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Zirconium & Hafnium: Inorganic & Coordination ChemistryBased in part on the article Zirconium & Hafnium: Inorganic & Coordination Chemistry by Grigorii L. Soloveichik which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.
Authors: Kozak Christopher M. , Mountford Philip
Published: December 2011

The inorganic and coordination chemistry of zirconium and hafnium is described. The physical properties of the metals, as well as their isolation, purification, and commercial and industrial uses are discussed. A section on the binary compounds of zirconium and hafnium containing hydride and main group elements follows. Oxoacid salts of the two metals are described, as well as aqueous chemistry. Examples of zirconium and hafnium coordination chemistry are presented according to ligand type: molecular and silica‐supported hydride complexes; ionic and neutral halide complexes; complexes with oxygen donor ligands; complexes with nitrogen donor ligands; dinitrogen complexes; complexes with phosphorus and sulphur donors ligands; complexes with mixed‐donor ligands. Lastly, lower oxidation state inorganic, coordination, and cluster compounds are discussed.
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: December 2011

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: December 2011

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.
Metal‐Related Diseases of Genetic Origin
Authors: Sarkar Bibudhendra
Published: December 2011

In humans, there are several genetic disorders of metal metabolism. Wilson and Menkes diseases are genetic disorders of copper transport. The genes from these two diseases encode copper‐transporting ATPases. Mutations in Wilson disease gene cause toxic accumulation of copper in the cells whereas mutations in Menkes disease gene cause copper deficiency and lack of its incorporation in important copper enzymes. Wilson disease is treated by chelating agents such asD‐penicillamine and trientine and zinc, which inhibits absorption of copper from the intestine. Current treatment for Menkes disease is subcutaneous injection of copper‐histidine. Hereditary hemochromatosis, thalassemia, and aceruloplasminemia are major disorders of iron metabolism. The gene responsible for hereditary hemochromatosis encodes a protein, which is very similar to major histocompatibility complex on chromosome 6, that has been proposed to down‐regulate iron absorption. The disease is treated by weekly venesection of approximately 0.5 L of blood. Thalassemia causes severe iron accumulation as a result of disturbances in the synthesis of one or more of the globin chains of hemoglobin. The chelating agent used for the treatment is deferoxamine administered subcutaneously. The search for an orally administered iron chelator has been intensified in recent years leading to chemical trials of chelator such as deferiprone (L1). Aceruloplasminemia is caused by mutations in the ceruloplasmin gene resulting in iron accumulation in various tissues. Deferoxamine has been used to treat this disease. Acrodermatitis enteropathica is a genetic disorder of zinc metabolism. Mutations in the gene responsible for zinc transport result in insufficient absorption of zinc in this disease, which can be ameliorated by zinc supplementation. Several less common genetic disorders of metal metabolism are discussed briefly.
Hard & Soft Acids and Bases
Authors: Martin R. Bruce
Published: December 2011

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‐Based Drugs
Authors: Shaw C. Frank
Published: December 2011

Metallopharmaceuticals have a long history in the development of chemotherapy. The more recent success of cisplatin and six related Pt‐based antitumor drugs, and longer histories of chrysotherapy (gold treatments) for arthritis, bismuth antiulcer agents, and silver‐, antimony‐ and arsenic‐based antimicrobial agents demonstrate that the periodic table represents a potential wealth of medicinal agents to be explored and developed in the future. This article reviews the use of twelve elements (Ag, As, Au, Bi, Ga, Li, Pt, Ru, Sb, Sn, Ti, V) for a wide variety of diseases and disorders. The current state of research on particular applications varies widely – from promising treatments that have not yet reached the clinic to those that are well established empirically despite uncertain mechanisms of action. The array of antitumor agents licensed or in clinical trials includes compounds of As, Ga, Ru, and Ti, in addition to platinum. There are also exciting efforts to apply known treatments or biological properties to new diseases by taking advantage of extensive databases, for example, developing antitumor agents from organotin complexes that have long been used as fungicides and antifouling agents, and antimicrobial agents from gold complexes. The ability to modulate the properties of metal complexes by choice of the oxidation state (AuIvs AuIII; PtIIvs PtIV; VIII, VIV& VV, etc.) and design of the medical carrier ligands (e.g. 1,2‐diaminocyclohexane vs two ammine ligands for Pt antitumor agents) allows targeting of particular tissues or cells and balancing of lipophilicity, solubility, and reactivity to balance therapeutic activity against toxicity. Many, if not most, metallopharmaceuticals are prodrugs that undergo redox changes and/or ligand exchange reactions in vivo to generate the active species. Hence, research on metallodrug metabolism and pharmacology is as important as the initial medicinal screening of the agents.
Metal‐Based Imaging Agents
Authors: Francesconi Lynn C. , Cantorias Melchor V. , Howell Robertha C.
Published: December 2011

