bluuebunny/crossref_metadata_2025 huggingface dataset:
Bonding Energetics of Organometallic Compounds
Authors: Dorta Reto , Nolan Steven P.
Published: March 2006

This contribution gives an overview of solution calorimetric studies carried out on organometallic complexes in the last decade. A special focus has been put on a close investigation of metal–ligand exchange reactions involving different ligand and metal systems. Ligand substitution reactions are at the heart of many catalytic transformations, since creation of vacant coordination sites often require liberation of one or more ligands from the coordination sphere of the metal. Detailed analyses of phosphine‐, phosphite‐, and NHC‐ (NHC = N‐heterocyclic carbene) ruthenium, rhodium, and platinum systems show that enthalpies of ligand substitution reactions are associated to electronic and/or steric variation of the ligands. They can be explained in terms of different amounts of these two contributions to the overall enthalpy. Extensive calorimetric studies on ligand substitutions showed that cis disposition of the ligands leads to an overwhelmingly steric contribution to the enthalpy of ligand substitution, whereas trans disposition of the ligands gives enthalpy values that are highly dependent on the electronic nature of the ligand systems studied. Thermodynamic studies of addition reactions of organic molecules to metal fragments are equally crucial in catalysis and gave valuable thermodynamic data for CO, H2, aldehyde, acyl chloride addition reactions to catalytically relevant rhodium systems. Thermodynamic data for different reductive cleavage reactions of organic fragments involving molybdenum systems are also presented.
Boron: Inorganic Chemistry
Authors: Schubert David M. , Brotherton Robert J.
Published: March 2006

Boron is unique among the elements and displays remarkable chemistry in all of its compounds. It is widely distributed at low concentrations on the Earth and is virtually always found bound to oxygen in its natural forms. Boron enters into the life cycle of and is an essential element for plants and animals. The vast majority of industrial uses of boron on a weight basis also involve boron–oxygen compounds, including important mineral sources, metal borates, boric acid, boric oxide, and boric acid esters. These find large‐scale applications in many industries, most notably in the manufacture of glass and other vitreous materials. Boric acid is a weak Lewis acid and does not display Brønsted acidity in aqueous solution. The condensation of B(OH)3and B(OH)4−species to form polyborates in relatively concentrated borate solutions has important implications for the solubilities and other solution properties of metal and nonmetal borate salts. Boric acid reacts with alcohols and other hydroxyl group‐bearing compounds to form esters, which find many industrial uses. Reversible borate ester crosslinking leads to polymer gel formation and provides the basis for the essential biological role of boron. Nonoxide boron compounds also have important industrial uses. These include refractory compounds, such as boron carbide, boron nitrides, boron phosphides, elemental boron, and metal–boron alloys, as well as nonrefractories, such as the boron halides. Nonrefractory boron–phosphorus and boron–sulfur compounds and the boron subhalides are yet to find significant industrial use, yet, nonetheless exhibit fascinating chemistries.
Boron: Metallacarbaboranes
Authors: Housecroft Catherine E.
Published: March 2006

Among cluster compounds of boron, the metallacarbaboranes form a well‐established family that has developed significantly over the last decade, for example, with regard to
f
‐block‐containing species and supramolecular systems. This article covers the basic principles of metallacarbaborane clusters: classification of clusters, structural, and bonding aspects. An attempt has been made to illustrate synthetic routes to metallacarbaboranes in a systematic manner, although it must be recognized that many other syntheses reported in the literature lack a strategic quality. Isomerism of the cluster skeleton is discussed with pertinent examples. Electrochemical properties of cluster and stacked metallacarbaboranes are summarized. An overview of typical patterns in reactivity is given, although, as with synthetic strategies, it is extremely difficult to generalize. Potential applications of metallacarboranes are covered in this article, but the discussion is selective in view of recent review articles.
Boron–Nitrogen CompoundsBased in part on the article Boron–Nitrogen Compounds by Robert T. Paine which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Bakus Ronald C. , Atwood David A.
Published: March 2006

The majority of this article details the many synthetic routes and compounds that can be accessed in combinations of boron reagents with amines. Lewis acid‐base adducts are the simplest type of compounds, and lead into the more complex aminoboranes and iminoboranes. Owing to their widespread importance and usage, the borazines, pyrazaboles, and polypyrazaolyborates are covered in separate sections. The newly emerging fields of boron–nitrogen cations and boron nitride solid‐state materials are covered in detail.
Cadmium: Inorganic & Coordination Chemistry
Authors: Borsari Marco
Published: March 2006

