bluuebunny/crossref_metadata_2025 huggingface dataset:
Stereochemical Nonrigidity of Organometallic Complexes
Authors: Faller John W.
Published: March 2006

Compounds that undergo intramolecular rearrangements at rates that affect the NMR line shapes in generally accessible temperature ranges are described as stereochemically nonrigid. Analysis of 1D and 2D NMR spectra as a function of temperature often allow the rates, as well as the mechanisms of interconversion of conformers or isomers, to be determined. Examples were selected to illustrate the importance of stereochemical nonrigidity in reactivity and the potential for affecting stereochemistry in the products of reactions.
Supported Organotransition Metal Compounds
Authors: Hanson Brian E.
Published: March 2006

Supported transition metal complexes were first prepared in the 1960s as a potential method for the immobilization of homogeneous catalysts. Although this goal is yet to be commercially realized, the chemistry of supported complexes has given insight to the preparation of heterogeneous catalysts as, for example, in the use of discrete metal complexes as precursors to heterogeneous polymerization catalysts. The purpose of this article is to review the general methods for the immobilization of transition metals with examples of each type of immobilization technique.
Technetium & Rhenium: Inorganic & Coordination Chemistry
Authors: Pombeiro Armando J. L. , Guedes da Silva M. Fátima C. , Crabtree Robert H.
Published: March 2006

The inorganic and coordination chemistry of both rhenium and technetium is described in a condensed way with presentation of significant and typical compounds and reactions and applications in nuclear medicine and catalysis. After an Introduction, with properties of those elements and their sources, a description of binary compounds (for example oxides, sulfides, halides, and nitrides), their syntheses, structures, and selected reactions are given. This is followed by a treatment of the coordination chemistry of those metals, comprising hydrides, metal–metal bonded species and complexes with ligands coordinated by donor‐atoms mainly of the following periodic groups: 15 (commonly nitrogen and phosphorus, including dinitrogen, nitrile, nitric oxide and related ligands, hydrazides, diazenides, nitride, imides, pyrazolylborates, and phosphine ligands), 16 (mainly oxo, thio, and other O‐ or S‐ligands), and 17 (halide ligands); the metal–carbon (group 14) bond complexes are usually not covered since they are dealt with separately. Preparative methods, structures, and reactivities are illustrated for those types of complexes.

Selected complexes of
99 m
Tc and
186
Re (or
188
Re) are described and their applications in nuclear medicine illustrated, as radiopharmaceuticals normally for clinical diagnosis. The application of inorganic rhenium complexes in catalysis constitutes a currently expanding area of high potential and their use as catalysts for the following reactions is reviewed: oxygen atom transfer (OAT) and related reactions; isomerization or etherification of allylic or propargylic alcohols; Beckmann rearrangement of oximes; dehydration of primary amides and aldoximes; dehydrogenation of alkanes, unconventional reduction of aldehydes and ketones; aldol‐type reactions; and photo‐ and electrocatalytic reduction of carbon dioxide. A few catalytic applications of technetium compounds are also indicated.
Tellurium: Inorganic ChemistryBased in part on the article Tellurium: Inorganic Chemistry by William R. McWhinnie which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Laitinen Risto S. , Oilunkaniemi Raija
Published: March 2006

The present chapter describes the main trends in the progress of inorganic tellurium chemistry during recent decades. The emphasis is on the synthesis and characterization of novel nonmetallic tellurium compounds and on transition metal compounds with tellurium‐containing ligands. Many inorganic tellurium compounds find use as single‐source precursors for the generation of binary telluride thin films for electronic applications, or as reagents in organic synthetic chemistry. The advent and progress in modern instrumental methods of analysis has shown that the structural features of tellurium compounds often differ from those of sulfur and selenium compounds.
Tin: Inorganic Chemistry
Authors: Wardell James L.
Published: March 2006

The alloying of copper with tin to produce bronze led to the formation of tools and weapons in what was later called the ‘Bronze Age’ from ≈2600–1500 BC. Since that time tin, in both the +2 and +4 oxidation states has figured prominently in inorganic compounds. This article will provide a summary of the types of inorganic compounds that tin forms, their spectroscopic characterization, and industrial uses.
Tin: Organometallic Chemistry
Authors: Wardell James L. , Spencer Gavin M.
Published: March 2006