Radiopharmaceuticals are drugs that contain a radionuclide and are routinely used for diagnosis and treatment of disease. This review discusses imaging agents that are useful for diagnosis using Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET). Moreover, this entry focuses on the growing class of metal‐based imaging agents, wherein the radionuclide is a transition metal or posttransition metal. The most widely used radionuclide for radiopharmaceuticals is technetium‐99m,
99m
Tc, owing its widespread availability from the parent–daughter generator and its excellent physical properties for imaging. The radionuclides
67
Ga,
111
In,
210
Tl,
57
Co, and
51
Cr are used in the clinic as well. There are a number of radioisotopes, including
64
Cu,
67
Cu,
68
Ga, that show promise for diagnostic purposes. An increased understanding of the coordination chemistry of
99m
Tc and other radiometals resulted in first‐generation metal‐essential imaging agents. This understanding of coordination chemistry is responsible for the introduction and development of targeted receptor‐specific agents, many of which are in clinical trials and some of which are on the market.
Metallocenter Biosynthesis & Assembly
Authors: Mulrooney Scott B. , Hausinger Robert P.
Published: December 2011

Metalloproteins have been characterized intensively for decades, yet only recently has progress been made in characterizing the mechanisms of biological metallocenter biosynthesis and assembly. Here, we describe how cellular synthesis of metal sites makes use of several recurring themes, often with multiple themes combined into a single pathway. In the simplest situation, binding of a metal ion to a biological ligand occurs by reversible thermodynamic control. However, the prevalence of metallocenters deeply buried within macromolecules, the exceedingly low concentrations of free metal ions within cells, and the sophisticated structures of many metal‐containing active sites and cofactors provide evidence that alternative and more complex approaches also must exist. In some cases, metallochaperones are used to deliver the metal of interest to an apoprotein. In other cases the target protein undergoes posttranslational modification at the metal‐binding site, either prior to or after the metal is bound. Many metallocenters contain additional components that are added along with the metal ion or used to form a preassembled metal‐containing cofactor. Scaffolding proteins may be used to provide a framework for construction of the cofactor. Electron transfer reactions involving the metal, an added compound, or the protein ligand can participate in activation. Finally, molecular chaperones that bind to and alter the conformation of the target apoprotein may be utilized. In many cases, the function of the molecular chaperone is coupled to nucleotide triphosphate hydrolysis.
Metalloprotein Design & Engineering
Authors: Lu Yi
Published: December 2011

This review covers recent advances in metalloprotein design, with focus on different approaches to the design. Impressive progress has been made in designing metal‐binding sites in peptides, de novo designed proteins, and native protein scaffolds. The approach can be rational or combinatorial. Under rational design, redesigning an existing metal‐binding site to a new site with dramatically different structure and function complements well the design of new metal‐binding sites by revealing the role of specific residues responsible for a particular structural or functional feature of the metal‐binding site of interest. To create a new metal‐binding site, several approaches have been used, including design based on structural homology, by inspection, using automated computer search algorithms, or combination of the above approaches. In addition, modular approach by transplanting a conserved structural unit from one protein into another has also been shown to be effective. Design through combinatorial and evolution methods has also been successful as it requires little prior knowledge of the protein structure. Finally, introducing unnatural amino acids or nonnative metal ions/prosthetic groups to expand the repertoires of metalloproteins have been demonstrated. Successful examples of each of the approaches are given; advantages and disadvantages of the approaches are discussed; the outlook for future research is also presented.
Organic Synthesis Using Metal‐Mediated Metathesis Reactions
Authors: Kissling Rebecca M. , Nolan Steven P.
Published: December 2011

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.
Organic Synthesis Using Metal‐Mediated Coupling Reactions
Authors: Goodson Felix E. , Cichowicz Melissa B.
Published: December 2011

