bluuebunny/crossref_metadata_2025 huggingface dataset:
Oxide Catalysts in Solid‐State ChemistryBased in part on the article Oxide Catalysts in Solid State Chemistry by Toshio Okuhara & Makoto Misono which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Okuhara Toshio , Kamiya Yuichi
Published: December 2011

This chapter deals with catalysis of metal oxides containing single elements and more than two metallic elements. The metal oxides catalyze typically selective oxidation and acid‐base reactions. The first section outlines factors controlling catalyst performance (activity, selectivity, and durability), which are categorized into geometric (crystal plane and surface defects) and material‐intrinsic factors. Catalysis of the metal oxides is illustrated in detail by examples such as BiMoO, MoVTeNbO, VPO, Perovskite‐type oxide, and heteropoly compounds. Catalyst performances of these metal oxides are discussed from the viewpoint of geometric aspects. For the heteropoly compounds, their synthetic methods, chemical properties, and porosities are also described. Solid acid and base catalysts are finally summarized.
Peptide–Metal InteractionsBased in part on the article Peptide–Metal Interactions by Justin K. Moran & Claude F. Meares which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Meares Claude F.
Published: December 2011

Copper–peptide complexes have been the most extensively studied of all the metal–peptide complexes. Copper(II) is the most important oxidation state in copper–peptide complexes, although copper(III) can be stabilized by coordination to peptides. The deprotonated peptide nitrogens help stabilize the higher oxidation state of copper; therefore the more deprotonated peptide nitrogens, the more stable the copper(III) complex. The copper(II) complexes of GlyGlyProGly and (GlyGlyProGly)2 are very similar. At low pH, both peptides bond through the amino nitrogen and carbonyl oxygen of the first peptide linkage. The peptides GlyGlyProLys and (GlyGlyProLys)2 initially form 1N complexes with copper(II). With luteinizing hormone‐releasing hormone the copper(II) is coordinated via the imidazole nitrogen and two deprotonated peptide nitrogens. With angiotensin II, the copper(II) is bonded via the imidazole nitrogen of the histidine residue and two deprotonated peptide nitrogens. The dipeptide TyrPro bonded copper(II) through the amino nitrogen and the peptide carbonyl. Since there was no ionizable hydrogen on the peptide nitrogen, there was no bonding by the peptide nitrogen. The copper(II)AVP (vasopressin analog) complex is one of the most stable 4N copper–peptide complexes known. Owing to their similarities, nickel(II)–peptide complexes are usually studied along with copper(II) complexes. In neutral to acidic solutions, the nickel is coordinated through the amino nitrogen and the carbonyl oxygen (1N). When the pH is raised to 9.0, nickel(II) substitutes for a peptide hydrogen, and the coordination changes. In NiH1L complexes of cysteine‐containing dipeptides, the nickel is coordinated via the amino and deprotonated peptide nitrogens and the thiol group. Zinc‐peptide complexes are less stable. With the dipeptides Ala–His and Gly–His at high pH, zinc(II) was shown to form complexes coordinated through the amino nitrogen, deprotonated peptide nitrogen, and the imidazole nitrogen. Unlike copper(II), both palladium(II) and platinum(II) are soft acceptors and as such are expected to coordinate to soft donors like sulfur and aromatic nitrogen donors in residues such as Met and His, and deprotonated peptide nitrogens. Early investigations of metals showed that divalent metals such as copper(II), cobalt(II), and nickel(II) could catalyze the hydrolysis of amino acid esters. Mechanisms of cobalt(III)‐mediated peptide‐bond cleavage have been investigated; because cobalt(III) complexes bind to the N‐terminal amino acid residue of peptides, only the N‐terminal peptide bond is hydrolyzed. Complexes containing iron can cleave peptide bonds, as can complexes of palladium or platinum.
Periodic Table: Trends in the Properties of the Elements
Authors: Wulfsberg G. P.
Published: December 2011