This article provides an overview on the chemistry of cadmium. Cadmium is a naturally occurring metal elements, being the 67th most abundant element in the earth's crust, and it is found in several minerals. Occurrence, production and applications have been surveyed, also in the light of toxicological and environmental risks joined to uses of cadmium. The usual oxidation state of cadmium is 2+ and its chemistry closely resembles that of zinc. The versatile coordination ability allows a wide variety of structure. Therefore, emphasis is placed on stereochemical aspects of cadmium chemistry, in particular on 1D, 2D, and 3D architectures resulting from assembly of Cd
2+
–ligand(s) units. The main compounds and complexes, ordered by metal oxidation state and ligand donor atom, have been examined in their properties and structural aspects, both in solution and in solid state.
Cadmium: Organometallic ChemistryBased in part on the article Cadmium: Organometallic Chemistry by Gerard Parkin which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Gupta Niladri Narayan , Atwood David A.
Published: March 2006

As a large, soft, heavy element, the chemistry of cadmium is marked by relatively weak bonding to carbon and other more electronegative atoms. Organocadmium compounds thus demonstrate diverse reaction chemistry. Furthermore, the large size and low valency of cadmium leads to interesting aggregated compounds. The reactivity and structural importance of organocadmium compounds and applications in solid‐state chemistry is offset by their high toxicity.
Carbides: Transition Metal Solid‐State ChemistryBased in part on the article Carbides: 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 use of transition metal carbides and carbonitrides are presented in this article. Starting from crystal chemistry, bonding, and preparation, the article also briefly discusses analytical chemistry of chemical and physical carbon analysis as well as thermodynamics. Updated versions of phase diagrams of group 4, 5, and 6 transition metals with carbon as well as phase diagrams of FeC and MnC are presented together with those of the most important transition metal carbonitride systems TiCN and ZrCN. A large part is devoted to the solid‐state properties because they make carbides and carbonitrides interesting materials for application. The presentation of these properties includes hardness, thermal properties, conductivities, carbon diffusivity, and elastic properties. Some ternary and quaternary carbonitrides are also discussed. Several of these properties are given as a function of temperature and composition. Examples for application of carbides in modern hardmetal types such as recent development in fine‐grained hardmetals and functional gradient hardmetals are included. The topic is closely connected to the article Nitrides: Transition Metal Solid State Chemistry.
Carbonyl Complexes of the Transition Metals
Authors: Calderazzo Fausto
Published: March 2006

Carbon monoxide is a versatile ligand as it forms compounds with both transition metals and main group elements. Transition d metals of electronic configuration dn(0 < n < 10) form the most numerous class of these compounds, although recent progress has been made in expanding the area of existence toward main group, lanthanide (4f) and actinide (5f) metals. The most extensively studied series of carbonyl derivatives have the general formula [M(CO)n]m, withnrepresenting the coordination number of the central metal atom M, andmbeing the nuclearity of the system. In these compounds, the central metal atom is in the zero oxidation state. Starting from readily available inorganic derivatives in a positive oxidation state, the synthesis of these compounds normally requires a reducing agent capable of acting in the presence of carbon monoxide: in fact, only nickel metal, as a finely divided activated powder, is reactive toward CO leading to the corresponding derivative Ni(CO)4. The selection of the reducing agent varies as a function of the position of the metal M within the periodic table. There is a general trend of an increase in the nuclearitymtoward the end of the transition d series, the nuclearity being related to the bonding arrangements of the CO ligand, which may act as terminal, doubly bridging or triply bridging. Metal carbonyls are characterized by IR‐active vibrations around 2000 cm−1associated with the carbonyl groups. Vibrational spectra in solution give important information about: (a) the electronic distribution within the M–CO bond in terminally bonded carbonyl groups; (b) the molecular structure associated with the number of bands observed; (c) the type of bonding in dinuclear or polynuclear compounds. X‐ray diffraction data have been extensively used in order to define the molecular structure in the solid state. The metal–CO combinations span a wide range of oxidation states, derivatives being known for oxidation states from +III to −IV. Metal carbonyls undergo three types of reaction: (a) substitution of the carbonyl groups, whereby the oxidation state of the central metal atom remains unchanged, (b) oxidation, and (c) reduction. Carbonyl derivatives of early‐ and late transition metals present specific bonding and spectroscopic properties. Concerning the transition elements of groups 6–9, the M–CO bond dissociation enthalpy increases down the group, and is normally greater than the metal–metal bond enthalpy, the latter being estimated from the heat of atomization. From a mechanistic viewpoint, substitution and redox reactions have been studied extensively. A special case of substitution is the migratory carbonyl insertion, by which an M–R bond to a carbyl group R is converted by CO into the M–(CO)R sequence.
Cation‐Activated Enzymes
Authors: Glusker Jenny P.
Published: March 2006