Frankland produced the first organometallic tin compound, Et
2
SnI
2
, in 1849. Since that time a wide range of tin compounds containing SnC bonds have been prepared. This article will detail the synthesis and characterization of these compounds. This will include a description of tetraorganotin compounds, halides, pseudohalides, hydrides, tin‐tin bonded compounds, radicals, group 15, and group 16 derivatives. The industrial and environmental aspects of these compounds will also be discussed.
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: March 2006

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.
Titanium: Organometallic Chemistry
Authors: Mintz Eric A.
Published: March 2006

The majority of titanium organometallic chemistry involves complexes in which the titanium is in its highest oxidation state (+4) with cyclopentadienyl derivatives as ancillary ligands. However, considerable chemistry has also been developed for complexes with titanium in the +3 and +2 oxidation state, with lesser amounts of chemistry developed for titanium in lower oxidation states (+1, 0). Since the early 1980s, chemists have placed considerable emphasis on the fine‐tuning of the structure and reactivity of titanium organometallic complexes. Particular emphasis has been devoted to tailoring the structure and reactivity of bis(cyclopentadienyl)titanium derivatives by incorporating electron‐donating, electron‐withdrawing, sterically demanding, or chiral substituents on the cyclopentadienyl ring. Considerable effort has gone into preparing ansa‐metallocenes with a wide variety of bridging groups and substituents to fine‐tune the reactivity catalysts prepared from them. In a similar manner, considerable effort has gone into the development of constrained geometry complexes. Several types of chiral substituted cyclopentadienyl, annulated cyclopentadienyl,ansa‐metallocenes, and constrained geometry complexes have been prepared and applied to olefin polymerization and organic synthesis. Additional efforts at modifying the structure and reactivity by focusing on varying the oxidation state or coordination geometry at the titanium center have expanded over the past decade.
Tungsten: Organometallic ChemistryBased in part on the article Tungsten: Organometallic Chemistry by Andreas Mayr which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Pamplin Craig B. , Hayton Trevor W. , Legzdins Peter
Published: March 2006

This article surveys the organometallic chemistry of tungsten in a systematic manner according to the various types of complexes known to contain tungsten–carbon bonds. The first section deals with complexes having the inorganic ligands CO (carbonyl), CNR (isocyanide), and CN (cyanide) in the metal's coordination sphere. The second and by far the largest section considers tungsten complexes containing organic ligands. This section is divided into subsections dealing with compounds having η1‐hydrocarbyl ligands (i.e. alkyl and aryl complexes, vinyl and alkynyl complexes, carbene complexes, and carbyne complexes), η2‐hydrocarbon ligands (i.e. alkene complexes and alkyne complexes), η3‐hydrocarbon ligands (i.e. allyl complexes), η4‐hydrocarbon ligands (i.e. diene complexes), η5‐hydrocarbon ligands (i.e. cyclopentadienyl complexes), and η6‐hydrocarbon ligands (i.e. arene complexes). Within each category of complex, the material is generally presented in the following order: common methods of synthesis of these complexes, their characteristic physical properties, and their representative chemical properties. The latter are usually illustrated by examples taken from the literature published since 1992, the final year of coverage by the first edition of the Encyclopedia of Inorganic Chemistry. Other articles in the current encyclopedia also cover some aspects of this chemistry, most notably, alkene and alkyne metathesis reactions mediated by organotungsten complexes, the organometallic chemistry of di‐ and polynuclear tungsten complexes, and tungsten compounds containing main group element ligands. Consequently, these topics are not specifically discussed in this article.
Tungsten Proteins
Authors: Adams Michael W. W. , Roy Roopali
Published: March 2006