The article gives examples of trends in properties of the elements, their ions, and their compounds, which can usefully be examined from the perspective of the periodic table as a whole, rather than by looking at one group of elements (or even individual elements) at a time. The properties discussed are those of great interest not only to inorganic chemists but also to research workers in allied fields, such as the other branches of chemistry, medicinal chemistry, environmental science, agricultural and geochemistry, industrial chemistry and safety, metallurgy, materials science, and surface chemistry. These trends can be organized, explained, and predicted on the basis of a small set of parameters of the elements and their ions, most notably the sizes of the ions, their charges (or oxidation numbers), and the Pauling electronegativities of the elements. Trends are emphasized in acid–base reactivity, precipitation and solubility trends, coordination chemistry, oxidation–reduction chemistry, and physical properties of the elements and their compounds. Much acid–base and precipitation chemistry can be predicted after calculating the p
K
a
values of cations and the p
K
b
values of anions, or simply predicting to which of the six acidity and basicity categories an ion belongs. Generally very useful in coordination chemistry are the chelate and macrocyclic effects and the Hard–Soft Acid–Base (HSAB) Principle. Oxidation–reduction chemistry of the elements can be summarized graphically in the form of Pourbaix diagrams or by redox predominance diagrams where the pH is set at the standard pH value of zero. Among physical properties, perhaps the most important for applications in materials science is whether a compound that might be synthesized is a solid with an extended structure; methods are given for predicting this on the basis of the radius ratios of the cations and anions (at least formally) contained in the compound. One physical property, metallic conduction, is discussed in further detail. As an example of chemical properties, trends in the acidity and basicity of oxides of the elements are discussed.
Phosphates: Solid‐State Chemistry
Authors: Attfield J. Paul
Published: December 2011

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: December 2011

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.
Photochemistry of Transition Metal ComplexesBased in part on the article Photochemistry of Transition Metal Complexes by Lisa McElwee‐White which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Bitterwolf Tom
Published: December 2011

Photochemistry has evolved into a powerful tool for the highly selective generation of reactive intermediates in inorganic and organometallic chemistry. This review examines the state of the art in the examination of mechanisms of photochemical reactions utilizing techniques such as frozen matrices and fast time‐resolved methods. The application of photochemistry to the preparation of dihydrogen, alkane, and noble gas intermediates is reviewed along with the significance of these intermediates in the understanding of the mechanism of oxidative addition processes. Experiment and theory of charge transfer phenomena are extensively discussed along with the application of these processes to semiconductor and biochemical studies. Applications of photocrystallography to the examination of linkage isomers and very short‐lived excited state species are reviewed. Finally, a number of examples of the application of photochemical reactions, such as the photolysis of Fisher carbene species to generate ketenelike intermediates, to synthesis are discussed. The literature is reviewed to the end of 2003.
Photoelectron Spectroscopy of Transition Metal Systems
Authors: Green Jennifer C.
Published: December 2011

Photoelectron spectroscopy measures the binding energies of electrons in molecules and solids. The article briefly considers experimental methods. A photoelectron spectrum is related to the molecular orbital structure of a system. Variation of the band intensities with photon energy gives information on the atomic orbital content of the originating molecular orbital. Band splitting may have a number of origins such as vibrational states, Jahn–Teller distortions, exchange splittings, and spin–orbit coupling. All of these provide additional information on electronic structure. Density functional calculations of ionization energies give good agreement with experiment. All these points are exemplified throughout and the article concludes with a number of case studies.
Polonium: Inorganic Chemistry
Authors: Zingaro Ralph A.
Published: December 2011

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.
Polonium: Organometallic Chemistry
Authors: Zingaro Ralph A.
Published: December 2011