This article provides an overview of the cross‐coupling protocols most commonly used in organic synthesis. Particular emphasis is placed upon topics likely to be of interest to the inorganic chemist, namely, history, catalyst development, and mechanistic studies. While not a comprehensive review, this treatment is intended to provide the reader with a summary of some successful catalyst design strategies, methodological improvements, and mechanistic insights that have been achieved over the past 10 years.
Porous Inorganic Materials
Authors: Wang Xiqing , Bu Xianhui , Feng Pingyun
Published: December 2011

Porous inorganic materials cover a range of solids, both oxides and nonoxides, crystalline and amorphous, that have a pore size extending from about 3 Å to over 500 Å. Crystalline microporous materials such as zeolites have been known for over 200 years, and the research in the past decade has extended types of microporous materials from traditional oxides to chalcogenides and metal–organic frameworks. Efforts aimed toward developing new microporous materials that integrate uniform microporosity with other functionality have also led to promising results.

Periodically ordered mesoporous materials are a recent addition to the family of ordered inorganic porous materials. They can be made in the presence of organized assemblies of organic molecules (e.g. amines, cationic, anionic, or nonionic surfactants) or polymers (e.g. amphiphilic block copolymers). Micellar solutions, lyotropic liquid crystals, and microemulsions can all serve as structure‐directing agents during the cocondensation of inorganic precursors with organic species. A number of synthetic pathways have been developed for the preparation of mesoporous solids in a variety of chemical compositions including silica, organosilica, transition metal oxides, chalcogenides, and even elemental forms such as carbon and platinum. Much progress has also been made in the control of macroscopic morphologies (e.g. films, fibers, monoliths, and spheres). The pore geometry is closely related to the nature of the organic assemblies and other synthetic parameters and can be controlled to a certain degree to allow the synthesis of mesoporous materials with the desirable architecture and properties.

The past decade has also witnessed a rapid development in the synthesis of ordered macroporous materials and porous materials that are ordered at multiple‐length scales. Commonly used templates for the control of the macroscopic ordering include colloidal crystals such as latex or silica spheres and emulsion droplets. With the availability of a variety of inorganic porous materials, numerous applications are awaiting the exploration of materials researchers.
Self‐Assembled Inorganic Architectures
Authors: Bera Jitendra K. , Bacsa John , Dunbar Kim R.
Published: December 2011

The field of inorganic chemistry is witnessing an enormous growth period in the area of molecular self‐assembly. This review highlights key advances in this area with an emphasis on novel structures and the dynamic nature of inorganic self‐assembly reactions. Multidentate N‐heterocylic, cyanide, and carboxylate ligands are discussed in detail with respect to their use in the design of discrete structures, including molecular triangles, squares, pentagon, cubes, as well as polyhedra, oligomers, and polymers. Inorganic structures that employ metal–coordination and hydrogen bonds are also highlighted. The examples described herein illustrate the fact that the available binding sites and the topicity of the ligands along with the coordination requirements of metal ions are the key elements for building desired architectures. The role of intrinsic factors such as geometrical preferences of the metal ion as well as extrinsic factors such as the presence of a template in stabilizing a particular product are discussed. Recent progress in the rational design of functional materials vis‐à‐vis the manipulation of supramolecular chemistry is also covered in brief.
Hydrogen Bonding & Dihydrogen Bonding
Authors: Crabtree Robert H.
Published: December 2011

The review begins with the historical development of the area, notably Pauling's contribution. The current interest is in part related to the rise of supramolecular chemistry and molecular recognition, as well as the significance of hydrogen bonding in biology and biochemistry. Strong and weak cases are distinguished. A relationship with hypervalency is proposed, in which the bridging proton of a FHF anion is considered as hypervalent hydrogen. Dihydrogen bonding is considered the latest hydrogen bond type. Structural and spectroscopic aspects are discussed, together with applications. Hydrogen bonding in metal complexes is covered together with the distinction between hydrogen bonding and sigma complexation.
Carbene Complexes
Authors: Crabtree Robert H.
Published: December 2011