Tungsten (W, atomic number 74) and molybdenum (Mo, atomic number 42) are very similar metals chemically, and are equally abundant elements on this planet. Molybdenum has been known for decades to play an essential role in biology, and molybdenum‐containing enzymes are ubiquitous in both prokaryotes and eukaryotes. In contrast, tungsten appears to play a much more limited role in biological systems, and one that was established relatively recently. So far, tungsten‐containing enzymes have been found only in prokaryotes. The best‐characterized organisms with regard to their metabolism of tungsten are certain species of anaerobic archaea that grow at extreme temperatures (the hyperthermophiles) or produce methane gas (the methanogens), and anaerobic bacteria that produce acetate (the acetogens) or utilize acetylene. Of these, only the hyperthermophilic archaea appear to be obligately tungsten‐dependent. That is, they do not synthesize molybdenum‐containing homologs of key tungstoenzymes when grown on molybdenum rather than tungsten. Three fundamentally different types of tungstoenzyme have been characterized so far. The best characterized is the Aldehyde ferredoxin oxidoreductase (AOR) family, represented by aldehyde ferredoxin oxidoreductase. Hyperthermophilic archaea such asPyrococcus furiosuscontain at least four members of the AOR family of tungstoenzymes. These appear to play key roles in the metabolism of sugar and peptides. The second family, termed F(M)DH, contains two types of tungstoenzyme, formate dehydrogenase and formyl methanofuran dehydrogenase. These are involved in CO2metabolism in acetogens and methanogens, respectively. The F(M)DH members are much more complex enzymes than the AOR family and show no amino acid sequence similarity to them. The third family is represented by the enzyme acetylene hydratase (AH). It catalyzes the hydration of acetylene and is found in acetylene‐utilizing bacteria. Members of the AOR and F(M)DH family catalyze redox reactions of very low potential (≤−420 mV), in which tungsten undergoes a two‐electron redox reaction [W(IV) to W(VI)]. Crystal structures are available for two members of the AOR family and reveal that each subunit of the proteins contains a single W atom coordinated to two organic pterin molecules, which are bridged by a magnesium ion. An iron–sulfur cluster is situated near the W atom and is thought to be involved in electron transfer to it. The geochemical, ecological, and biochemical basis for tungsten‐ rather than molybdenum‐dependent organisms is discussed.
Vanadium in Biology
Authors: Wever Ron , Renirie Rokus , Hasan Zulfiqar
Published: March 2006

In this article, the role of vanadium in a number of biological systems will be discussed. The metal oxide (vanadate) is present as the prosthetic group in the haloperoxidases, enzymes that are able to oxidize halides in the presence of hydrogen peroxide to hypohalous acids. The bromoperoxidases seem to be involved in the production of the huge amounts of brominated compounds formed by several seaweeds. The enzymes have unusual properties in terms of stability and structural organization. X‐ray structures at high resolution are available now for four vanadium haloperoxidases and details of the events occurring at the active site during catalysis are known. The metal oxide binds peroxide in a side‐on fashion and seems to acts as a Lewis acid in polarizing the bound peroxide for further nucleophilic attack by an incoming halide. These enzymes are also able to mediate in the presence of hydrogen peroxide, the enantioselective sulfoxidation of organic sulfides yielding chiral sulfoxides with high enantiomeric excess. The active site of the haloperoxidases is very similar to that found in acid phosphatases that hydrolyze phosphate monoesters and there is some evidence that the haloperoxidases have evolved from the phosphatases. Vanadium is found in the vanadiumIIIstate in certain blood cells of tunicates (Ascidians) under very acidic conditions. Despite many studies, there is no clue as to the physiological role of the metal and there are many questions regarding the mechanism of accumulation and reduction of the metal in these organisms. The metal also seems to be used as an electron sink by certain bacteria under anaerobic conditions. Furthermore, vanadium is accumulated in Amanita species. These toadstools contain up to 10 mM of a vanadiumIVcompound with very special structure in which the vanadiumIVis eightfold coordinated in an unusual geometry. Although the compound has been reported to carry out redox reactions, the physiological function is elusive.
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: March 2006

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.
ZeolitesBased in part on the article Zeolites by Alan Dyer which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Williams Craig D.
Published: March 2006

This work looks at the natural formation of zeolites in our environment. It details the steps required in the laboratory to successfully manufacture zeolites, producing both novel and nature identical crystal forms. The classification of zeolites based on their aluminium content, pore/channel size is also covered in some detail. Finally the various physical and chemical properties that make zeolites so useful to everyday life are examined. These include their absorptive properties, ion‐exchange properties, desiccant properties, and catalytic properties.
Zinc: Inorganic & Coordination ChemistryBased in part on the article Zinc: Inorganic & Coordination Chemistry by Reg H. Prince which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Burgess John , Prince Reg H.
Published: March 2006