Cation‐activated enzymes are those that require a cation for maximal activity but which still have some activity in the absence of the cation, that is, the activating cation is not an essential part of the enzyme mechanism. The cations involved are generally metal or ammonium ions. The metal ions used are generally Na+, K+, Mg2+, or Ca2+, those that are abundant in nature. In the last 10 years, many crystal structures of cation‐activated enzymes have been determined and, in these studies, potassium is the most common activating cation. The geometry of binding and possible conclusions about this activity are the subject of this review. Chosen for description are thrombin (a sodium‐activated enzyme), pyruvate kinase, dialkylglycine decarboxylase, diol dehydrase, inosine monophosphate dehydrogenase, and tryptophanase (all potassium‐activated enzymes), and alkaline phosphatase (a magnesium‐activated enzyme). In some enzymes, the activating cation is near the active site, but in others it is distant. This implies that its function in these two possibilities involves the modification of the charge distribution in the reaction center or else stabilization of the enzyme conformation most suited to the reaction to be catalyzed. These studies provide information on one way in which nature controls enzyme activity, and hopefully further studies will help us learn more possibilities for utilizing these enzyme‐control strategies in medicine.
Chromium: Biological Relevance
Authors: Vincent John B.
Published: March 2006

The biochemistry of CrIIIhas been a poorly understood field of endeavor; studies of the biochemistry of none of the other transition metals have been as problematic. Despite four decades of investigation, only recently has a clear picture of the role of Cr been defined potentially. The biologically important form is the trivalent ion, although not in the form of the purported glucose tolerance factor. Apparently, CrIIIis required for proper carbohydrate and lipid metabolism in mammals, although Cr deficiency is difficult to achieve. Conditions that increase circulating glucose and insulin concentrations increase urinary chromium output. Cr is excreted after an insulin challenge, probably in the form of the oligopeptide chromodulin. Chromodulin may be the key to understanding the role of Cr at a molecular level as the molecule has been found to bind to the activated insulin receptor, stimulating its kinase activity. A mechanism for the action of chromodulin has been proposed recently. The popular nutritional supplement chromium picolinate has been associated with potentially deleterious effects. The mechanism of toxicity of chromate (CrVI) is still an area of intense investigation.
Chromium: Inorganic & Coordination Chemistry
Authors: Theopold Klaus H.
Published: March 2006

The element chromium (Cr, atomic number 24) belongs to the first row of the transition metals, the properties of which are marked by partial occupation of their 3d orbitals. Together with molybdenum (Mo), tungsten (W), and unstable seaborgium (Sg), it makes up group 6 of the periodic table. Chromium is relatively abundant, occurring naturally in various oxide minerals (notably chromite, FeCr
2
O
4
). Because of its resistance to reaction with air and water, thin coatings of chromium are used to protect other metals against corrosion. Chromium features a great variety of accessible oxidation states; among these the +VI, +III, and +II states are of prime importance. Octahedral coordination compounds of Cr(III) are very stable and numerous; they have played a prominent role in the development of coordination chemistry. Hexavalent chromium is a very strong oxidant. It has traditionally been used in organic synthesis, but the inevitable metal wastes have discouraged its use as a stoichiometric oxidant. Cr(VI) is a known carcinogen, but there are no indications of adverse health effects of Cr(III); the latter is indeed an essential trace mineral implicated in glucose metabolism. Heterogeneous chromium catalysts play an important role in the catalysis of ethylene polymerization.
Chromium: Organometallic Chemistry
Authors: Theopold Klaus H. , Kucharczyk Robin R.
Published: March 2006

Starting with Hein's investigation of the reaction of CrCl3with PhMgBr in the beginning of the twentieth century, chromium organometallic chemistry is now marked by an ever increasing diversity of compound types and formal oxidation states. Organochromium compounds such as Cr(CO)6, (η6‐C6C6)2Cr, and (OC)5CrC(OMe)Ph represent milestones in the development of organometallic chemistry as a whole. The lower oxidation states (−IV to +II) are dominated by chromium carbonyl complexes with 18 electron configurations, while midvalent compounds (+II, +III) often feature cyclopentadienyl rings and exhibit unpaired electrons. Organometallic compounds of chromium in its highest oxidation states (+IV to +VI) are still relatively rare, but this has been a growing area of late. Much of the interest in chromium alkyls is due to the metal's important role in the catalysis of olefin polymerization. Homogeneous model systems for the Union Carbide catalyst (Cp2Cr/SiO2) and the Phillips catalyst (CrO3/SiO2) still motivate much synthetic work. Applications of organochromium chemistry in organic synthesis are also becoming more numerous and useful.
Cobalt:
B
12
Enzymes & Coenzymes
Authors: Kräutler Bernhard
Published: March 2006