The organometallic chemistry of polonium has received limited research effort. The element is very rare, and all of its isotopes are radioactive. Radioactivity causes rupture of the bonds between carbon and polonium. Also, because polonium is a heavy metal, chemical bonding to carbon is thermodynamically very weak. It is rare for an organometallic compound of polonium to be isolated in weighable quantities and characterized using the conventional techniques of organic chemistry. The identification of organopolonium compounds relies largely on chromatographic behavior and on the use of tellurium compounds having analogous chemical structures as reference compounds.
Polyhedra
Authors: King R. Bruce
Published: December 2011

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.
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: December 2011

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.
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: December 2011

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.
Scandium, Yttrium & the Lanthanides: Inorganic & Coordination Chemistry
Authors: Cotton Simon A.
Published: December 2011

The lanthanides are a block of elements with atomic numbers 57–71, which exhibit subtle gradations in their chemical properties, since the ionic radii gradually decrease across the series (the ‘lanthanide contraction’). In reactivity, they resemble the group 2 elements, but are much more diverse; although the aqueous chemistry is essentially that of the +3 oxidation state, the +2 and +4 states are significant for certain metals. The 4f orbitals are well shielded and play little part in lanthanide chemistry, so lanthanides have virtually no chemistry in the zerovalent state and do not form multiple bonds to atoms like N and O. As with the transition metals, lanthanide ions usually have a partly filled subshell (4f), but, in contrast, crystal‐field splittings are weak compared with spin‐orbit coupling, and thus spectroscopic properties of lanthanide complexes strongly resemble those of the free ions; sharp f‐f‐transitions in both excitation and emission spectra confer valuable properties upon certain ions. Lanthanides form a full range of binary compounds with nonmetals, especially the group 16 and 17 elements. The halides LnX3, in particular, demonstrate both the high coordination numbers and the lanthanide contraction. The complex chemistry in aqueous solution is dominated by ‘hard’ donors, especially oxygen‐based ligands (lanthanide ions have very high hydration energies), but ligands with other donor atoms, such as nitrogen, form stable complexes in nonaqueous solvents. The lanthanide aqua ion has a coordination number of 9, decreasing to 8 for the later metals; higher coordination numbers of up to 12 are possible with ligands with a small ‘bite’, such as nitrate. Many complexes exist of oxygen‐donor ligands such as THF and phosphine oxides, with formulae of the type [La(THF)nCl3] (n, e.g. 3, 4) and [Ln(R3PO)n(NO3)3] (n, e.g. 2, 3). Diketonate complexes are important; stable adducts with molecules like polyethers are often volatile enough to be MOCVD precursors. Bi‐ and tridentate N‐donor ligands like bipy and terpy form high coordination number complexes that often exhibit subtle variations in structure with choice of lanthanide and solvent. Low‐coordination numbers are achieved using very bulky amide, alkoxide, and alkyl ligands, when values as low as 2 and 3 are possible. Polyaminepolycarboxylic acids such as EDTA and DTPA form very stable complexes that are highly important in separating mixtures of the lanthanides and also, in the case of [Gd(DTPA)(H2O)]3−, as a contrast enhancing agent in MRI, one of an increasing number of applications that rely upon their electronic and magnetic properties. Yttrium has very similar properties to the later lanthanides, but scandium is significantly smaller. Although the first 3d metal, scandium is not a transition metal, but has significantly different properties, partly on account of its slightly greater size, forming complexes with higher coordination numbers (e.g. the aqua ion [Sc(H2O)7]3+). Its developing chemistry is that of a slightly smaller version of lutetium.
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: December 2011

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.
Selenium Proteins Containing Selenocysteine
Authors: Böck August
Published: December 2011