Alloys and salts of zinc have been known for many centuries; commercial production of the metal dates from early in the eighteenth century. Of the several million tons now produced annually, by electrolytic and smelting processes, much is used in alloys, in batteries, in corrosion prevention, and in the manufacture of a range of compounds with various uses. In particular, zinc oxide and zinc chloride have a large number of uses, detailed in this article. Zinc sulfide is both one of the main zinc ores and the archetype for the two main structural forms of 4:4‐coordinated structures (zinc blende and wurtzite). Many zinc compounds and complexes have interesting structural, spectroscopic, optical, magnetic, and conductivity properties, some of which are of actual or potential value in technical and industrial applications. Zinc‐containing species have also proved valuable in synthesis and self‐assembly of a range of supramolecular structures. Both in the solid state and in solution zinc exhibits a range of coordination numbers, though tetrahedral and, especially in solution, octahedral stereochemistries dominate. Zinc forms a variety of complexes, especially with nitrogen, oxygen, sulfur, and halide donor ligands. Many of these complexes show high stability, but they are generally labile in solution. Zinc plays an essential role in biology, being a key constituent of at least 200 metalloproteins. It is an essential element in human nutrition, but toxic in large quantities. Many zinc complexes have been synthesized as biochemical models; a few zinc salts are used in dietary supplements.
Zintl Compounds
Authors: Kauzlarich Susan M.
Published: March 2006

The term Zintl phases was originally used to recognize contributions to the field of solid‐state inorganic compounds by Eduard Zintl. They were defined as a subgroup of intermetallic phases where the anions or anionic network could be considered formally to be valence satisfied. These compounds are made up of electropositive elements from groups 1, 2 that, in a formal sense, donate their electrons to the electronegative elements from groups 13, 14, and 15, which use them to form the correct number of bonds such that each element has a filled shell. The historical definition has been expanded from binary to ternary phases and enlarged to include transition metals and rare earth elements in some cases. The compounds that make up this class are described along with the corresponding Zintl anions arising from solution chemistry. Recent results in the field such as the use of Zintl phases for novel materials is also described.
Zirconium & Hafnium: Organometallic ChemistryBased in part on the article Zirconium & Hafnium: Organometallic Chemistry by Bernard Gautheron, Roland Broussier, & Philippe Meunierk which appeared in the
Encyclopedia of Inorganic Chemistry, First Edition
.
Authors: Meunier Philippe , Pirio Nadine
Published: March 2006

This review considers the main aspects of organometallic chemistry of zirconium and hafnium during the last 10 years. It is complementary to the first edition of Encyclopedia of Inorganic Chemistry. The material is presented in two major parts divided in five and three sections respectively. The first part is devoted to syntheses: the first section, by far the largest, covers in a systematic manner bis and mono cyclopentadienyl complexes with +4, +3, and +2 oxidation states and more especially constrained‐geometry (CG) complexes, cationic complexes, and anionic or zwitterionic 18‐electron complexes; unusual coordination geometries at carbon stabilized by group 4 metallocenes are presented in the second section; other anionic π‐ligands complexes and also lacking π‐ligands complexes are considered in two different sections and bimetallics are developed in the last section. Applications in metal‐mediated organic synthesis and catalysis are reviewed in the second part: neutral organozirconium complexes in organic and organometallic syntheses (hydrozirconation and carbozirconation), cationic zirconocenes in organic synthesis and catalysis, and organozirconium complexes in asymmetric catalysis.
Bond Lengths in Inorganic Solids & Liquids
Authors: Brown I. David
Published: March 2006