The most prominent biological role associated with cobalt concerns vitamin B
12
, its natural variants, and its coenzyme forms. The B
12
‐derivatives are cobalt complexes of the unique and remarkably complex corrin ligand that belong to the larger class of the tetrapyrrolic natural compounds. The ‘antipernicious anemia factor’ vitamin B
12
and its cofactor forms are, first of all, required for human and animal health. However, except for the higher plants, all forms of life appear to depend upon the natural variants of vitamin B
12
. The main chemical reactivities of the corrinoids (needed to fulfill their essential biological roles) may reflect their possible corresponding chemical contributions to the development of the early stages of life on earth. Indeed, the corrinoids play a particularly basic role in carbon metabolism in microorganisms, which also are the only natural sources of the B
12
‐derivatives. Microorganisms have developed extraordinary means for the biosynthesis, and for the regulation of the biosynthesis of the B
12
‐derivatives, in order to cope with the structural and functional complexity of these tetrapyrrolic cobalt‐compounds, while most (other) forms of life have evolved intricate strategies for the economic and efficient uptake and transport of the corrinoids. The known cofactor functions of the cobalt‐corrinoids are due to their extraordinary organometallic chemistry and their redox chemistry under physiological conditions: Coenzyme B
12
, methylcorrinoids, and reduced corrinoids are the cofactors in various important and unique organometallic enzymes, whose intricate structural and mechanistic properties are the current focus of strong and broad research activities. The essential role of the corrinoids in the remarkable organometallic paths of carbon fixation and further carbon metabolism in anaerobic microorganisms has attracted particular attention. Vitamin B
12
and its derivatives thus hold an important position in all of the life sciences and are at the center of particular interdisciplinary interest from such diverse areas as medicine, biology, chemistry, and physics.
Cobalt: Inorganic & Coordination ChemistryBased in part on the article Cobalt: Inorganic & Coordination Chemistry by David A. Buckingham which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Blackman Allan G.
Published: March 2006

This article provides a summary of the important and interesting aspects of the inorganic and coordination chemistry of cobalt. A brief history of the element, detailing its discovery and historical uses, is given, and some general information concerning the properties of elemental cobalt is detailed. The general chemistry of cobalt is then outlined, including an overview of the common oxidation states found in inorganic compounds and complexes, a discussion of the stereochemistry, spectra and magnetism of cobalt complexes, and an outline of some analytical methods used for cobalt. The inorganic chemistry of simple cobalt salts is then described, and the coordination chemistry of cobalt, which constitutes the majority of the article, is then outlined in some detail. Coordination complexes of cobalt are detailed according to the oxidation state of the metal ion, with complexes ranging from Co(−I) to Co(V) discussed. Brief descriptions of the chemistry of each oxidation state are given, and specific examples are then outlined according to the nature of the ligand donor atom(s), with extensive references to both the primary literature and to review articles given. Thus, Co(I), Co(0), and Co(−I) complexes containing aliphatic, aromatic, and macrocyclic amine ligands, as well as Schiff‐base, porphyrin, and P‐donor ligands are discussed, while Co(II) complexes of carboxylate, heterocyclic, and oxygen‐derived ligands are summarized. Co(III) ammine and amine complexes are described in detail, while Co(III) complexes containing O‐donor ligands are also discussed. The few known Co(IV) and Co(V) complexes are outlined.
Coordination & Organometallic Chemistry: Principles
Authors: Crabtree Robert H.
Published: March 2006

The title topic is reviewed. Under coordination chemistry, special attention is given to Werner complexes, the trans effect, soft and hard ligands, crystal and ligand field (LF) models, together with oxidation state and coordination number. Under organometallic chemistry, the main topics include electroneutrality, back bonding, the 18‐electron rule, and effects of complexation.
Copper: Hemocyanin/Tyrosinase ModelsBased in part on the article Copper: Hemocyanin/Tyrosinase Models by Nobumasa Kitajima which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Shearer Jason , Karlin Kenneth D.
Published: March 2006

Coupled dinuclear copper(II) centers are found in metalloenzymes such as hemocyanin (Hc), tyrosinase (Tyr), and catechol oxidase (Co), and perform functions such as dioxygen transport,
o
‐phenol aromatic hydroxylations, and dehydrogenation of
o
‐catechols. These enzymes all contain two‐copper centers, each ligated by imidazole nitrogen ligands from histidine residues, and in their oxyforms are bridged by a μ‐η
2
:η
2
‐peroxo moiety. Synthetic model compounds have proven useful in elucidating a number of different aspects of the protein chemistry, such as spectroscopic and structural properties. Also, new coordination chemistry has been developed, such as a new and possibly important equilibrium form of the μ‐η
2
:η
2
‐peroxo‐

$\hbox{Cu}^{{\rm II}}_{2}$

species, namely, the

$\hbox{Cu}^{{\rm III}}_{2}$

‐bis‐μ‐oxo moiety. In conjunction with stopped‐flow kinetic studies, new fundamental reactions involving Cu
I
complexes and dioxygen have also been elucidated, such as the demonstration that an initial Cu
II
‐superoxo species is formed prior to

$\hbox{Cu}^{{\rm II}}_{2}$

‐peroxo formation. Both species are enthalpically favored at low temperatures, but an unfavorable entropy term precludes the isolation of many of these complexes near room temperature. Considerable efforts are directed towards elucidation of detailed mechanistic pictures for substrate oxidations with copper–dioxygen adducts.
Ortho
‐phenol hydroxylations proceed
via
an electrophilic aromatic substitution reaction. The mechanism of Co activity is still widely debated with evidence in model systems pointing towards both proton‐coupled electron transfer pathways, as well as direct hydrogen atom transfer reaction. Such mechanistic inquiries, and determining the differential reactivity of μ‐η
2
:η
2
‐peroxo‐