Selenium is incorporated in the form of the nonstandard amino acid selenocysteine into selected proteins of organisms belonging to all three lines of descent. The majority of these proteins, which contain selenocysteine in the active site, catalyze oxidation–reduction reactions and are involved in numerous biochemical and regulatory processes, which are indispensable for the organism. Because of the higher chemical reactivity of selenocysteine in comparison to cysteine, selenoenzymes display a greatly increased rate of catalysis. Apart from its biochemical function, selenocysteine is also unique since its incorporation is DNA‐encoded and its cotranslational insertion follows a route independent in many features from the path of insertion of the 20 classical amino acids. Selenocysteine, therefore, can be considered the 21st amino acid.
Sol–Gel Synthesis of Solids
Authors: Coradin Thibaud , Livage Jacques
Published: December 2011

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.
Symmetry Point Groups
Authors: Bruce King R.
Published: December 2011

The symmetry of a molecule can be described by its symmetry point group, which is a mathematical group consisting of the symmetry operations of the molecule. This symmetry point group thus consists of the identity plus proper rotations, reflections, an inversion, and/or improper rotations.
Titanium: Inorganic & Coordination ChemistryBased in part on the article Titanium: Inorganic & Coordination Chemistry by Charles A. McAuliffe & Neil Bricklebank which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Valentine Ann M.
Published: December 2011

This article describes the chemistry of titanium, with special focus on its inorganic coordination chemistry. This field has a rich history and has flourished in the years since the first industrial processes for isolation of titanium were introduced. Covered are the discovery of the metal, some of the physical properties of titanium and its complexes, and the unique coordination chemistry of titanium in its various oxidation states. The metal is commonly found in the +4 oxidation state, in which it behaves as a hard Lewis acid. Titanium complexes with formal oxidation states +3, and, much less commonly, +2, 0, and −1 are also known. The oxides, sulfides, halides, and complex halides are discussed. Complexes with carbon, nitrogen, oxygen, silicon, phosphorus, sulfur, and arsenic donor ligands are described. Structures and spectroscopic data are given for selected compounds. Interest in the coordination chemistry of titanium has been driven by important applications in materials chemistry and in catalysis.
Vanadium: Inorganic & Coordination ChemistryBased in part on the article Vanadium: Inorganic & Coordination Chemistry by Elizabeth M. Page & Sherilyn A. Wass which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Richards Raymond L.
Published: December 2011

The chemistry of vanadium and its complexes with various ligands is reviewed, following the format used in the corresponding article in the first edition of this Encyclopedia. The article begins with an update of the element itself. This is followed by a description of the binary compounds formed with hydride, halide, oxide, chalcogenide, nitride, and phosphide ligands. Compounds formed with carbide ligands are briefly mentioned, but thereafter carbon‐donor ligands are not included in this article.

Then follows a review of the coordination chemistry of vanadium, which, after a brief introduction, is grouped into sections according to the oxidation state of vanadium in the compounds under discussion. The first section describes compounds in low oxidation states, [vanadium(−1), vanadium(0), and vanadium(1)], in which nitrogen‐donor ligands such as N
2
and NO and tertiary phosphine ligands are prominent. The second section describes the coordination chemistry of vanadium(II). This section is subdivided according to the donor atom of the various ligands that bind the metal, that is, oxygen, nitrogen, and phosphorus ligands and other ligand types, such as halides. Vanadium(III) coordination complexes comprise the next section, which is subdivided according to oxygen ligands, nitrogen ligands, sulfur ligands, and halide and other ligands. It is to be noted that in general the compounds that have thus far been described tend to be oxidized rather easily to compounds of higher oxidation states.

The coordination chemistry of vanadium(IV), which comprises the next section, is dominated by the very stable VO
2+
(vanadyl) group and a separate subsection is devoted to coordination compounds that contain this grouping. The other subsections describe nonoxo complexes and halide, nitrogen, and sulfur and selenium ligands; the thiovanadyl group, VS
2+
is included in the last subsection. Oxygen‐donor ligands are also prominent in the chemistry of the highest oxidation state of vanadium(V), whose coordination chemistry is covered in the final section. Thus, separate subsections describe the coordination chemistry of the VO
3+
group, the VO
2
+
unit, and peroxo‐complexes. The remaining subsections describe nonoxo complexes, nitrogen, sulfur, and selenium ligands.

