bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetism of Extended Arrays in Inorganic Solids
Authors: Landee Christopher P. , Turnbull Mark M.
Published: March 2006

The dominant magnetic interaction in insulators is magnetic exchange. This quantum mechanical effect arises from the overlap of wavefunctions of two different paramagnetic centers, either directly (direct exchange) or through an intervening ligand (superexchange). The associated exchange energy is electrostatic in origin but formally equivalent to the difference between the single and triplet states for the two interacting electrons. Magnetic exchange, plus other effects arising from the orientation of an electronic moment to the lattice through spin‐orbit coupling, leads to a wide variety of magnetic behavior, as illustrated with examples of magnetically one‐dimensional lattices.
Magnetism of Transition Metal Ions
Authors: Hatfield William E.
Published: March 2006

The title topic is discussed with emphasis on the interpretation of experimental data. Typical examples are also discussed.
Manganese: Inorganic & Coordination ChemistryBased in part on the article Manganese: Inorganic & Coordination Chemistry by Charles A. McAuliffe, Stephen M. Godfrey, & Michael Watkinson which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Collomb Marie‐Noëlle , Deronzier Alain
Published: March 2006

This article covers the last 10 years of the inorganic and coordination chemistry of manganese at oxidation states (II), (III), (IV), (V), (VI), and (VII), as well as multinuclear compounds having oxidation states (II)–(IV) with both unitary and mixed valencies. The common ligands for manganese compounds involve the donor atoms nitrogen, oxygen, and the halogens while only few are known with other group V and group VI donors. One of the major areas of expansion has been the multinuclear complexes that are relevant to such diverse fields as metalloenzymes and single‐molecule magnets. Besides the small clusters, comprising two to four metal ions synthesized to model the structures and functions of manganese metalloenzymes, a lot of interest has been invested in larger clusters, not only for their biological relevance, but also for the development of materials with novel magnetic properties. Significant advances have been also been made in the chemistry of higher oxidation state manganese complexes (V), (VI), and (VII).
Manganese Proteins with Mono‐ & Dinuclear Sites
Authors: Penner‐Hahn James E.
Published: March 2006

Although manganese is not as widely recognized as iron and copper, it nevertheless plays an essential role in several key biological processes. Consequently, manganese concentrations are tightly regulated in most cells. Several Mn‐specific transporters have been identified and a novel Mn‐regulatory protein has been characterized. Manganese‐activated enzymes are proteins that have relatively weak affinities for Mn; in many cases these are isolated without any metal bound. Manganese‐activated enzymes are especially important in hydrolytic reactions, and appear, in at least some cases, to have physiologically significant changes in activity that respond to changes in Mn concentration. The best characterized Mn proteins are redox‐active enzymes, where Mn typically cycles between the MnIIand MnIIIoxidation states, although some MnIV‐containing species are known.
Metal Vapor Synthesis of Transition Metal Compounds
Authors: Schmidt Eckhardt , Klabunde Kenneth J. , Ponce Aldo , Smetana Alexander , Heroux David
Published: March 2006

The title topic is reviewed with special reference to the reactivity of metal atoms, their energetics of vaporization, vaporization techniques, and some detailed examples of reaction products. Recent advances in nanoparticle work are also discussed.
Metallic Materials Deposition: Metal‐Organic PrecursorsBased in part on the article Metallic Materials Deposition: Metal–Organic Precursors by Herbert D. Kaesz, Alfred Zinn, & Lutz Brandt which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Winter Charles H. , Zheng Wenjun , El‐Kaderi Hani M.
Published: March 2006

This review describes metal‐organic precursors for the growth of metal‐containing thin films by chemical vapor deposition methods. The major emphasis is on precursors that have been reported since 1994, and corresponds to a time of major growth in this field. Progress in the development of metal‐organic precursors is documented for the main group, lanthanide, and group 4–11 elements. In the main group elements, there has been considerable research activity directed toward the identification of strontium and barium precursors, due both to the technological importance of mixed oxide phases and the inherent difficulties in obtaining volatile, complexes of these large metal ions. Aluminum, gallium, and indium have also been the subject of intense investigation, owing to the importance of many phases containing these elements. The group 4 and 5 elements titanium, zirconium, hafnium, niobium, and tantalum have been the subject of considerable precursor development activity, owing to the importance of several mixed oxide phases and the importance of zirconium and hafnium oxide for high permittivity gate materials in microelectronics devices. Growth of metal nitride films of these elements has also been an active area of research for use as barrier materials in microelectronics devices. The deposition of copper metal films from metal‐organic precursors is driven by the urgent need for copper metallization procedures in microelectronics device manufacturing. The current state of metal‐organic precursor development is presented for each of the other metallic elements.
Molecular Orbital Theory
Authors: Czech Paul T.
Published: March 2006