$\hbox{Cu}^{{\rm II}}_{2}$

and

$\hbox{Cu}^{{\rm III}}_{2}$

‐bis‐μ‐oxo complexes, are of continuing and future interest.
Copper: Organometallic ChemistryBased in part on the article Copper: Organometallic Chemistry by Bruce H. Lipshutz & Michael R. Wood which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Lipshutz Bruce H. , Pfeiffer Steven S.
Published: March 2006

Much of copper chemistry over the past decade has transitioned toward catalysis, driven in large measure by environmental rather than economic concerns. Catalytic processes have appeared in large numbers, in particular due to the advances made in both the design of new ligands that effectively coordinate copper, but also due to their increased availability. Applications of copper‐catalyzed processes to bond formations have gone well‐beyond those of a CC nature, nowadays including CN, CO, CS, CP, and CH types. Asymmetric variants based on nonracemically ligated copper salts and copper hydride have led to highly valued entries to chiral products of great use in synthesis. Transmetalations from various metals (e.g. Zn, Zr, etc.) to copper also continue to play a vital role in organic synthesis, and are discussed herein as well.

Considerable progress on many physical organic fronts relating to copper chemistry has been made. New advances in technologies that shed light on structural aspects of copper reagents are now available (extended X‐ray absorption fine structure, EXAFS, X‐ray absorption near edge structure, XANES) and are being used to accompany X‐ray analyses and highly sophisticated NMR techniques. High‐level computer calculations have led to theoretical ‘pictures’ illustrating the mechanistic pathways that may be followed by some of the most commonly used reactions in copper chemistry (e.g. 1,4‐additions). Taken in their entirety, these developments suggest that this past decade has been a most productive period in the continuing evolution of copper chemistry.
Copper Proteins with Dinuclear Active SitesBased in part on the article Copper Proteins with Dinuclear Active Sites by Konrad Lerch which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Decker Heinz
Published: March 2006

Copper proteins with dinuclear active sites comprise proteins with different structures and functions. The phenoloxidase, tyrosinase, and catecholoxidase are responsible for browning by starting the synthesis of melanin. These enzymes are involved in the primary immune response in invertebrates, plants, fungi as well as in the sclerotization of arthropods. The respiratory proteins hemocyanins are responsible for oxygen transport in some arthropods and molluscs. However, they can be converted to enzymes exhibiting phenoloxidase activity. Based on X‐ray structures of hemocyanins and a catecholoxidase, large parts of folding motifs are very similar although the sequence identities are far below 30%. A model for the activation process of phenoloxidase activity is presented.
Cyanide Complexes of the Transition MetalsBased in part on the article Cyanide Complexes of the Transition Metals by Timothy P. Hanusa & David J. Burkey which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Hanusa Timothy P.
Published: March 2006

Cyano metal complexes contain one or more bound cyanide ligands, CN
−
, and constitute one of the largest and longest‐known classes of compounds in inorganic chemistry. Like carbon monoxide, the cyanide ion can function as a π‐acid ligand, but because of its negative charge, the cyanide ion can also form strong sigma bonds. This bifunctional behavior allows CN
−
to stabilize both high and low oxidation states of metals. The cyanide ion can bind to metals in both terminal and bridging (MCNM′) modes; the bridges are commonly linear, and are present in Prussian blue and many other polymeric metal cyanides. Framework solids with high degrees of void space can be formed with bridging cyanide ligands. Cyano metal complexes undergo a variety of oxidation‐reduction reactions, but owing to the strong σ‐donor properties of CN
−
, only a few ligands are capable of directly replacing metal‐bound cyanide. For most substitution reactions, photochemically induced dissociation of one or more of the CN
−
ligands is needed. Cyano complexes are used as pigments and dyes, and are being increasingly explored as electroactive, zeolitic, and chemical sensing materials.
Cytochrome OxidaseBased in part on the article Cytochrome Oxidase by Gerald T. Babcock which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Wikström Mårten
Published: March 2006

Heme‐copper oxidase is the current generic name for the superfamily of respiratory enzymes that are responsible for cell respiration in the mitochondria of eukaryotic cells as well as in aerobic prokaryotes, and which shows substantial structural and functional homology among its members. This group of enzymes is responsible for more than 90% of the consumption of dioxygen by living organisms on this planet. Harnessing the high oxidizing power of the O2/H2O redox couple some 2.5–3 billion years ago made a revolutionary impact on evolution. The oxidation of foodstuffs by O2gave an approx. 15‐fold energetic advantage over previously predominant glycolytic oxidations, and was the prerequisite for the development and evolution of all higher multicellular organisms. Apart from mastering the O2reduction chemistry in a safe and economical fashion, these enzymes also function as remarkable energy transducers that convert much of the free energy released in the process into a proton gradient, which is subsequently used to drive virtually all energy‐requiring processes in the cell. This article reviews the major advances in the last 10 years of research on this group of enzymes. The major highlights are the elucidation of the crystal structures of several oxidases, major advances in the understanding of the mechanism of O2activation and reduction at the bimetallic heme iron‐copper active site, and recent progress in the ongoing research on the perhaps most intriguing but least known function, redox‐coupled proton translocation.
Defects in Solids
Authors: Tilley Richard J. D.
Published: March 2006