A feature of the development of vanadium chemistry during the years since the original article appeared has been the very large increase in the number of coordination complexes that have been prepared and structurally characterized. The content and relative sizes of the subsections of this article and the figures that are included reflect this trend.
Vanadium: Organometallic ChemistryBased in part on the article Vanadium: Organometallic Chemistry by Dieter Rehder which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Hirao Toshikazu
Published: December 2011

This review surveys the chemistry of organovanadium compounds. Their syntheses and reactivities are described depending on the coordination‐binding mode.
Water &O‐Donor Ligands
Authors: Ogden Mark I. , Beer Paul D.
Published: December 2011

The chemistry of a range of O‐donor ligands is described, with an emphasis on the various ligand coordination modes and different structural types of the complexes formed. Water as a ligand is discussed along with the related hydroxide and oxide species. The structure, preparation, and reactions of alkoxide and aryloxide complexes are described. Similar coverage is provided for the diketone ligands. The rich variety of coordination modes of oxoanions, carboxylates, and related ligands, is emphasized. Complexation and structural chemistry of macrocyclic O‐donor ligands are discussed, particularly focusing on crown ether and calixarene metal complexes. The final section provides an overview of the chemistry of other commonly encountered η1O‐donor ligands.
Zinc Enzymes
Authors: Auld David S.
Published: December 2011

A remarkable mosaic of zinc‐binding motifs influence the function of proteins involved in all aspects of metabolism, providing evidence on the molecular level for the indispensability of zinc to growth and development and transmission of the genetic message. There are now about 400 three‐dimensional structures for zinc proteins, representing all six classes of enzymes and covering a wide range of phyla and species. These structures provide standards of reference for the identity and nature of zinc ligands in other proteins for which only the primary structure is known. Three primary types of zinc sites are apparent from examination of these structures: structural, catalytic, and cocatalytic. The most common amino acids that supply ligands to these sites are His, Glu, Asp, and Cys. In catalytic sites, zinc generally forms complexes with water and any three nitrogen, oxygen, and sulfur donors with His being the predominant amino acid chosen. Water is always a ligand to such sites. Structural zinc sites have four protein ligands and no bound water molecule. Twelve of the twenty‐two permutations of four Cys, His, Glu, and Asp ligands have been observed. Cys is the preferred ligand in such sites. Cocatalytic sites contain two or three metals in close proximity with two of the metals bridged by a side‐chain moiety of a single amino acid residue, such as Asp, Glu, or His and sometimes a water molecule. Asp and His are the preferred amino acids for these sites. The properties of both H‐bonding and hydrophobic amino acid ‘outer shell ligands’ and the secondary support structure of the zinc sites is important to the function, stability, and reactivity of the bound metal. The influence of zinc on quaternary protein structure has led to the identification of a fourth type of zinc‐binding site, protein interface. In this case, zinc sites are formed from ligands supplied from amino acid residues residing in the binding surface of two proteins. The resulting zinc site usually has the coordination properties of a catalytic or structural zinc‐binding site.
Alkaline Earth Metals: Organometallic Chemistry
Authors: Alexander Jacob S. , Guino‐o Marites , Zuniga Maria Felisa , Hahn Roger C. , Ruhlandt‐Senge Karin
Published: December 2011

The organometallic chemistry of the alkaline earth metals has undergone a renaissance during the last decade with exciting novel uses of the organomagnesium reagents in synthetic chemistry to significantly extend the utility of these well‐developed reagents. With the development of the organometallic chemistry of the heavier metals significantly lagging behind, and past work mainly focusing on and establishing π‐bonded metal–ligand systems, intense research efforts during the last decade have contributed significantly to expand the understanding of the organometallic compounds of these metals, establishing σ‐bonded organometallic compounds for the heavy alkaline earth metals. This improved insight did not only provide the basis for a more detailed understanding of the metal–ligand bonding characteristics, but also paved the way towards the successful applications of the compounds as polymerization initiators and in organic synthetic chemistry. This article summarizes some of the recent, exciting developments in regards to alkaline earth organometallics.
Biomimetic Synthesis of Nanoparticles
Authors: Slocik Joseph M. , Knecht Marc R. , Wright David W.
Published: December 2011