The synthesis, structure, reactions, and applications of carbenes are discussed. Fischer and Schrock carbenes are distinguished. N‐heterocyclic carbenes (NHCs) are considered as a class of Fischer carbenes with promise as spectator ligands in homogeneous catalysis. Nucleophilic attack on carbonyls and alpha elimination are noted as leading pathways leading to Fischer and Schrock carbenes, respectively. Criteria are discussed for characterization of carbenes, notably nuclear magnetic resonance (NMR) spectroscopy. The Grubbs metathesis catalyst and alkene metathesis catalysis are also mentioned. Bridging carbenes and carbynes are briefly considered.
Low Oxidation State Main Group
Authors: Macdonald Charles L. B. , Ellis Bobby D.
Published: December 2011

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: December 2011

The allyl anion (C3H5−) 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. – SiMe3) 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‐(SiMe3)2C3H3]2M; M = Cr, Fe, Co) have been synthesized that have no (C3H5)2M 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.
Subvalent Compounds
Authors: Brothers Penelope J.
Published: December 2011

Subvalent compounds are those in which a main group element occurs in an oxidation state lower than its normal group oxidation state. Traditionally, these have been identified in solid‐state inorganic phases, but more recently there have been major advances in the preparation of stable, molecular compounds containing subvalent main group elements. This has occurred in some cases through the use of specially designed ligands able to stabilize low oxidation state species through electronic stabilization or steric protection using bulky substituents. Ingenious synthetic routes such as the preparation of solutions in organic solvents of metastable aluminum(I) and gallium(I) halides have allowed access to other novel species. Apart from a relatively small number of types of mononuclear complexes, most subvalent molecular compounds involve homoatomic element–element bonds in the form of rings, chains, and clusters. Many of these have unusual structural and electronic properties that challenge established, classical ideas of bonding and electronic structure. This discussion focuses on recent advances in the structure and chemistry of molecular subvalent compounds, with an emphasis on the groups 13 and 14 elements. In addition to mononuclear and simple homonuclear single‐bonded compounds, several classes of cluster compounds are examined, including electron‐precise, electron‐rich, and electron‐deficient clusters. The latter include the Zintl ions. A very new class of aluminum and gallium metalloid clusters containing metal–metal bonds unsupported by ligands represents species on the pathway to formation of the bulk metal. An area in which exciting developments have taken place in the last decade encompasses subvalent compounds that are heavier main group element analogs of alkenes and alkynes, and significant structural features and new bonding models for these are discussed.
Metal Nanoparticles, Synthesis of
Authors: Schmid Günter
Published: December 2011

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.
Carbon: Nanotubes
Authors: Baskaran Durairaj
Published: December 2011

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.
High‐Energy Electron Diffraction
Authors: Zuo Jian‐Min
Published: December 2011

High‐energy electrons have very short wavelengths associated with particle waves and electron diffraction is used to probe the atomic structure of both inorganic and organic materials. Electrons have a large scattering cross section and can be focused into small probes by magnetic lenses. This makes electron diffraction very useful for studying “small” crystals, surfaces, or material's microstructure. The principles and the modern aspects of electron diffraction are described here with the intention to provide an introduction that both the novice and the expert will find useful as a guide for electron diffraction. The chapter starts with an introduction to different electron diffraction techniques. This is followed by the background theories which are useful for the interpretation of diffraction patterns. Experimental procedures are described at the end of the technical background. The applications section has a large collection of examples from relatively straightforward tasks to more advanced ones that are under development for their promises. Experimental details are provided when it is appropriate to give some idea of what it takes to do electron diffraction.
High Resolution Electron Energy‐Loss Spectroscopy
Authors: Soriaga Manuel P. , Chen Xiaole , Li Ding , Stickney John L.
Published: December 2011

The determination of the identity and structure of molecular species present in no more than monolayer quantities at solid surfaces is one of the more difficult tasks in interfacial science. In this regard, vibrational spectroscopy is an invaluable technique; in the evaluation of the molecular integrity of surface‐bound compounds, it has few equals. Optical methods such as infrared spectroscopy, while rather uncomplicated for bulk materials, lack the high sensitivity needed for most surface species due to minuscule molecular amounts. A much more responsive alternative is based on the use of low‐energy electrons to interact with the molecular oscillators on the surface. If inelastic interactions occur, the energy of the backscattered electrons will be lower than that of the incident electrons, and the energy loss will correspond to a vibrational‐mode frequency. The present article briefly describes the fundamental principles of and instrumental considerations in high‐resolution electron energy‐loss spectroscopy (HREELS), and cites seminal applications in the study of the adsorption of organic and inorganic molecules at metal and semiconductor surfaces. A limited list of HREELS work published since the turn of the century is added in the last section.
Magnetic Circular Dichroism (
MCD
) Spectroscopy
Authors: Mack John , Stillman Martin J.
Published: December 2011