This entry outlines the development of theoretical chemistry from its origins to the present highlighting important several theories, techniques, and applications to inorganic chemistry. The field has evolved from the series of postulates that provide the framework for quantum mechanics and which culminated in the Schrödinger equation proposed in 1926. Soon thereafter, the application to chemical systems began. Valence Bond Theory (VBT) and Molecular Orbital (MO) Theory were initial techniques for explaining what for chemists was of critical importance, that is, the nature of the chemical bond in molecules. Each approach is still useful today. Molecular Orbital Theory, however, has enjoyed wider applicability owing to being much more easily handled quantitatively. The explosive growth of digital computing capabilities over the last half‐century can no doubt be traced in part to the intensive number crunching requirements of this type. The article reviews the early work of VBT and MO theory, and the treatment of H2, homonuclear, and heteronuclear diatomic molecules in some detail. Sections concerning the important application of mathematical group theory conventions and several techniques that use the self‐consistent field approach define much of the terminology that is commonly used in the field. An up‐to‐date list of references and related articles provided interested readers with much more detail on the techniques and applications in this enormous field.
Molybdenum:MPT‐Containing Enzymes
Authors: Young Charles G.
Published: March 2006

Molybdenum, an essential trace element for all forms of life, is found in a wide range of Mo‐MPT enzymes important in the metabolism of small molecules and anions of nitrogen, sulfur, carbon, chlorine, selenium, and arsenic, and in the global biogeochemical cycles of these elements. These enzymes feature active sites composed of a single (mononuclear) Mo atom coordinated by one or two MPT‐based ligands, L (MPT = ‘molybdopterin′ orMetal‐bindingPterin ene‐1,2‐diThiolate). These ligands contain a pyranopterin or simple pterin nucleus, an ene‐1,2‐dithiolate (dithiolene) unit, and a pendant phosphate group; in enzymes from prokaryotes, the phosphate is generally attached to a nucleotide, giving ‘dinucleotide’ forms of MPT. The dithiolene moiety functions as a bidentate S2‐donor ligand to Mo. The Mo‐MPT enzymes are classified into two broad functional types: oxotransferases and hydroxylases. The oxotransferases generally catalyze oxygen atom transfer reactions and fall into two families: the sulfite oxidase and dimethyl sulfoxide (DMSO) reductase families. Enzymes from these families contain square‐pyramidalcis‐[MoVIO2L(cysteinate)]−and distorted trigonal prismatic [MoVIO(AA)L2]−(AA = serinate, cysteinate, selenocysteinate, etc.) oxidized active sites, respectively. The hydroxylases all belong to the xanthine oxidase family and generally catalyze net insertion of oxygen into the CH bonds of heterocyclic aromatic compounds and aldehydes. They contain biologically unique, square‐pyramidal [MoVIOS(OH)L]−oxidized active sites featuring a catalytically essential terminal sulfido ligand. The sulfido ligand can act in a bridging capacity to Cu, as recently demonstrated for CO dehydrogenase. In most reactions catalyzed by Mo‐MPT enzymes, an oxygen atom is exchanged between substrate and water with concomitant consumption (for reductases) or production (for oxidases) of reducing equivalents (2e−, 2H+). Synthetic analogue (model) studies have produced complexes replicating or informing the composition, structures, spectroscopy, and chemistry of the enzymes. This account focuses on the Mo bioinorganic chemistry of the Mo‐MPT enzymes and selected synthetic analogues. It is divided into two main parts. Part 1 provides an overview of the enzyme systems, their distribution, roles, and essential chemistry, as well as a discussion of consensus structures and likely mechanisms. The genes and proteins involved in Mo trafficking and Mo‐cofactor biosynthesis are also briefly described. Part 2 summarizes important developments in the production of synthetic analogues, with a focus on new dithiolene, scorpionate, and other complexes sharing the structural, spectroscopic, or chemical attributes of the enzymes.
Nickel: Models of Protein Active SitesBased in part on the article Nickel: Models of Protein Active Sites by Michael J. Maroney which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Liaw Wen‐Feng
Published: March 2006

During the past decade, structures and functions of the active sites have been established for the nickel‐based metalloenzymes present in the hydrogenases, ureases, carbon monoxide dehydrogenases/acetyl coenzyme A synthase, and methyl coenzyme M reductase. Roles of nickel in these Ni‐containing enzymes serve as catalytic or redox centers. Model compounds and various biophysical techniques have been lent to elucidate aspects of the structure and function (catalytic mechanism) of the Ni sites in these enzymes. Structures/functions of these Ni‐containing enzymes, biomimetic model compound studies, and the proposed catalytic mechanisms are presented.
Niobium & Tantalum: Organometallic ChemistryBased in part on the article Niobium & Tantalum: Organometallic Chemistry by David E. Wigley which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Wang Ruitao , Xue Ziling
Published: March 2006

This review highlights leading literature in syntheses, characterization, structures, and reactivities of organometallic complexes of niobium and tantalum between 1993 and early 2003. Such complexes include monometallic and bimetallic species. The applications of niobium and tantalum organometallic complexes in catalysis and metal‐mediated organic synthesis are discussed as well.
Nutritional Aspects of Metals & Trace ElementsBased in part on the article Nutritional Aspects of Metals & Trace Elements by Paul Saltman which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Yokoi Katsuhiko
Published: March 2006