Biological systems provide numerous examples of highly controlled biomineralization processes that result in stabilized nanoparticles with superior optical properties, crystallinity, dispersity, and morphology. This is best exemplified by the selective mineralization of ferrihydrite (FeOOH·3H
2
O) as a labile source of iron within the iron storage protein ferritin. Consequently, these processes have inspired the use of biomolecular templates to mediate and mimic the synthesis of nano‐materials with a remarkable degree of success. Biomimetic approaches to nanoparticle synthesis include the use of single amino acids, small synthetic peptides (3–18 residues), phage displayed peptides, combinatorial libraries, native proteins, and plant viruses. In the following, we present these biologically inspired templates and methodologies; in addition to describing nanoparticle characterization, resultant properties, and implications for the nanoscale synthesis of materials.
Long‐Range Electron Transfer in Biology
Authors: Winkler Jay R.
Published: December 2011

Electron‐transfer processes are vital elements of energy transduction pathways in living cells. A unique synergy between theory and experiment has produced an extraordinarily detailed understanding of the factors that regulate this ‘current of life’. Studies of Ru‐modified proteins have provided insights into the distance‐ and driving‐force dependences of intraprotein electron transfer rates. Electron transfer across protein–protein interfaces has been probed both in solution and in structurally characterized crystals. It is now clear that electrons tunnel between sites in biological redox chains, and that protein structures tune thermodynamic properties and electronic coupling interactions to facilitate these reactions. This research has produced an experimentally validated, electron tunneling timetable that predicts the time required for an electron to transfer across a specified distance in a protein. Many of the electron tunneling rates measured in cytochromecoxidase and photosynthetic reaction centers agree well with timetable predictions, indicating that the natural reactions are highly optimized, both in terms of thermodynamics and electronic coupling.
Metal‐Mediated Protein Modification
Authors: Whittaker James W.
Published: December 2011

Posttranslational modification of proteins is a general mechanism for regulating biological activity and expanding the functionality in this family of biopolymers. Metal ions have been found to be intimately involved in many of these protein‐processing reactions, whether as cofactors of processing metalloenzymes, intrinsic cofactors for self‐processing proteins, or protein‐targeting solution complexes. A broad range of covalent modifications have been identified as the products of these metal‐mediated reactions. Novel amino acid residues resulting from oxygenation, oxidation, or cross‐linking side chains play important roles in the structure and reactivity of proteins. In a number of cases, the modified amino acids serve as built‐in catalytic cofactors. Reactions of certain metal complexes have been implicated in biomedically important protein modifications, and synthetic complexes have been developed as probes of macromolecular structure. These links underscore the close connections between inorganic and biological chemistry.
Multi‐Heme Cytochromes & Enzymes
Authors: Pereira Inês A. C. , Xavier António V.
Published: December 2011

Multihemeccytochromes and enzymes are a diverse group of proteins having several hemesccovalently bound to the polypeptide chain that are either enzymes involved in redox chemistry or electron transfer proteins. Most known members of this group are associated with the respiratory chains of proteobacteria. These proteins are characterized by a low amino acid to heme ratio, bis‐histidinyl coordination (with a few exceptions), and low redox potentials of the hemes. The hemes are usually tightly packed in close contact, allowing very fast electron transfer. Despite the absence of overall sequence identity between proteins, the hemes are found in quite conserved structural motifs, which tend to be organized in diheme pairs that are either parallel or perpendicular to each other. From the structures available, two families can be recognized in terms of the heme structural arrangement: the cytochromec3family where the perpendicular motif is predominant, and the hydroxylamine oxidoreductase (HAO) family where the stacked parallel motif alternates with the perpendicular motifs. Other multiheme cytochromes for which the structure is not yet available may be part of novel structural families.
Dihydrogen Complexes & Related Sigma Complexes
Authors: Crabtree Robert H.
Published: December 2011