The chemical aspects of defects of relevance to inorganic crystalline solids are described, especially the close links with the composition of crystalline nonmetallic inorganic nonstoichiometric compounds. The discussion begins with a description of simple point defects in stoichiometric materials and the relationship between the population of these defects and the Gibbs energy of formation of the defects. This includes defects in monatomic structures and balanced populations of point defects, Frenkel and Schottky defects, in binary compounds. Superionic compounds that contain quasi‐liquid cation arrays are considered as extreme cases of these simpler defect populations. Nonstoichiometric compounds treated from the standpoint of small populations of point defect populations are considered, including color centers. Defect chemistry, the incorporation of point defects into chemical nomenclature, is described, including the Kroger–Vink notation and the construction of Brouwer diagrams. Grossly nonstoichiometric compounds, including those that can be considered to contain point defects or structurally organised defect clusters, such as the iron oxide wüstite, and the defect fluorite structures are illustrated. Extended defects portray a wider view of defect chemistry, in which planar faults and intergrowths are prominent structural features of many complex inorganic solids. To illustrate planar defects, examples are taken from crystallographic shear phases, twinned structures, intergrowth structures, and the niobium oxide block structures. Structures containing pentagonal columns linked by vertices provide an example of structures that contain columnar defects. Examples of inorganic solids that possess more than one defect type are taken from layered perovskite phases, tungsten bronzes, and high‐temperature ceramic superconductors. The final section is concerned with structures that were once described as containing defects but that have recently been found to be essentially defect free. These materials, in which defects are eliminated by the device of continuous structural modulation affecting one part of the structure, are widespread over many classes of inorganic solids. Examples drawn from oxides related to tantalum pentoxide, layer misfit phases, and anion‐excess fluorite‐related materials.
Electrochemistry: Applications in Inorganic ChemistryBased in part on the article Electrochemistry: Applications in Inorganic Chemistry by Robert H. Crabtree which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Holden Thorp H.
Published: March 2006

Because of the redox versatility of many transition metal complexes, electrochemical methods are an important part of the inorganic chemist's experimental repertoire. This article centers mostly on the use of cyclic voltammetry, where current is measured as the potential is varied linearly with time. Cyclic voltammetry is the primary method used by inorganic chemists, but many pulsed voltammetric provide complementary, and often useful, information. Theoretical methods now offer the means to calculate redox potentials of metal complexes, which can be correlated with values determined by cyclic voltammetry. Digital simulation techniques also now provide the ability to determine mechanistic information from voltammograms by directly calculating expected responses based on input mechanisms. Frontier applications of electrochemistry in inorganic chemistry involve the application of redox cycling to molecular memory.
Electron Transfer in Coordination Compounds
Authors: Endicott John F.
Published: March 2006

The transfer of an electron between a transition metal donor and a transition metal acceptor can be promoted by the absorption of either thermal or light energy. The thermal processes have been experimentally determined to span a range of about 10
20
in their ambient rates of electron transfer. The same molecular properties that determine the electron‐transfer reactivity also determine the correlated electron‐transfer absorption or emission band energy, band shape, and absorptivity. The relationships between the thermal‐kinetic and the spectroscopic observations are relatively simple when there is very little electron delocalization between the donor and acceptor, the weak‐coupling limit. This limit applies to most bimolecular electron‐transfer reactions of transition metals and to the spectroscopy of transition metal ion pairs; this is the limit treated well by Marcus theory. When the donor and acceptor are covalently linked, interactions of the donor and acceptor with the linker often increase the coupling in such a way as to enhance the reactivity and to alter the spectroscopy. Most of these linker‐induced changes of reactivity and spectroscopy can be treated in terms of perturbation theory‐based alterations of the behavior in the weak‐coupling limit. Reactivity in some very strongly coupled complexes seems to fall outside the standard theoretical framework; many such systems seem to implicate nuclear motions of the linker in facilitating the donor–acceptor electronic coupling. Extensions to biological, multicentered, and heterogeneous systems containing transition metal complexes are briefly considered.
Electronic Structure of Organometallic Compounds
Authors: Albright Thomas A.
Published: March 2006