The length of a chemical bond is determined by the number of valence electron pairs associated with it. In a simple electrostatic picture, this number is equal to the electrostatic flux linking the bonded atoms. The flux obeys a number of chemically useful theorems, and since it is independent of the ionic or covalent character of the bond, any bond can be transformed to the ionic limit where the flux can be calculated using Coulomb's law. Alternatively, by recognizing that a network of ionically bonded atoms is a capacitive electric circuit, the flux can be determined using the Kirchhoff equations. The correlation between the bond length and bond flux (or bond valence) allows the bond geometry to be predicted even for unknown compounds. A comparison of the observed bond lengths with those expected is useful in validating and analyzing an observed structure. Atomic radii can also be used to predict bond lengths in regular coordination spheres, but they are unable to predict the distortions from regularity typically found in low symmetry structures.
Hard & Soft Acids and Bases
Authors: Martin R. Bruce
Published: March 2006

Practical metal ion hardness–softness scales applicable to aqueous solutions and corresponding to what most chemists probably imply when they use the terms hardness and softness are presented. The scales are derived from experimentally determined log stability constant differences for substitution reactions of one ligand for another on a metal ion. Correlations between scales for different ligand systems are high but not perfect. In an analogous ligand scale, all oxygen donors are hard and all nitrogen donors borderline. The principle of hard and soft acids and bases finds its most consistent application, not in direct stability constant comparisons, but rather in log stability constant differences of appropriate substitution reactions.
Metal‐Mediated Protein Modification
Authors: Whittaker James W.
Published: March 2006

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.
Organic Synthesis Using Metal‐Mediated Metathesis Reactions
Authors: Kissling Rebecca M. , Nolan Steven P.
Published: March 2006

Olefin metathesis is an established reaction, yet it is still growing and diversifying at a remarkable rate. Since 2000, there has been notable progress in a number of areas, especially the development of new catalysts, many of which have N‐heterocyclic carbenes modifying ruthenium. These include phosphine‐free ruthenium catalysts that broaden the scope of efficient ruthenium‐catalyzed olefin metathesis, as well as catalysts with N‐heterocyclic carbenes and electron‐poor phosphines having turnover frequencies >100‐fold faster than the original Grubbs catalyst. In concert with this, some excellent mechanistic experiments and theoretical work have better elucidated the structure‐reactivity relationship and subtle but important points about mechanism relative rates of phosphine. Association, dissociation, and formation of the metallacyclobutane of the ruthenium‐catalyzed reactions are leading the way to further improvements in ruthenium catalysts.

Many methodological applications have also been developed, including improvements in the scope of hetereocycle and macrocycle formation. Cross metathesis (CM) has also undergone a transformation to a high yielding and stereoselective reaction that can be performed with predictable outcomes of yield and alkene stereochemistry. Great strides have been made in asymmetric olefin metathesis reactions, especially desymmetrization and kinetic resolution, with the majority of the results reported using molybdenum‐based catalysts with
C
2
‐symmetric chiral bisaryloxy based ligands. In addition, successful ruthenium‐based catalysts with
C
2
‐ and
C
1
‐symmetric ligands have been reported. Numerous reports of Tandem and sequential olefin metathesis reactions have been published, often in one‐pot, and sometimes utilizing the same metal complex for multiple operations, like ring closure/hydrogenation. A number of examples of tandem reactions utilizing a palladium catalyst and ruthenium‐catalyzed olefin metathesis have also been reported as very efficient routes to desirable organic building blocks.

One of the areas that exploited olefin metathesis is natural product synthesis. There are an ever increasing number of reports that cite olefin metathesis as a key, often step saving, reaction in the successful completion of target molecules by ring closing or cross metathesis. The synthetic advantages the olefin metathesis reaction have also been employed by materials and polymer chemists as part of design strategies for constructing well‐defined polymers, substrates for combinatorial chemistry, and metal‐containing rings.

The olefin metathesis catalysts have been attached to a variety of supports for the purpose of recycling. Many groups have examined methods for tethering both molybdenum and ruthenium‐based catalysts through reversibly coordinating ligands, the alkylidene and the backbone of nonlabile ligands, with reports of high efficiency recycling of up to 10 times.
Low Coordinated Group 13 Chelates
Authors: Dagorne Samuel
Published: March 2006

Recent developments in the area of low‐coordinate group 13 chelates are the focus of this article. Emphasis is placed on the synthesis and reactivity of two new classes of compounds, that is, tri‐coordinate cationic chelates and monovalent two/three‐coordinate chelate species.