The title topic is reviewed with emphasis on structure and bonding, dynamics, synthesis, and reactivity. Spectroscopic characterization is considered with special reference to diffraction, NMR, and IR methods.
Carbon–Carbon & Carbon–Heteroatom Activation
Authors: Crabtree Robert H.
Published: December 2011

Activation involves binding of the intact CC or CX bond to the metal followed by CC or CX bond cleavage. Both aspects are considered here. The activation of CC bonds is facilitated by an enforced close approach of the CC bond to the metal or by the presence of a strong driving force such as a strained ring starting material or aromatization during the reaction. The reactivity order is: sp–sp > sp2–sp2 > sp3–sp3. The activation of CX bonds occurs by a number of mechanistic pathways depending on the system. Halocarbon complexes of type RXM are often intermediates and can be isolated in favorable cases.
Main Group Carbenes
Authors: Carmalt Claire J.
Published: December 2011

This article describes the chemistry of main group carbene complexes. Although not limited to, the article particularly summarizes the coordination of N‐heterocyclic carbenes to main group species. Usually, carbene ligands cannot be stabilized by electron‐poor coordination fragments because traditional carbenes show a pronounced tendency toward metal‐to‐ligand π back‐donation. However, N‐heterocyclic carbenes do not rely on back‐donation for stabilization because of the delocalization of the nitrogen lone pair in these carbenes. Thus, they are able to form stable complexes with metals that normally show no tendency for π back‐donation. In general, the carbene acts as a two‐electron donor ligand in a similar fashion to phosphines.

Carbene complexes of the alkali and alkaline earth metals, which show a preference for σ‐bonding, are described. The high nucleophilicity of the N‐heterocyclic carbenes is shown by alane, gallane, and indane complexes that have much greater stability than normally observed for Lewis base adducts of group 13. Nucleophilic carbenes also react with Lewis acidic group 14 complexes with the formation of neutral or ionic compounds via simple adduct formation or displacement of a halide ion. Furthermore, N‐heterocyclic carbenes are sufficiently nucleophilic to enable depolymerization of cyclopolyphosphines and cyclopolyarsines to occur with the isolation of carbene–pnictinidene adducts. These carbenes can also form stable adducts with strong electron acceptors, such as pnictogen pentafluorides, if the reactivity is moderated. In addition, stable chalcogen–carbene and halide–carbene complexes are also discussed.
Copper Proteins with Type 2 Sites
Authors: McGuirl Michele A. , Dooley David M.
Published: December 2011

The copper enzymes covered in this article span a wide range in terms of both their structural and reactivity properties. Yet the copper centers in each share similar coordination geometries and have been designated ‘Type 2 (or II)’ copper proteins. Type 2 copper proteins lack the unique spectroscopic or magnetic signatures that typify the so‐called Type 1 and Type 3 copper proteins, and the unique tetranuclear cluster found in nitrous oxide reductase. Indeed, the basis for classifying proteins as Type 2 was that the EPR (Electron Paramagnetic Resonance) spectra of the oxidized (resting) forms of these proteins closely resembled the spectra of ‘normal’ tetragonal CuIIcomplexes, withg∥ > g⊥ > 2.00, andA∥ ≥ 120 G at X‐band. It was generally surmised that the coordination environments of the copper sites in such enzymes were unexceptional, with five‐ or six‐coordinate structures composed of N, O ligands. As is often the case in science, many fascinating and totally unexpected structural and chemical phenomena turned out to be concealed behind this deceptively ‘normal’ facade. Each of the Type 2 enzymes covered here (superoxide dismutase, amine oxidase, lysyl oxidase, galactose oxidase, dopamine‐β‐monooxygenase, peptidylglycine α‐hydroxylating monooxygenase, and quercetinase) displays unique active‐site properties. Elucidating and elaborating structure/function relationships for these enzymes has been greatly aided by the availability of high‐resolution crystal structures for one or more enzymes from each group. These have generally corroborated the fairly detailed structural models that were developed from extensive spectroscopic studies. At the same time, the 3D structures have revealed many surprising details of the active sites. Indeed, the roles of copper in these ‘normal’ Type 2 copper centers are more diverse than ever imagined.
Copper Enzymes in Denitrification
Authors: Dooley David M. , Chan Jeannine M.
Published: December 2011