A brief treatment of the concept of orbital interaction is given, particularly in reference to the transition metal‐ligand bond. This is then applied to the σ splitting pattern for the ML

n

series where
n
 = 6, 5, 4 (square planar and tetrahedral), 3, and 2 cases. Here M is a transition metal and L is a generic ligand with a σ donor function. A description is given for how the splitting pattern is modified when one or more of the ligands have π acceptor or donor functions. Several examples illustrate how symmetry can sometimes create modification to the normal electron counting rules. A detailed discussion is given of how to create the important valence orbitals of an ML

n

fragment and several examples are given of their use to build up the orbitals of large organometallic molecules. Finally, a detailed discussion is given of the construction and use of the isolobal analogy.
Electronic Structure of Solids
Authors: Canadell Enric
Published: March 2006

The basic language needed to understand the electronic structure of solids is introduced. The approach is chemically oriented and the notion of orbital interaction, so useful to understand the electronic structure of molecules, is used as much as possible. A step‐by‐step approach to the electronic structure of several solids is discussed. Notions like Density of states (DOS), Fermi surface (and nesting), Peierls distortions, charge and spin density waves (CDW/SDW), and COOP (crystal orbital overlap population) curves are presented and analyzed with reference to selected examples. The role of electron repulsions is also considered. The relationship between the electronic structure and the structural and transport properties of solids is discussed. A brief account of the more commonly used computational approaches for solids is also presented.
Fluorine: Inorganic ChemistryBased in part on the article Fluorine: Inorganic Chemistry by Darryl D. DesMarteau, Charles W. Bauknight, Jr., & Todd E. Mlsna which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Atwood David A.
Published: March 2006

This article provides an overview of the extensive and diverse inorganic chemistry of fluorine. Key topics are covered, such as the methods used for the formation of element–fluorine bonds with hydrogen fluoride and elemental fluorine. The article includes a general description of the structure and bonding of common fluorine compounds and the various spectroscopic techniques for their characterization. More specific details are provided for the fluoride compounds of boron, and the groups 14–17 elements. New topics such as the means by which carbon–fluorine bonds may be broken and the fluorination of carbon‐based materials such as fullerenes and nanotubes are included in the appropriate sections.
Gallium: Inorganic ChemistryBased in part on the article Gallium: Inorganic Chemistry by Andrew R. Barron which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Banger Kulbinder Kumar , Hepp Aloysius F.
Published: March 2006

An overview of the inorganic chemistry of gallium and its compounds is reported in detail with an attempt to bring up to date current studies in this field. In addition, the use of various Ga related alloys as either binary or ternary semiconducting material for bulk, thin film, and nanotechnology is reported in detail pertaining to their synthesis, properties, and application. The chemistry of Ga inorganic and coordination derivatives is also reviewed and discussed.
Gallium: Organometallic ChemistryBased in part on the article Gallium: Organometallic Chemistry by Andrew R. Barron which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Robinson Gregory H.
Published: March 2006

The organometallic chemistry of gallium has experienced substantial growth over the past decade. Much of this work concerned the development of the organometallic chemistry of compounds containing GaGa bonds. This would include simple dimeric compounds, gallium ring compounds, and small gallium clusters. This chapter will discuss the synthesis and structural aspects of these compounds.
Germanium: Organometallic Chemistry
Authors: Banaszak Holl Mark , Peck David R.
Published: March 2006

This article provides an overview of the organometallic chemistry from 1994 to 2004. Major advances in the formation and breaking of GeC bonds are discussed, as are advances in the physical characterization of these compounds. Recent advances in the formation of organogermanium species containing multiple bonds to germanium and the formation of germanium‐centered radicals and cations are delineated. Applications to the synthesis of germanium‐containing polymers are overviewed.
Halides: Solid‐State ChemistryBased in part on the article Halides: Solid State Chemistry by Gordon J. Miller which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Köhler J.
Published: March 2006

This review on binary metal halides demonstrates their enormous richness of structural topologies as well as compositions. It emphasizes thermodynamic stabilities, influences of size and electronegativity effects, the intricate balance of ionic and covalent metal–halide and metal–metal interactions, and effects of excess valence electrons of atoms with nonclosed shells, in order to outline relationships of structure and bonding. The most important structure types of the main group metal halides are given and pressure effects are discussed. Selected examples show the influence of the stereochemical activity of lone pairs of electrons in reduced halides. Halides of the transition elements are discussed according to their empirical formulas in order to correlate structure and properties with d electron concentration at the metal atoms, which are based on typical Jahn‐Teller distortions or the formation of metal–metal bonded units. Finally, a summary of the rare earth element halides is given.
Hydride Complexes of the Transition Metals
Authors: Crabtree Robert H.
Published: March 2006

The topic Hydride Complexes of the Transition Metals is reviewed with emphasis on structure and bonding, spectroscopy, energetics, synthesis, and reactions. Terminal, bridging, and cluster hydrides are covered, together with dihydrogen complexes and nonclassical hydrides.
Hydrides: Solid State Transition Metal Complexes
Authors: Yvon Klaus , Renaudin Guillaume
Published: March 2006