Unknown 10 years ago, tri‐coordinate chelate cations {L‐X}MR
+
(M = Al, Ga, In; R = Me, Et, Pr) could be obtained by reaction of the neutral precursors {L‐X}MR
2
with a strong Lewis acid such as [Ph
3
C][B(C
6
F
5
)
4
], provided an extremely crowded L‐X
−
bidentate ligand and an inert counterion are used. Reactivity studies on the Al derivatives show that these highly electrophilic species are potent Lewis acids as expected by the combination of the cationic charge and a low‐coordination number, but are less reactive than neutral AlR
3
compounds with regard to insertion of unsaturated substrates into the AlC bond.

The use of bulky tri‐ and bidentate ligands has also allowed access to unprecedented monovalent and monomeric low‐coordinate group 13 compounds. Tri‐coordinate Ga(I) and In(I) complexes incorporating a sterically encumbered HB(Pz
R, R′
)
−
tridentate ligand were obtained and are quite stable. Similarly, the use of the very crowded HC(CMeNAr)
2
−
and

t

BuNCCN

t

Bu
2−
dinitrogen ligands with an appropriate synthetic methodology has opened the way to stable two‐coordinate and monovalent species: {HC(CMeNAr)
2
}M (M = Al, Ga) and the {

t

BuNCCN

t

Bu}Ga
−
anion, respectively. Such species may behave either as Lewis acids or as Lewis bases due to the intrinsic properties of the metal center (a sextet configuration and the presence of a lone pair of electrons). Reactivity studies on these two‐coordinate complexes have just begun but have already permitted the synthesis of the first monomeric Al and Ga imide complexes {HC(CMeNAr)
2
}M(N‐Trip) (M = Al, Ga).

Finally, representative examples of recent new catalytic applications of tri‐coordinate Al(III) neutral species containing a chiral or achiral X
2
2−
‐type chelating ligand, used as catalysts, are mentioned.
Main Group Carbenes
Authors: Carmalt Claire J.
Published: March 2006

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 Enzymes in Denitrification
Authors: Dooley David M. , Chan Jeannine M.
Published: March 2006

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.
Low Oxidation State Main Group
Authors: Macdonald Charles L. B. , Ellis Bobby D.
Published: March 2006

The synthesis and chemistry of compounds containing groups 13–15 elements in unusually low oxidation states is presented. In general, the oxidation state of an element influences the reactivity of the compounds in which the atom is found. For the main group elements, compounds containing elements in lower than usual oxidation states exhibit significantly different structural features, modes of bonding, and chemistry than is observed for compounds containing the element in a more typical oxidation state. The elements of the s‐block do not form low oxidation state compounds and the elements of groups 16–18 are generally found in the lowest available oxidation state. Thus, this article provides an overview of the following classes of low‐valent main group species: the chemistry of group 13 elements in the +1 and +2 oxidation states, the chemistry of group 14 elements in the 0 oxidation state, and the chemistry of the group 15 elements in oxidation states lower than +3. Numerous exemplary modes of reactivity are presented and the utility of some low oxidation state compounds as catalysts or precursors to materials is acknowledged.
Transition Metal Complexes with Bulky Allyl Ligands
Authors: Hanusa Timothy P. , Carlson Christin N.
Published: March 2006

The allyl anion (C
3
H
5
−
) is often found in combination with other ligands in organometallic complexes. Its compact size, however, means that compounds in which it is the only or principal type of ligand are frequently coordinatively unsaturated and prone to decomposition. Sterically bulky substituents (e.g. – SiMe
3
) placed on the allyl group will increase its effective size, dramatically enhancing the thermal, and in some cases oxidative, stability of the associated metal complexes. Solubility in nonpolar solvents such as aromatic hydrocarbons and alkanes is also improved. Electron‐deficient monomeric first row transition metal allyl complexes (e.g. [π‐(SiMe
3
)
2
C
3
H
3
]
2
M; M = Cr, Fe, Co) have been synthesized that have no (C
3
H
5
)
2
M counterparts. New varieties of early transition metal and f‐element complexes have been made with bulky allyl ligands, some of which are active polymerization catalysts. Research in this area has markedly expanded the range of metals, structural types, and associated reactions that can be incorporated into metal allyl chemistry.
Group 14 Multiple Bonding
Authors: Sasamori Takahiro , Tokitoh Norihiro
Published: March 2006