The essential inorganic nutrients are dietary components that exert nutritional function when they are consumed as inorganic substances. Some essential inorganic nutrients such as selenium and iodine act as organic active constituents after metabolic conversionin vivowhen they are consumed as organic as well as inorganic substances. Inadequate intake of essential inorganic nutrients results in deficiency diseases and/or interruption of life cycles. The essential inorganic nutrients consist of the major elements (sodium, potassium, calcium, magnesium, chlorine, phosphorus, and sulfur) and the trace elements (iron, copper, zinc, manganese, cobalt, chromium, molybdenum, vanadium, nickel, arsenic, selenium, iodine, fluorine, silicon, boron, tin, cadmium, lead, rubidium, and lithium). Evidence of essentiality is limited for arsenic, boron, cadmium, fluorine, lead, lithium, nickel, rubidium, silicon, tin, and vanadium. If the requirement is less than 1 mg day−1for humans or 1 mg kg−1diet for animals, the inorganic nutrients are called ultratrace elements. Further investigation may increase the number of essential inorganic nutrients. Deficiencies of inorganic nutrients such as iodine, iron, and zinc occur worldwide. To prevent deficiencies and toxicosis, the US and other nations have established the Dietary Reference Intakes (DRIs) or the equivalents that are composed of reference values including Estimated Average Requirements (EARs), Recommended Dietary Allowances (RDAs), Adequate Intakes (AIs) and Tolerable Upper Intake Levels (ULs). Essential inorganic nutrients function in vivo through various modalities such as catalytic centers of enzymes, metabolic regulation, modulation of ion channels and receptors. The regulation and control of inorganic nutrients are a function of eight parameters: availability, solubility, permeability, ligand exchange, transport, assimilation, storage, and excretion. Excessive intakes, genetic abnormalities, and other etiologies cause pathologies of metal overload. Functions of selenium and nickel were reviewed. Selenium manifests antioxidant activity as an active site of glutathione peroxidase. Other important selenium‐containing proteins include thioredoxin reductase, iodothyronine deiodinases, selenoprotein P. Nickel occupies the metal centers of urease, carbon monoxide dehydrogenase, and other enzymes. Nickel deprivation results in impaired reproduction (rats and goats) and decreased spermatozoa motility and density (rats). It is our challenge to determine optimal intakes of inorganic nutrients for ensuring optimal health for all.
Organic Synthesis Using Transition Metal Complexes Containing π‐Bonded LigandsBased in part on the article Organic Synthesis using Transition Metal Complexes Containing π‐Bonded Ligands by Martin F. Semmelhack which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Morris W. H. , Sullivan Lisa M. , Wellons Matthew S. , Lukehart C. M.
Published: March 2006

A selective overview of reactions of transition metal π‐complexes pertaining to organic synthesis applications is provided. While reactions involving nucleophilic addition to pi‐coordinated ligands are emphasized, alkene insertion/elimination reactions and allyl coupling reactions are also discussed.
Osmium: Inorganic & Coordination Chemistry
Authors: Collinson Simon R. , Schröder Martin
Published: March 2006

The general features and coordination chemistry of Os are discussed in the context of the use of different ligand types to stabilize complexes of OsVIIIthrough to Os−II. The complexation chemistry with cyanide, N‐, P‐, O‐, S‐, and halide‐donor ligands is detailed. The asymmetric dihydroxylation and aminohydroxylation of alkenes, general coordination chemistry with macrocyclic and open chain ligands, porphyrin complexes, photophysical properties and electrochemistry of OsIIcomplexes of heterocyclic ligands and comparisons to their RuIIanalogues, and electron transfer in heterobimetallic and dendrimer complexes of OsIIare all highlighted.
Oxidation Catalysis by Transition Metal Complexes
Authors: Crabtree Robert H.
Published: March 2006

Oxidation catalysis is reviewed with emphasis on radical versus nonradical pathways, autoxidation, and mechanistic studies. Bioinorganic model catalysts are discussed, including the distinction between monooxygenase and dioxygenase pathways. Some organic synthetic applications as well as applications to green chemistry are discussed. Finally some organometallic oxidation catalysts are discussed.
Oxides: Solid‐State Chemistry
Authors: McCarroll William H. , Ramanujachary Kandalam V.
Published: March 2006

This article is an overview of the solid‐state chemistry of oxides. It starts with a brief description of the periodic trends associated with compounds of oxygen and is followed by a discussion of the principle of the closest packing of spheres that can be used to describe ionic structures. An in depth description of several structure types, important in solid‐state oxide chemistry, follows, and covers the important binary structures as well as several ternary structures including the Perovskite, Spinel, Garnet, Pyrochlore, and Scheelite structures.

Selected special topics in solid‐state oxide chemistry include a discussion of crystallographic shear structures, defect oxides, and tungsten and molybdenum bronzes, intergrowth structures of which the high‐temperature superconductor YBa2Cu3O7is an example. The importance of structure‐property relations in oxides is illustrated by a discussion of metal–metal bonds in oxides, electronic conduction in a variety of oxides, and the catalytic properties of defect scheelites.

A lengthy discussion is devoted to the various means used by solid‐state chemists to synthesize oxides not only as bulk polycrystalline phases but as single crystals and thin films as well that are so important to electronic applications. The article concludes with a discussion of several important practical applications of oxides including magnetic oxides, oxide sensors, luminescent materials, ferroelectrics, CMR oxides, optical materials, ceramics, and refractory materials as well as a brief overview of other important uses.

