will play a feature role in the organometallic chemistry of the future. Herein, new developments in organomanganese chemistry are described. There have been significant advances in insertion reactions, nucleophilic additions to coordinated arenes, and the chemistry of manganese‐coordinated carbenes, vinylidenes, and cymantrenes. Of special interest are the recent demonstration of manganese‐mediated activation of strong covalent bonds in sulfur and nitrogen heterocycles and the construction of organomanganese coordination networks via self‐assembly of modular quinonoid complexes.
Manganese: The Oxygen‐Evolving Complex & ModelsBased in part on the article Manganese: Oxygen‐Evolving Complex & Models by Lars‐Erik Andréasson & Tore Vänngård which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Charles Dismukes G. , van Willigen Rogier T.
Published: March 2006

In this article, we give an overview of nature's singular biological process for producing oxygen gas by the oxidation of water in photosynthetic organisms. The harnessing of light to accomplish the splitting of water was arguably nature's most successful experiment in biological innovation. It enabled global proliferation of oxygenic photosynthesis and created the biogeochemical cycles of oxygen and carbon on earth. We review the atomic structure of the metalloenzyme, the photosystem II water splitting complex (PSII‐WOC), as revealed by X‐ray diffraction and spectroscopic techniques. We describe: 1) The electronic structure of its inorganic core, Mn4Ca1OxCl1–2(HCO3)y, based upon spectroscopic and magnetic susceptibility measurements, 2) the organization of the protein subunits as revealed by X‐ray diffraction and the role of select amino acid residues as revealed by site‐directed mutagenesis, and 3) the kinetic sequence of steps during assembly of the inorganic core to the cofactor‐depleted apo‐protein and the functional consequences of substitution of the inorganic cofactors. We use these data together with physico‐chemical data describing the formation of light‐induced intermediates, the exchange rates between substrate and free water molecules, the stoichiometry of electron and proton release and the activation energies to discuss possible mechanisms for photosynthetic water splitting. The chemistry of photosynthetic water splitting is energetically demanding and mechanistically complex. The lessons learned from nature have guided chemists seeking to incorporate these design principles within catalysts suitable for abiotic water splitting. We end with a description of the few synthetic manganese complexes that have been found to produce oxygen gas from water and discuss the chemical mechanisms by which they appear to function.
Mechanisms of Reaction of Organometallic Complexes
Authors: Atwood Jim D. , Rosenberg Edward
Published: March 2006

An overview of the mechanistic features of organometallic reactions is presented. The primary mechanistic types discussed are ligand substitution, oxidative addition, reductive elimination, and electron transfer. Each reaction pathway is discussed within the context of the common electron counts encountered for most organometallic complexes. Examples of how these common reaction pathways extend to polynuclear complexes are also presented. Although reasonable generalizations with regard to overall reaction rates and stepwise mechanisms using the electron count formalism can be made, many recent reports point out that the details of ligand and metal center structure can override these general trends. A considerable amount of quantitative rate data is provided and computational methodologies for understanding the observed rates are 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: March 2006

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.
MetallothioneinsBased in part on the article Metallothioneins by Milan Vašák & Jeremias H. R. Kägi which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Vašák Milan , Romero‐Isart Núria
Published: March 2006

Metallothionein (MT) is a generic name for a superfamily of ubiquitous low molecular weight cysteine‐ and metal‐rich proteins or polypeptides possessing sulfur‐based metal clusters. In these compounds, clusters are usually formed through the preferential coordination of d
10
metal ions by an array of closely spaced cysteine thiolate ligands. Their preservation in an increasing number of three‐dimensional structures of these proteins signifies the importance of this structural motif. The versatile and striking chemical reactivity of clusters in MTs and their unprecedented dynamics make this class of proteins highly unusual. A primary role of MTs has not been identified, and remains elusive, as further functions continue to be discovered.
Mixed Donor LigandsBased in part on the article Mixed Donor Ligands by Nikolay N. Gerasimchuk & Kristin Bowman‐James which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Lawrance Geoffrey A.
Published: March 2006

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: March 2006

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: Organometallic ChemistryBased in part on the article Molybdenum: Organometallic Chemistry by M. David Curtis which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Flower Kevin R.
Published: March 2006