At present, some 50 different homoleptic transition (T) metal hydride complexes are known to occur in over 127 compounds that cover 47 structure types. The complexes form with T elements from group seven (Mn) to closed d‐shell elements of group twelve (Zn). While the great majority are centred by one T‐metal atom and display up to nine terminal hydrogen ligands, some are centred by two (Ru, Pt), four (Rh, Ni), or more (Ru, Rh) and display both terminal and bridging hydrogen ligands, and/or TT metal bonds. The structural and electronic configurations are consistent with those usually found in coordination compounds. The complexes are stabilized by charge transfer from the surrounding cation matrix, are often 18‐electron, and less often 16‐ or 14‐electron. Some hydride structures show evidence for metal–metal interactions as in typically interstitial metal hydrides while others contain additional hydride anions (H−) bonded to electropositive metals only, such as in saline hydrides. No HH bond formation is observed. Most hydrides are nonmetallic and many are colored. Some, however, show hydrogenation induced complex formation and metal‐to‐insulator transitions. The systems generally display very high volume efficiencies for hydrogen storage but are thermally too stable and/or too expensive for practical applications. Thermodynamic data suggest that their enthalpy of formation scales with the thermal stability of the binary hydrides of the electropositive metal constituents.
Hydrocyanation by Homogeneous Catalysis
Authors: Casalnuovo Albert L. , McKinney Ronald J. , Tolman Chadwick A.
Published: March 2006

The homogeneous catalysis of hydrocyanation is discussed with particular reference to the hydrocyanation of butadiene to adiponitrile, as employed in the commercial synthesis of nylon. Nickel complexes with bulky phosphite ligands are most effective and the mechanism is discussed. Asymmetric versions of the reaction are also discussed.
Hydrodesulfurization & Hydrodenitrogenation
Authors: Angelici Robert J.
Published: March 2006

Hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) are large‐scale commercial processes that occur when petroleum feedstocks are treated with hydrogen (H
2
) at moderately high pressures and temperatures. During the reactions, the organosulfur and organonitrogen molecules liberate H
2
S and NH
3
. The process results in the production of hydrocarbon fuels with low levels of sulfur and nitrogen. Such fuels are necessary in order to prevent pollution of the atmosphere by sulfur and nitrogen oxides. The HDS and HDN processes are catalyzed by heterogeneous catalysts consisting of molybdenum and cobalt sulfides supported on alumina. Because it is experimentally impossible to determine details of the HDS and HDN processes as they occur on catalyst surfaces, detailed transformations that may occur on the catalyst are modeled by reactions that occur in transition metal complexes that can be fully characterized. As models for the adsorption of thiophenes, benzothiophenes, indoles, and quinolines on metal sites of the catalyst surface, these organosulfur and organonitrogen compounds are coordinated to metals in a variety of complexes. This coordination occurs through the sulfur or nitrogen atom or through π‐bonds in the hydrocarbon portion of the molecules and depends on the electronic state of the metal center. Once coordinated to a metal center, the organosulfur or organonitrogen compound is activated in ways that lead to the cleavage of CS and CN bonds, which are also reactions that occur in the heterogeneous HDS and HDN reactions. Patterns of reactivity of organosulfur and organonitrogen compounds in their metal complexes are used to propose detailed mechanisms for the steps that lead to the formation of H
2
S and NH
3
in the catalytic processes. The validity of these mechanisms has been tested in a range of isotopic labeling, spectroscopic, and reactivity investigations on the heterogeneous catalysts. Not only have the studies of organosulfur and organonitrogen complexes provided a basis for understanding and potentially improving the HDS and HDN processes, they have led to new approaches to the removal of sulfur and nitrogen and to new transformations of organosulfur and organonitrogen compounds.
Hydrogen: Inorganic Chemistry
Authors: Kakiuchi Masahisa
Published: March 2006

The properties of behavior of hydrogen and its isotopes, and the various applications of hydrogen are presented and discussed. The natural occurrence, preparation, use, and analysis of hydrogen are described. The atomic properties of protium (H), deuterium (D), and tritium (T) and the major physical properties of isotopic molecular hydrogen and isotopic water molecules are compared, and the distribution of deuterium and tritium in nature is geochemically discussed. Isotope effects concerning the vapor pressure and the fractionation factor of hydrogen and water molecules in equilibrium are explained, and are expanded in aqueous solutions. The chemical properties of atomic and ionized hydrogen, the reactions of molecular hydrogen, and the absorption and adsorption of hydrogen are compiled. The topics of the hydrogen spectroscopy, the ortho‐ and para‐hydrogen, the hydrogen in semiconductor, and the hydrogen energy are also compiled.
Hydrosilation CatalysisBased in part on the article Hydrosilation Catalysis by John F. Harrod which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Marschner Christoph
Published: March 2006

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

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

The hydrosilation reaction can promote the transformation of a prochiral (alkene, ketone) to a potentially chiral molecule. It bears potential for asymmetric synthesis, which is currently heavily investigated. The reaction is equally popular for classical organic synthesis as well as for material science tasks such as the chemical bonding of a polymer to a surface.