Because this is a survey article, in depth discussion of many properties of oxides is not possible. However, many of these topics are covered in depth in other parts of the Encyclopedia and the reader will find the cross references helpful. A substantial list of references also provides a gateway to the solid‐state literature.
Oxygen: Inorganic ChemistryBased in part on the article Oxygen: Inorganic Chemistry by Donald T. Sawyer which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Bharara Mohan S. , Atwood David A.
Published: March 2006

The present review will provide a systematic and complete coverage of the formation, chemical properties, and reactivity of a wide range of common oxygen‐containing species. These include dioxygen, ozone, hydrogen peroxide, oxide, superoxide, and hydroxide, as well as the radical forms of most of these species.
Palladium: Inorganic & Coordination Chemistry
Authors: Albéniz Ana C. , Espinet Pablo
Published: March 2006

Palladium is a group 10 metal used in the manufacture of exhaust autocatalysts in the automobile industry, in dental palladium alloys, in the electronics industry, in jewelry alloys, and in the manufacture of industrial catalysts. Although the latter represents only about 5% of Pd use, it is extremely important. A remarkable property of metallic palladium is its ability to absorb considerable amounts of hydrogen. The inorganic and coordination chemistry of palladium is mostly that of their oxidation states 0 and +2, but the oxidation states +1 and +4 are also well represented. Only in a few examples Pd can be guaranteed as Pd +3, while in others the ligands of Pd are noninnocent and the apparent oxidation state assigned to Pd can be deceptive. Only one Pd(V) and one Pd(VI) compound have been reported but not confirmed by independent research groups. The Pd(0) complexes reported include compounds with many possible ligands, which are known in [PdL4], [PdL3], and [PdL2] stoichiometries. The former are formed with conventional ligands and have the tendency to dissociate at least one ligand in solution, while the latter are formed with bulky ligands and are more reactive toward oxidative addition. In addition to Pd on carbon, colloids and polymer‐stabilized forms of Pd are catalytically important. All Pd(I) compounds reported feature PdPd or PdM bonds, with or without supporting bridges, displaying quite a large range of distances. Pd(II) compounds are commonly four‐coordinated square planar, but there are a considerable number of five‐coordinated compounds. Three‐coordinated Pd(II) compounds are not known. One doubtless case of a four‐coordinated tetrahedral (and accordingly paramagnetic) complex has been reported. Pdf2is a rare example of six‐coordinated Pd(II). Square‐planar Pd(II) fragments are building blocks in the construction of extended self‐assembled structures. Coordination tocis‐‘PdL2’ fragments can be used to define corners, whiletrans‐‘PdL2’ moieties can help build linear edges. Mononuclear complexes of Pd(III) can be stabilized by some tridentate macrocyclic ligands containing S and N as donors. A binuclear Pd(III) compound, displaying the shortest PdPd bond is also known. In both cases, the coordination number for Pd is six. Pd(IV) is not an uncommon oxidation state, although it is quite oxidant. Complexes with many different ligands, which show octahedral coordination are known.
Paramagnetic Organometallic Complexes
Authors: Leznoff Daniel B. , Mund Garry
Published: March 2006

The synthesis, characterization, and reactivity of paramagnetic (or open‐shell) organometallic species are described. Many stable complexes featuring transition metals, lanthanides, and actinides have been reported. These systems, which are exceptions to the 18‐electron rule (or 16‐electron rule), are increasingly being used in the catalytic realm, and also have different reactivity patterns compared to their diamagnetic counterparts due to the presence of the unpaired electrons. The formation and subsequent stability of paramagnetic organometallic complexes in violation of the 18‐electron rule can be explained via molecular orbital theory in terms of (1) the use of π‐donor ligands (as opposed to the ubiquitous π‐acceptor ligands usually found in organometallic chemistry); (2) kinetic stabilization with sterically demanding ligands; (3) partially filled molecular orbitals being nonbonding or slightly antibonding/bonding. Although metallocene‐based complexes are probably the most well studied of all paramagnetic organometallic complexes, homoleptic σ‐alkyl complexes and other non‐Cp systems have also been explored. Such non‐Cp ancillary ligands include amido‐, alkoxo‐, and thiolato‐ligands, β‐diketiminate ligands, and tris(pyrazolylborate) ligands. f‐electron organometallic systems also utilize a similar range of ligands, but due to their larger size, they can accommodate more sterically congested coordination spheres; for example, Cp*3M complexes can be prepared, and they show unusual reactivity pathways.

Open‐shell organometallic molecules are known to span a range of electron counts from 8–20. Nineteen‐electron organometallic complexes are of particular interest with respect to the location of the extra electron residing primarily on the metal or on the ligand (in this case, the complex is referred to as an 18 + δ system).

The discussion of short‐lived paramagnetic organometallic systems revolves around 17‐ and 19‐electron radicals, and their use in catalytic processes and as ‘super reducing agents.’ Many reactivity pathways for such organometallic radicals (or ‘metalloradicals’) have been identified.