This article considers only compounds that contain a direct molybdenum‐to‐carbon bond. Because of the size of the topic and the large number of publications each year, the article gives a general overview of major current areas of interest in molybdenum‐centered organometallic chemistry. The material is presented in three major sections: σ‐ligand‐containing complexes, which covers alkyl‐, carbonyl‐, halocarbonyl‐, alkylidenes‐, and alkylidyne complexes, heavier group 14 analogs of alkylidene and alkylidyne complexes; π‐ligand‐containing complexes, which covers alkene‐, polyene‐, alkyne‐, phosphaalkyne‐containing complexes, along with π‐bound cyclic organic systems such as cyclobutadiene, cyclopentadienyl, and arenes; polymetallic complexes and multiply bonded systems are also discussed. Each section considers the major methods of preparation of compounds containing that ligand class and surveys the major types of reaction they undergo, both metal‐ and ligand‐centered, along with the factors that influence the products that form. Where data is available, solid‐state structural data are presented to aid in the discussion of the material presented.
Niobium & Tantalum: Inorganic & Coordination Chemistry
Authors: Hubert‐Pfalzgraf Liliane G.
Published: March 2006

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.
Nitrides: Transition Metal Solid‐State ChemistryBased in part on the article Nitrides: Transition Metal Solid State Chemistry by Peter Ettmayer & Walter Lengauer which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Lengauer Walter , Eder Alexander
Published: March 2006

Basic properties and properties interesting for applications of transition metal nitrides are presented in this article. Starting from crystal chemistry, bonding, and preparation, the article also briefly discusses analytical chemistry of chemical and physical nitrogen analysis and standardization as well as thermodynamics. Updated portions of phase diagrams of group 4–6 transition metals with nitrogen are presented. A substantial part is devoted to the solid‐state properties because they make nitrides interesting for application. The presentation of these properties includes hardness, thermal expansion, nitrogen diffusivity, thermal properties, electrical conductivities, superconductivities, and elastic properties. Several of these properties are given as a function of temperature and composition. The topic is closely connected to article (
see
Carbides: Transition Metal Solid‐state Chemistry).
Noble Gases: Inorganic ChemistryBased in part on the article Noble Gases: Inorganic Chemistry by Boris Žemva which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Atwood David A.
Published: March 2006

The noble gas (Ng) elements have a rich reaction chemistry despite having a filled valence shell that would indicate otherwise. Halide compounds, usually with fluorine, and various oxides are common. There is also a growing class of carbon and nitrogen derivatives. A wide range of neutral and cationic noble gas compounds can be formed as long as the added elements, or combinations of elements, possess sufficient electron affinity. The range of compounds, along with characterization data and brief bonding descriptions will be presented for each element. Many of the compounds serve as starting materials for other derivatives, including the rapidly growing field of noble gas containing transition metal complexes. For example, there is now a broad range of salts containing transition metals. The article will conclude with a brief overview of the many unusual and disparate applications being found for these elements.
Noncrystalline Solids
Authors: Adam Jean‐Luc , Lucas Jacques
Published: March 2006

This review deals with the formation and physical properties of noncrystalline materials in oxide and nonoxide systems. Structural aspects are also developed. Oxide materials are silica‐based glasses, borate, and phosphate glasses, while nonoxide systems include halide and chalcogenide glasses. They are described in terms of chemical compositions and thermal and optical properties. Special attention is paid to the unique properties of nonoxide glasses for infrared‐light transmission and rare earth emissions. Some applications in the fields of infrared optics, infrared fiber sensors, fiber lasers, and amplifiers are shown. In a special section, the formation and properties of metallic glasses are presented, with special focus on chemical, magnetic, and mechanical properties.
Nucleic Acid–Metal Ion Interactions
Authors: Kazakov Sergei A. , Hecht Sidney M.
Published: March 2006