Multiply bonded compounds between heavier group 14 elements cannot exist as monomers under normal conditions owing to their inherent high reactivity, that is, they readily undergo oligomerization, oxidation, hydrolysis, and so on. The bulkiness of substituents on the reactive sites shows a great effect on the stabilization of such highly reactive compounds. Since the synthesis and isolation of the first stable disilene, Mes
2
SiSiMes
2
(Mes = mesityl), by R. West
et al.
in 1981, the idea of kinetic stabilization using bulky substituents have led to the successful isolation of several types of unsaturated compounds of group 14 elements, such as
heavy alkenes
(double bonds between heavier group 14 elements),
heavy ketones
(heavier group 14–16 double bonds),
heavy imines
(heavier group 14–15 double bonds),
heavy aromatics
(aromatic systems containing heavier group 14 elements),
heavy butadienes
, and triply bonded systems containing heavier group 14 elements, and so on. Now, unsaturated compounds containing heavier group 14 elements are no more imaginary species but are those of real existence even in the case of lead, the heaviest group 14 element, when they are appropriately protected by bulky substituents. The synthesis and properties of the kinetically stabilized unsaturated compounds containing heavier group 14 elements will be described in this section. Although a number of multiple bondings between heavier group 14 elements and transition metal elements have been reported so far, only unsaturated compounds containing main group elements should be considered here from the viewpoint of comparisons with carbon chemistry.
Sol–Gel Encapsulation of Metal and Semiconductor Nanocrystals
Authors: Petruska Melissa A. , Klimov Victor I.
Published: March 2006

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.
Metal Nanoparticles, Synthesis of
Authors: Schmid Günter
Published: March 2006

The successful synthesis of metal nanoparticles decisively depends on sufficient stabilisation by protecting materials. This can be reached either by electrostatic repulsion of charged species or by covering the surface atoms with polymers, surfactants, or especially ligand molecules binding to the particles via covalent bonds. Noble metals are in focus, since their salts or complexes can easily be reduced to the oxidation state 0 and the sensitivity towards oxygen is reduced or even avoided like in the case of gold. Nevertheless, nanoparticles of less noble metals can be prepared. Above all, there is an increasing need to get monodispersed particles, since their properties depend on size and shape. There exist so‐called physical methods to generate metal atoms from bulk metals that are then allowed to coalesce to nanoparticles. High temperatures or laser ablations are examples. Chemically based procedures use in any case stabilizing agencies. Among numerous methods, salt reduction dominates. Thermal decomposition of appropriate precursor complexes and soft ligand removal from organometallics have become known as alternative routes. Photolytic and radiolytic techniques exist, but do not play important roles. The synthesis of magnetic metal nanoparticles is of increasing interest with respect to applications. The syntheses mainly follow recipes known from noble metals. Fe, Co, Ni as well as magnetic bimetallic alloy‐like nanoparticles are available. The magnetic characteristics of nanoparticles again depend decisively on size, structure, and shape, even more than in case of the nonmagnetic species. Therefore, synthetic procedures resulting in uniform particles are of great importance.
Metal Nanoparticles, Organization & Applications of
Authors: Schmid Günter
Published: March 2006

The need to organize metal nanoparticles in three, two, or one dimension(s) is linked with their properties, the use of which is only possible if they are organized. The most important properties of nonmagnetic and magnetic metal nanoparticles are therefore briefly discussed. Organization in three dimensions (3D) may happen by naturally happening crystallization processes or by linking the particles with the help of spacer molecules of various length and composition. 3D organized metal particles are quite well investigated and electron transport between the building blocks is satisfyingly understood. 2D arrangements result either by spontaneous self‐assembly from solution on appropriate supports, by guided self‐assembly, supporting the organization process, for instance, by means of pressure (Langmuir–Blodgett) or pre‐prepared surfaces, or the most attractive kind of organization, the aimed one, since it results in artificial structures that would be necessary if nanoparticles should find application in future nanodevices. Two routes to aimed structures have become known up to now: the dip‐pen nanolithography and the nanoelectrical modification of surfaces. Dip‐pen nanolithography uses the transport of special molecules from a moving AFM tip to suited surfaces. Chemical modifications of as‐prepared structures end up with formation of aimed patterns of metal nanoparticles. Nanoelectrical modification of CH3‐terminated surfaces, also using an AFM tip, give COOH functions that can further be modified to bind metal nanoparticles. Architectures performed by one of these methods simply depend on the software the AFM is moving on the surfaces.
Carbon: Nanotubes
Authors: Baskaran Durairaj
Published: March 2006