Characterization tools for paramagnetic organometallic complexes, such as NMR and ESR spectroscopy, DFT calculations, and magnetic measurements are presented. Variable temperature NMR studies and2H NMR can be of significant use in interpreting paramagnetic NMR spectra. Solid‐state NMR spectroscopy of paramagnetic organometallic systems is still in its infancy. The applications of paramagnetic organometallic complexes span many areas including catalysis (e.g. olefin polymerization), synthetic organic chemistry, and the development of molecule‐based magnetic materials.
Phosphates: Solid‐State Chemistry
Authors: Attfield J. Paul
Published: March 2006

Phosphates contain oxyanions of phosphorus(V). The most common anion is the orthophosphate group PO
4
3−
, which is found in many solid phosphates including hydrated and acid phosphate salts. Oligomeric polyphosphate chains P

n

O

3
n
+1

(
n
+2)−

up to
n
 = 6 and cyclophosphate rings P

n

O

3
n

n
−

up to
n
 = 12 are also observed, and a unique cage ultraphosphate anion P
8
O
23
6−
is found in the Na
3
FeP
8
O
23
structure. Polymeric phosphate anions include the infinite chain catenaphosphate anion (PO
3
−
)
∞
and the ultraphosphate networks found in CaP
4
O
11
and the lanthanide MP
5
O
14
structures. Many compound anions and neutral frameworks such as silicophosphates, aluminophosphates, and molybdenum phosphates also exist. Phosphate species are characterized through PO stretching and bending vibrations in infrared and Raman spectroscopies, and by
31
P NMR. Phosphate structures are generally rigid, resistant to chemical attack, and (when anhydrous) insoluble and thermally stable. Some phosphate crystals have important physical properties such as ferrolectricity in KH
2
PO
4
(potassium dihydrogenphosphate, KDP). Diffusion of extra‐framework ions leads to potential uses of solid phosphates as ion exchangers and conductors, for example, NASICON based on NaZr
2
(PO
4
)
3
, and as cathode materials for lithium batteries. Vanadium phosphates are important selective oxidation catalysts. Large pore phosphates in which molecules can be adsorbed and undergo acid‐catalyzed reactions have been intensively studied. These materials are crystalline microporous solids such as aluminophosphates, or lamellar structures, typically based on α‐Zr(HPO
4
)
2
·H
2
O that can be pillared. This area of interest has led to the synthesis of many new organically templated metal phosphates in recent years, usually through hydrothermal reactions. Hydrated and acid orthophosphates can also be prepared in solution, and anhydrous phosphates are synthesized by direct high‐temperature reactions and from phosphate‐rich melts or fluxes. Slow cooling of melts is used to grow crystals, and quenching can result in many stable phosphate glasses. Phosphate anions do not absorb significantly in the UV‐visible region and so solid phosphates can also find use as optical materials such as phosphors, nonlinear crystals, for example, KTiOPO
4
, and lasers. Solid phosphates constitute many minerals, notably Apatites which are also found in living organisms as rigid components such as bones and teeth. Synthetic calcium phosphate biomaterials are used for bone implants.
Phosphides: Solid‐State Chemistry
Authors: Pöttgen Rainer , Hönle Wolfgang , von Schnering Hans Georg
Published: March 2006

The solid‐state chemistry of phosphides, phosphide oxides, and phosphide halides is reviewed. Phosphorus is outstanding, since no other element approaches phosphorus in the variety of homoatomic connected polyanions formed, as a result of its relatively low electronegativity and stereochemical features. The article covers the various synthesis techniques for solid‐state phosphides, the crystal chemistry, and chemical bonding. Besides the metal‐rich phosphides with pronounced metal–metal bonding, the large family of polyphosphides, which exhibits an unexpected wealth of chemical composition, is discussed. Furthermore, the largely varying magnetic, electronic, and catalytic properties are reviewed.
Polonium: Inorganic Chemistry
Authors: Zingaro Ralph A.
Published: March 2006

The chemistry of polonium is quite limited. Because all of the isotopes of this element are radioactive, and because it is available in such limited quantities, research with polonium is difficult. The metal has been isolated and characterized. Although a number of polonium compounds, such as PoO2 have been well characterized, the evidence for many is insubstantial. Among the halide derivatives, PoCl4 has been studied in greatest detail. A number of other halides, and their complex ions such as PoBr6‐2, have been studied with considerable success. A simple binary nitride has never been prepared, and only indirect evidence for the existence of H2Po has been reported.
Polyhedra
Authors: King R. Bruce
Published: March 2006

The concept of a polyhedron is a useful way of describing diverse chemical structures, including those of coordination compounds and metal clusters. In this context, a polyhedron may be regarded as a set consisting of vertices, edges, and faces. Familiar examples of polyhedra are the five regular (Platonic) polyhedra, that is, the tetrahedron, cube, octahedron, dodecahedron, and icosahedron. The relationship between the numbers of vertices (
v
), edges (
e
), and faces (
f
) in a polyhedron can be described by Euler's theorem, that is,
v
 − 
e
 + 
f
 = 2. A given polyhedron can be converted to larger polyhedra by capping one or more of its faces. Interchanging the vertices and faces of a polyhedron leads to its dual, which has the same symmetry as the original polyhedron. A projection of a polyhedron onto a plane, known as a Schlegel diagram, is often a useful way of visualizing polyhedra, which supplements the usual perspective drawings. There are no formulas, direct or recursive, for which the number of distinct polyhedra having a given number of vertices, edges, or faces can be calculated.
Polynuclear Organometallic Cluster Complexes
Authors: Keister Jerome B.
Published: March 2006