Denitrification is an intrinsic part of the global nitrogen cycle, balancing the cycle by returning fixed nitrogen to the atmosphere. During denitrification, a bacterial respiratory process, nitrate or nitrite is reduced to nitrogenous oxides. All of the redox reactions of denitrification are catalyzed by metalloenzymes, two of which contain copper. This chapter reviews the structures, mechanisms, and assembly of these copper‐containing enzymes, copper nitrite reductase and nitrous oxide reductase.
Beryllium: Inorganic Chemistry
Authors: Keizer Timothy S. , McCleskey T. Mark
Published: December 2011

The speciation and molecular interactions of beryllium have remained relatively unexplored compared to most metals, despite its high toxicity and extensive use in industry. One reason for this fact is the difficulty in characterizing beryllium species in solution. Herein, the chemistry of beryllium, and its interaction with hydroxo, inorganic and organic ligands are discussed. The structure and formation of beryllium species are communicated with emphasis on the stability of species in solution.
Sol–Gel Encapsulation of Metal and Semiconductor Nanocrystals
Authors: Petruska Melissa A. , Klimov Victor I.
Published: December 2011

The ability to encapsulate nanocrystals in glass matrices affords the opportunity to fabricate a variety of robust materials exhibiting properties tunable with the size, shape, or composition of the dopant. Among the various methods to these composites, sol–gel processing has the distinction of being a ‘wet chemistry’ approach, as it relies on high purity chemical reagents and mild reaction conditions to generate three‐dimensional metal‐oxide polymeric networks. Two general routes to noble metal and semiconductor nanocrystal‐sol–gel composites are explored. In one strategy, the synthesis of the nanocrystal occurs during the fabrication of the host matrix. This direct growth method has been investigated as an express route to nanocrystal‐doped glasses that typically involves the addition of metal salts to sol–gel precursors. Subsequent reduction or precipitation of the embedded metal salts, either with gases or other chemical reagents, leads to the formation of the desired nanocrystals. The second approach exploits the synthetic procedures already available for making high quality, crystalline nanoscale materials and focuses on a decoupled process in which the synthesis of the colloidal nanocrystals is independent of the fabrication of the composite network. In this way, the size, shape, and composition of the nanocrystals can be tailored for specific applications, promising a precise control over the dopant that is unavailable in the direct growth method. Improvements in these procedures have led to the preparation of uniformly dispersed nanocrystal‐sol–gel composites with high nanocrystal volume loadings and narrow nanocrystal size dispersities that have aroused considerable interest for their unique optical, catalytic, and electronic behaviors.
Inorganic Ring Systems
Authors: Haiduc Ionel
Published: December 2011

Inorganic rings of various types can be formed as a result of the broad range of bond types available. Cyclic structures formed by self‐assembly of molecular building units (tectons) are discussed in terms of supramolecular chemistry concepts. The formation of covalent rings can be understood in terms of combining various fragments (synthons) in which the central atoms can display various oxidation states, coordination numbers, and geometries, leading to a broad structural diversity of cyclic structures for a given composition of the ring skeleton (as illustrated for P2N2and Si2N2rings).