Nucleic acids (DNA and RNA) are actually salts (or complexes) of metal ions from a chemical point of view. It is impossible to separate the behavior of DNA and RNA from their interactions with metal ions. Metal ions are usually required to promote and stabilize functionally active or native conformations of nucleic acids, as well as to mediate nucleic acid‐protein interactions. However, certain metal ions can also cause structural transformation of nucleic acids, and induce their chemical modification and cleavage. Metal‐nucleic acid interactions are involved in nucleotide biochemistry; genetic information storage and transfer, and control of gene expression, as well as in mutagenesis and carcinogenesis. However, in most cases, it is not currently clear whether the observed effects actually result from direct metal ion‐nucleic acid interaction, since other species, such as specific proteins, may be mediators between the metal ion and the nucleic acid in these processes
in vivo
. Interest in metal‐nucleic acid interactions is also enhanced by studies of the mechanisms of antitumor activity of certain platinum metal compounds, finding metal ions in crystal structure of natural RNA molecules, studies of the mechanism of RNA catalysis and possible involvement of metal ions in origin of life, as well as by the use of metal ions as structural probes of nucleic acids and as biotechnology tools. This article does not attempt to be comprehensive, but rather presents selected examples to illustrate key facets of metal ion‐nucleic acid interaction. The focus is on the chemical and structural properties of both nucleic acids and metal ions relevant to their interactions. Nucleic acid‐metal interactions can be either nonspecific or dependent on the chemical nature, sequence, and structure of nucleic acids. However, the specificity of these interactions is dependent on structural and chemical properties of both the nucleic acids and the metal ions. Both nucleic acids and metal ions exhibit considerable complexity in their interactions, and such interactions could affect the chemical and biochemical properties of both species.
Oligomerization & Polymerization by Homogeneous Catalysis
Authors: Hlatky Gregory G.
Published: March 2006

The metal‐catalyzed oligomerization and polymerization of alkenes by homogeneous metal complexes has not only provided extraordinary insights into the chain‐growth process, it has also become of increasing commercial importance in delivering products of greater purity or improved properties. Novel materials not accessible using conventional heterogeneous catalysts can now be synthesized. Metals are no longer constrained to certain functions (titanium for polymerization, nickel for oligomerization) but span the range of molecular weights and polymer structures. This review discusses the fundamentals of chain‐growth polymerization catalyzed by homogeneous complexes and new developments in the field.
Osmium: Organometallic Chemistry
Authors: Shapley Patricia A.
Published: March 2006

Organometallic complexes of osmium include those with sigma bonds between the metal and the organic moiety, such as metal–alkyl complexes, or π bonds, such as metal–alkene complexes. The osmium alkyl complexes include homoleptic alkyls and complexes with ancillary carbonyl, cyclopentadienyl, arene, or alkene ligands. Because osmium complexes are stable in higher oxidation states than either ruthenium or iron, oxidative addition of CH bonds to form alkylidene and alkylidyne complexes is very common. Metal–ligand bonds also tend to be stronger in osmium complexes than in complexes of the lighter elements of this group. Because of the stability of OsOs bonds, there are many bimetallic and cluster complexes of osmium and these species have an extensive reaction chemistry. Selective oxidation reactions of unsaturated organic molecules dominates the catalytic chemistry of this metal. Mechanistic studies on the activation of molecular oxygen and oxygen transfer reactions by organoosmium complexes have provided insights into catalytic oxidation chemistry.
P‐Donor LigandsBased in part on the article P‐Donor Ligands by Charles A. McAuliffe & Anthony G. Mackie which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Schier Annette , Schmidbaur Hubert
Published: March 2006

Organophosphorus compounds of various types are among the most important classes of ligands in the coordination chemistry of the transition elements. This is particularly true for the tertiary phosphines R3P, which are known to stabilize complexes of metals in low oxidation states, while secondary and primary phosphines, R2HP and RPH2, are less common. The donor properties and the stereochemistry can be tuned by using carefully designed substitution patterns. More recently, organophosphorus compounds with low‐coordinate phosphorus atoms have also been employed as ligands in metal complexes. These include mainly phosphaalkenes and phosphaalkynes, phosphinidenes, diphosphenes, phosphinines, phosphonium cations, and even elemental phosphorus.
Palladium: Organometallic Chemistry
Authors: William Suggs J.
Published: March 2006

Four oxidation states of palladium are encountered in organometallic chemistry. In order of importance, they are Pd
II
, Pd
0
, Pd
IV
, and Pd
I
. With the reduction of palladium from Pd
II
to Pd
0
, the metal changes its reactivity from electrophile to nucleophile. Ligands π‐bound to Pd
II
, such as π‐allyl, alkene, and alkyne, show enhanced reactivity toward nucleophiles, particularly less basic nucleophiles such as malonates. Pd
0
species, such as (PR
3
)