The title refers to molecular compounds containing three or more transition metal atoms and hydrocarbyl ligands as a part of the metal–carbon cluster core. Because the chemistry of trimetallic clusters is the most well developed, these examples are to be stressed. Selected examples are presented to illustrate the principles of structure and bonding of organometallic clusters, synthetic approaches, and chemical reactions, including ligand substitution, oxidative addition, intramolecular insertion, protonation of metal–metal bonds and coordinated ligands, nucleophilic attack on coordinated ligands, and ligand fluxionality. The chemistry of these clusters is exemplified by trimetallic clusters containing hydrocarbon fragments of one, two, or three carbon atoms. While, many of the features of cluster chemistry are common with those of organometallic complexes containing a single metal atom, there are notable differences in structures and reactivities owing to the tendencies of ligands to bridge between two or more metal atoms and for ligands bound to only a single metal atom to undergo facile intramolecular migrations between metal atoms. These distinctions give rise to the ‘metal‐cluster‐metal surface analogy’ in which structural and spectroscopic features of organometallic clusters are used as models for hydrocarbons chemisorbed on metal surfaces.
Polyoxometalates
Authors: Pope Michael T.
Published: March 2006

The elements of groups 5 and 6, especially vanadium, molybdenum, and tungsten in their highest oxidation states, form a vast range of polyoxoanions that can incorporate most other positive‐valent elements of the periodic table. The first examples of these complexes were discovered in the latter half of the nineteenth century, but it is only within the last few decades that the extraordinary variety and numbers of polyoxoanion structures, properties, and applications have begun to be recognized. The field has historically been divided into isopoly‐ and heteropolyanions according to whether one or more than one type of positive‐valent element is incorporated, respectively. The largest variety of isopolyanions is found with V, Mo, and W, and the results of speciation studies in aqueous solution are summarized. The structures of heteropolyanions are dependent upon the coordination demands of the heteroatom, and the major types based on 4‐, 6‐, 8‐, and 12‐coordination are described. In virtually every case, the V, Mo, and W atoms (addenda atoms) occupy MO6octahedra with one or two (cis) unshared oxygen atoms. Such octahedra share vertices and edges with their neighbors. Structures in which one or more (adjacent) MO6octahedra have been ‘removed’ can be prepared by the control of solution pH, and the resulting lacunary anions are used as precursors to generate new, larger structures and derivatives. Since the addenda atoms are in their highest oxidation states (d0), many polyoxoanions can reversibly accept variable numbers of electrons to yield mixed‐valence heteropoly ‘blues’ (d0,d1) with delocalized hopping electrons and ‘browns’ (d0,d2) with metal–metal bonds. Several strategies for the synthesis of derivatized and functionalized polyoxoanions with organic and organometallic groups are reviewed since this has been a major thrust in the field in recent years. Also, a recent development has been the discovery of extremely large polyoxoanions with 100 + metal atoms that are readily prepared and are stable in aqueous solution. Examples are the mixed‐valence polymolybdate wheels (Mo154and Mo176) and tungstate(VI) assemblies (W184Ln16). Polyoxoanions have long been used in analytical chemistry, and their use as heterogeneous and homogeneous acid and oxidation catalysts continues to be developed. Other applications, especially in structural biology (imaging, crystallographic phasing) and medicine (antiviral and antitumoral agents) are also important.
Polysiloxanes & PolysilanesBased in part on the article Polysiloxanes & Polysilanes by Robert West which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Mitra Amitabha , Atwood David A.
Published: March 2006

Silicon‐containing polymers, of which polysiloxanes and polysilanes are the most important ones, are becoming more and more important industrially because of their unique properties. Polysiloxanes that contain SiOSi linkage in their structure are most commonly prepared by ring‐opening polymerization of cyclic siloxanes. The monomer for this process is prepared by the ‘Direct Process’. The polysiloxanes are hydrophobic despite having polar SiO bonds in their structures. They possess highly flexible structures with large SiOSi bond angles. They are chemically inert and have good thermal stability, low viscosity, and good gas permeability. These polymers and related copolymers find use in a variety of fields as silicone fluids, elastomers, and resins. Polysilanes consisting of chains exclusively made up of silicon atoms are most commonly prepared by Wurtz coupling reaction. through sodium condensation of dichlorosilanes. The most interesting aspect of their properties is σ‐electron delocalization, which gives rise to many interesting photochemical and photophysical effects like thermochromism, solvatochromism, and ionochromism. These polymers find use in preparation of silicon carbide and as photoresists, photoinitiators, and hole conductors in electrophotography. In this article, the history, synthesis, properties, and application of polysiloxane and polysilane polymers have been reviewed.
Rhodium: Organometallic Chemistry
Authors: Mague Joel T.
Published: March 2006