A brief introduction to carbon nanotubes with basic structural classification, synthesis, properties, and surface modification is presented. The growth process of carbon nanotubes and the role of metals catalyst in the arc‐discharge, the laser‐vaporization, and the chemical vapor deposition (CVD) techniques are discussed. The synthesis of aligned carbon nanotubes in a preferred orientation from various substrates and the formation of branched carbon nanotubes are unique features of the CVD technique. The method of purification of carbon nanotubes is chosen on the basis of the amount of catalyst impurities and amorphous carbon present in them that are dependent on the techniques used to produce carbon nanotubes. Oxidative purification methods and the introduction of carboxylic acid on the surface of carbon nanotubes have been described. Optical, electrical, and mechanical properties of carbon nantubes have been highlighted. An overview of the surface modification of carbon nanotubes with covalent and noncovalent chemistry to enhance the dispersion and dissolution properties of carbon nantotubes and the problem associated with size, length, and the functional groups distribution affecting the solution properties of carbon nanotubes have been discussed.
Aktuelle Entwicklungen in der optischen Datenspeicherung mit organischen Farbstoffen
Authors: Mustroph Heinz , Stollenwerk Manfred , Bressau Volker
Published: March 2006

Die wesentlichen Motivationen für die Entwicklung digitaler Datenspeicher sind die Verbesserung der Wiedergabequalität und die Erhöhung der Speicherkapazität. Im Jahre 1982 stellten Philips und Sony mit der Compact Disc (CD) und einem entsprechenden Abspielgerät das erste technisch und wirtschaftlich erfolgreiche System dieser Art vor. Heute steht eine Vielfalt an optischen Datenträgerformaten zur Verfügung. Dabei wird zwischen vorbespielten, einmal beschreibbaren und wiederbeschreibbaren Medien unterschieden. Der Aufsatz gibt zunächst eine Übersicht über diese Systeme und die Grundzüge ihrer Herstellung und konzentriert sich dann auf die Eigenschaften der organischen Farbstoffe, die in einmal beschreibbaren Systemen verwendet werden. Die Farbstoffchemie wird im Allgemeinen zu den reifen Arbeitsgebieten gezählt. – Ist dies ein begründeter Abgesang oder nur ein Vorurteil? Anhand von Anwendungen in der optischen Datenspeicherung wird illustriert, dass Fortschritte bei CD‐R und DVD/R auch heute ohne funktionelle Farbstoffe undenkbar wären.
Turn and helical peptide spacers: Combined distance and angular dependencies in the exciton‐coupled circular dichroism of intramolecularly interacting bis‐porphyrins
Authors: Guryanov Ivan , Moretto Alessandro , Campestrini Sandro , Broxterman Quirinus B. , Kaptein Bernard , Peggion Cristina , Formaggio Fernando , Toniolo Claudio
Published: March 2006

In a previous study we examined by the exciton‐coupled circular dichroic method the distance effect generated by three‐rigid‐turn and helical‐peptide spacers. In this connection porphyrins were confirmed to be excellent reporter chromophores. In the present investigation we have completed this research by expanding the original analysis to the assessment of the combined role of the chromophore distance and orientation with use of the same porphyrin derivatives and additional four analogous spacers of different main‐chain lengths. We find that not only the intramolecular separation of the chromophores, but the angular dependence between the directions of their effective transition moments as well, are responsible for the onset and modulation of the intensity of the exciton‐coupling phenomenon of the porphyrin Soret band. © 2006 Wiley Periodicals, Inc. Biopolymers 82: 482–490, 2006

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