n

Pd, are powerful nucleophiles. They undergo oxidative addition to alkyl, aryl, and vinyl halides to produce LnPd
II
‐R species. If the R group is an alkyl group with β‐hydrogens, a beta‐elimination of H usually takes place, giving a palladium hydride and an alkene. If the R group is aryl or vinyl, the species does not undergo beta‐elimination. Instead, if a second R′ group is bonded to the palladium center, reductive elimination can take place, forming R–R′. Alternatively, the PdR bond can add across an unsaturated group, such as an alkene, forming a new carbon–carbon bond. New carbon–palladium bonds can also form through transmetalation. Organotin, organoboron, organosilicon (and other main group and transition metal organometallic reagents) will react with Pd
II
halides to give new Pd
II
‐R bonds and R
3
SnX (or analogous) species. Because Pd
II
‐C bonds are neither very strong nor very weak (on the order of 160 kJ mol
−1
), palladium can catalyze many organic reactions. For example, in the Suzuki reaction, an aryl boronic acid (ArB(OH)
2
) and an aryl halide (Ar′X) are coupled to form a biaryl, Ar–Ar′. The mechanism involves a Pd
0
species undergoing oxidative addition to the Ar′X, forming L

n

Pd
II
Ar′(X), then transmetalation giving a L
n
Pd
II
Ar(Ar′) and, finally, reductive elimination forming Ar–Ar′ and regenerating L

n

Pd
0
. Palladium‐catalyzed reactions are increasingly being used in the synthesis of high value‐added chemicals such as pharmaceuticals. With basic, hindered phosphine ligands, such as P(
t
‐Bu)
3
, Pd
0
reagents undergo oxidative addition even with aryl chlorides, and new reactions, such as the arylation of amines (ArX + HNR
2
giving ArNR
2
) are possible.
Peptide–Metal InteractionsBased in part on the article Peptide–Metal Interactions by Justin K. Moran & Claude F. Meares which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Meares Claude F.
Published: March 2006

Copper–peptide complexes have been the most extensively studied of all the metal–peptide complexes. Copper(II) is the most important oxidation state in copper–peptide complexes, although copper(III) can be stabilized by coordination to peptides. The deprotonated peptide nitrogens help stabilize the higher oxidation state of copper; therefore the more deprotonated peptide nitrogens, the more stable the copper(III) complex. The copper(II) complexes of GlyGlyProGly and (GlyGlyProGly)2 are very similar. At low pH, both peptides bond through the amino nitrogen and carbonyl oxygen of the first peptide linkage. The peptides GlyGlyProLys and (GlyGlyProLys)2 initially form 1N complexes with copper(II). With luteinizing hormone‐releasing hormone the copper(II) is coordinated via the imidazole nitrogen and two deprotonated peptide nitrogens. With angiotensin II, the copper(II) is bonded via the imidazole nitrogen of the histidine residue and two deprotonated peptide nitrogens. The dipeptide TyrPro bonded copper(II) through the amino nitrogen and the peptide carbonyl. Since there was no ionizable hydrogen on the peptide nitrogen, there was no bonding by the peptide nitrogen. The copper(II)AVP (vasopressin analog) complex is one of the most stable 4N copper–peptide complexes known. Owing to their similarities, nickel(II)–peptide complexes are usually studied along with copper(II) complexes. In neutral to acidic solutions, the nickel is coordinated through the amino nitrogen and the carbonyl oxygen (1N). When the pH is raised to 9.0, nickel(II) substitutes for a peptide hydrogen, and the coordination changes. In NiH1L complexes of cysteine‐containing dipeptides, the nickel is coordinated via the amino and deprotonated peptide nitrogens and the thiol group. Zinc‐peptide complexes are less stable. With the dipeptides Ala–His and Gly–His at high pH, zinc(II) was shown to form complexes coordinated through the amino nitrogen, deprotonated peptide nitrogen, and the imidazole nitrogen. Unlike copper(II), both palladium(II) and platinum(II) are soft acceptors and as such are expected to coordinate to soft donors like sulfur and aromatic nitrogen donors in residues such as Met and His, and deprotonated peptide nitrogens. Early investigations of metals showed that divalent metals such as copper(II), cobalt(II), and nickel(II) could catalyze the hydrolysis of amino acid esters. Mechanisms of cobalt(III)‐mediated peptide‐bond cleavage have been investigated; because cobalt(III) complexes bind to the N‐terminal amino acid residue of peptides, only the N‐terminal peptide bond is hydrolyzed. Complexes containing iron can cleave peptide bonds, as can complexes of palladium or platinum.