A review of the major advances in the organometallic chemistry of rhodium covering the period 1993–2003 is presented. Highlighted are complexes containing N‐donor ligands such as triazacyclononanes and tris‐(pyrazolyl)borates, those containing vinylidene and allenylidene ligands, and the formation of metallamacrocyclic complexes. Also featured are results of detailed computational studies on the activation of CH bonds and the course of olefin hydroformylation and hydrogenation processes. Classes of organorhodium compounds covered include alkyl, aryl, vinylidene, allenylidene, alkynyl, and carbene complexes. Also reviewed are alkene, alkyne, isocyanide, and arene complexes, as well as carbonyl cluster compounds together with significant coverage of dinuclear complexes stabilized by bis(diphenylphosphino)methane ligands.
Ruthenium: Inorganic & Coordination ChemistryBased in part on the article Ruthenium: Inorganic & Coordination Chemistry by Bruno Chaudret & Sylviane Sabo‐Etienne which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Sabo‐Etienne Sylviane , Grellier Mary
Published: March 2006

This article presents an overview of the inorganic and coordination chemistry of ruthenium. Since the first edition of the encyclopedia in 1994, the field has seen a huge growth. We have attempted to select the most useful data for the readers not familiar with this topic and to detail the most promising systems for future developments. Key data and recent highlights are thus presented for seven classes of compounds incorporating the following ligands: (1) Halides: RuCl3·xH2O is by far the most useful starting material in ruthenium chemistry. (2) Oxygen‐donor ligands: this area is dominated by the chemistry of oxo complexes, with many applications in catalysis. (3) Sulfur‐donor ligands: potential applications are found in medicinal chemistry as antitumor agents and as radiosensitizers. (4) Nitrogen‐donor ligands: they represent the most important class of compounds in ruthenium chemistry. Supramolecular chemistry dominates, with photo‐redox processes and a wide range of applications being investigated from solar energy conversion to bioapplications. Ammines and bipyridines (and related ligands) are key ligands in this area. A great variety of inorganic architectures have been designed to assemble molecular machines and to favor interactions with DNA. (5) Phosphorus‐donor ligands: this class of compounds is at the origin of a rich organometallic chemistry, and the resulting complexes are often used as precursors in homogeneous catalysis. The design of new ligands for the improvement of catalytic performances is an active area of research. (6) Group 14 ligands: this field is dominated by the reactivity with silanes. New bonding modes, concepts, and applications have been disclosed. (7) Hydride ligands: the most recent developments concern the chemistry of dihydrogen complexes. The formation of dihydrogen bonds should have important consequences on the selectivity and stereochemistry in many catalytic processes. Finally, the last section is devoted to polynuclear complexes, with the new developments in the field of nanoparticles.
S
‐Donor Ligands
Authors: Collinson Simon R. , Schröder Martin
Published: March 2006

The chemistry of S‐donor ligands with metal centres is reviewed with emphasis placed on the structures and redox activity of the resultant complexes. Sulfides and thiolates bind to a wide range of metal ions. The oxidation of thiolates to sulfones and sulfoxides and the structures and chemistry of metal‐thiolene complexes are detailed. Thioethers are generally poor ligands for transition metal ions but macrocylic ligands afford stable complexes. Many metalloproteins contain metal–sulfur bonds and the structures of many of these have now been elucidated. This has sparked a renaissance in the synthesis of low‐molecular weight metal complex mimics.
Scandium, Yttrium & The Lanthanides: Organometallic ChemistryBased in part on the article Scandium, Yttrium & The Lanthanides: Organometallic Chemistry by R. D. Köhn, G. Kociok‐Köhn, & H. Schumann which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Schumann Herbert , Fedushkin Igor L.
Published: March 2006

The group 3 metals scandium and yttrium and the 4f elements lanthanum to lutetium build up a big variety of organometallic compounds, in general with the metal in the oxidation state +3. Most of them are sandwich complexes Cp3Ln, Cp2LnX, and CpLnX2containing unsubstituted or substituted cyclopentadienyl ligands. In addition, cyclopentadienyl‐free organo rare earth compounds, like alkyl or aryl derivatives, alkenyl, allyl, alkynyl, hydrido, and carbene complexes, and organolanthanide compounds with indenyl, fluorenyl, cyclooctatetraenyl, or with heterocyclic ligands are described. Samarium, europium, and ytterbium form reasonable stable organic compounds in the oxidation state +2, whereas only a few rare earth organometallics with the metal in the oxidation state 0, +1, and +4 are known. All organo rare earth complexes are extremely sensitive to oxygen and water. The development of various synthetic methods as well as intensive single‐crystal X‐ray structure investigations made these highly reactive organometallic compounds available for use as organic synthons, efficient catalysts for polymerization, hydrogenation, hydroamination, and hydrosilylation as well as precursors for Metal Organic Chemical Vapor Deposition (MOCVD) and other material science applications.
Semiconductor Interfaces
Authors: Tan Ming X. , Bansal Ashish , Lauermann Iver , Shreve Gary A. , Lewis Nathan S.
Published: March 2006

This article describes the properties of semiconductors that are responsible for the operation of rectifiers, transistors, photovoltaic devices, and light emitting. The penetration of the electric field and the electric potential into a bulk semiconductor phase yields a nonuniform carrier concentration in the sample, and produces contacts that either attract or repel specific types of charge carriers. Chemical control over these electric fields is crucial to the technology of semiconductor contacts. In principle, such control can be established through a simple variation in the electrochemical potential of the contacting phase. In practice, this ideal model often does not apply, and other factors influence or completely determine the interfacial electric field strength. The application of these principles is described for semiconductor/semiconductor, semiconductor/metal, semiconductor/liquid, and semiconductor/polymer contacts, making the approach valuable for understanding the operational principles of a variety of technologically important semiconductor devices.
Silicon: Inorganic Chemistry
Authors: Lickiss Paul D.
Published: March 2006

Silicon is an element of vital importance in many areas of modern synthetic chemistry, the electronics industry and in materials science. This article attempts to summarize the many types of important inorganic derivatives of silicon but excludes the chemistry of silicates and zeolites, which are covered in separate articles. The isolation of elemental silicon from sand is described together with the various methods of purification of the element for use in the semiconductor industry. A brief description of the occurrence of silicon in biology is also given. The bulk of the article deals with compounds of silicon organized according to coordination number. One‐coordinate compounds are short‐lived intermediates and are described briefly. Two‐coordinate compounds, silylenes, have become increasingly important in recent years as are transition metal complexes in which they act as ligands. The use of bulky substituents has allowed stable silylenes to be isolated and their chemistry is described. Three‐coordinate compounds comprise radicals, cations, anions, and silanes (compounds containing a double bond to Si). These species are also generally short‐lived and reactive. The bulk of the article deals with four‐coordinate compounds and is organized according to the substituents at silicon. The industrially important hydrosilanes and halosilanes are described together with the mixed halohydrosilanes and the less common pseudohalosilanes. Alkoxysilanes are also industrially important and these are discussed with other oxygen derivatives such as esters and silanols. Compounds containing SiN bonds are also common and have interesting structural features. The preparation and structures of transition metal complexes containing silyl ligands is described. This is followed by a description of compounds containing silicon with coordination number five or six. The final section describes the analytical methods for the determination of the composition and structure of inorganic silicon compounds. Compounds containing SiC bonds are excluded throughout.
Silicon: Organosilicon Chemistry
Authors: Sakurai Hideki
Published: March 2006

An overview of the chemistry of organosilicon compounds from the first synthesis in 1863 to the latest results is presented. After an introduction, classes of compounds and nomenclature are briefly described. Synthesis of organosilicon compounds is then disucssed systematically. Characteristic points of structure and bonding of organosilicon compounds, including tetrahedral and polyhedral compounds are described. Important σ–π conjugation (sigma‐pi conjugation) and σ*–π conjugation (sigma*‐pi conjugation) in organosilicon compounds are discussed in relation to structure and reactions. Reactions of organosilicon compounds are classifed into ionic and hemolytic (nonionic) reactions and various reactive intermediates including silyl radicals, silylium ions (silyl cations), silyl anions, and silylenes are described. Recent developments in kinetic stablization of these reactive intermediates are then summarized. In relation to kinetic stablization, silicon‐containing multiple bonds, silaaromatics, and silicon polyhedranes are described. Lastly, two important applications of organosilicon compounds, namely polymers and organic synthesis, are described briefly.
Silver: Organometallic Chemistry
Authors: Fackler John P.
Published: March 2006

The organometallic chemistry of silver and copper are in many ways similar. Simple alkyls are not thermally stable but fluorination of the hydrocarbon imparts considerable stability to the silver(I) products. Except for the special anion, [Ag(CF
3
)
4
]
−
, and derivatives such as Ag(CF
3
)
3
L, L = phosphine, which are stable, the fluorocarbon products generally are light and air sensitive, but less so than the nonfluorinated alkyls. Organometallic stability is increased when the silver is bonded to alkenes, alkynes, and other unsaturated species. Alkynylsilver compounds are generally polymeric solids. Alkenes π‐bond to the silver in Ag(I) salts. This feature has led to the well‐established use of silver catalysts for olefin epoxidation.
Sol–Gel Synthesis of Solids
Authors: Coradin Thibaud , Livage Jacques
Published: March 2006

The synthesis of glasses and ceramics via the sol–gel route is based on the inorganic polymerization of molecular precursors such as metal alkoxides. Precursor structure and reactivity can be chemically controlled via complexation or acid–base catalysis. Sol‐gel chemistry allows the powderless processing of glasses and ceramics, at much lower temperatures than usual solid‐state reactions. It provides a convenient and flexible access to nanoparticles, films, and fibres. Moreover, precursors are mixed at the molecular level and multicomponent materials can be formed. Thus, hybrid organic–inorganic materials have been made via the sol–gel route. The organic component can be used as a network modifier during the sol–gel synthesis and then withdrawn, allowing the formation of structured porous materials. It can also remain entrapped in the mineral matrix via weak interactions. Alternatively, organoalkoxysilanes allow covalent binding of organic moieties to silica networks. Such composites fill the gap between polymers and glasses and open new fields in materials science such as ‘sol–gel optics’ or bioencapsulation in mineral hosts.
Solids: Computer Modeling
Authors: Catlow Richard
Published: March 2006

We review the role of computer modeling techniques in solid state chemistry. The basic aims and objectives of the field are summarised. We then describe basic aspects of the methodologies both of techniques based on interatomic potentials and electronic structure methods. We consider the range of applicability of each technique. Later sections of the chapter describe recent topical examples including modeling and prediction of crystal structures, simulation of nano‐cluster structures and energies and modeling of reaction mechanisms in heterogeneously catalysed reactions.