Magnetic circular dichroism (MCD) spectroscopy provides information about the spin and orbital angular momenta associated with ground and excited electronic states. This can provide the key to understanding the optical spectroscopy of high symmetry ions and complexes, which provides an important insight into the electronic structure. MCD spectra contain the same set of spectral bands as the corresponding electronic absorption spectrum, with significantly different band morphologies due to the effect of the applied magnetic field and the use of a differential absorbance intensity scale.

MCD spectra are recorded by mounting a superconducting magnet (
B
 ≈ 0–11 T), an electromagnet (
B
 ≈ 0–1 T), or a permanent magnet into the sample compartment of a CD spectrometer with the field applied parallel to the axis of light propagation. Parallel applied fields are often referred to as the Faraday magnet alignment, since it formed the basis of Faraday's pioneering experiments in which plane polarized beams of light were found to be rotated during transmission through certain substances, due to the differential absorbance of left circularly polarized (lcp) and right circularly polarized (rcp) light. The Faraday 𝒜

1

, ℬ

0

, and 𝒞

0

terms form the basis for the analysis of MCD spectra based on the Zeeman splitting of the absorption bands for lcp and rcp light, the field‐induced mixing of zero‐field states, and the Zeeman splitting‐based ground state population adjustment, respectively.
Mass Spectrometry in Inorganic and Bioinorganic Chemistry
Authors: Becker Johanna Sabine , Becker Julia Susanne
Published: December 2011

Mass spectrometry (MS) in inorganic and bioinorganic chemistry has been established using sensitive analytical techniques in quite different application fields: the determination of major, minor, and trace element concentrations; for the characterization of surfaces; for precise and accurate isotope ratio measurements; and for the structural analysis of organic and bioorganic compounds. The review gives an overview on different mass spectrometric techniques and their applications
Microwave Rotational Spectroscopy
Authors: Xu Yunjie , Jäger Wolfgang
Published: December 2011

Since its inception in 1934, microwave rotational spectroscopy has provided some of the most accurate data for numerous small to medium sized molecules, including structural parameters, force field parameters, and electronic charge distributions. We give a very brief review of the history of microwave spectroscopy, leading up to the most recent developments, that is, instruments that use Fourier transform techniques. Since these Fourier transform microwave spectrometers are not commercially available, a brief discussion about their general design is given. Analyses of the recorded rotational spectra can yield a number of spectroscopic parameters, depending on the molecular symmetry and the various angular momenta present, such as nuclear spins and electronic momenta. The physical significance of the more common spectroscopic parameters, such as rotational constants, centrifugal distortion constants, nuclear quadrupole coupling constants, etc. is given. We also indicate how these parameters can be used to characterize the molecule in terms of, for example, structural parameters, dynamical behavior, and bond character. The remainder of the article is devoted to case studies. These include the discovery of covalent bonds involving the noble gas atom argon, structural characterization of organometallic compounds, such as ferrocenes, bonding in diatomic transition metal‐containing compounds, a description of laboratory studies of molecules which are of atmospheric and astrophysical interest, and studies of weakly bound complexes with relevance to inorganic chemistry.
Neutron Diffraction
Authors: Yousufuddin Muhammed , Bau Robert
Published: December 2011

The following article reviews the technique of neutron diffraction and its effectiveness in probing materials of chemical interest. The main advantage of neutrons is their ability to detect hydrogen atoms. We describe in this article several examples where neutrons offered additional information that was otherwise unavailable from X‐rays. We have summarized a number of structures studied using neutron diffraction. Most of these structures showed the precise location of chemically interesting hydrogen atoms. We have also summarized structures where neutron diffraction allowed for the assignment of absolute stereochemistry in deuterated systems. Lastly, we have discussed the usefulness of neutron diffraction in protein crystallography.

One of the major disadvantages of neutron diffraction is the immensely low number of neutron sources available as compared to X‐ray sources. We have detailed the neutron sources around the world capable of neutron diffraction experiments, including sources that are currently under construction such as the Spallation Neutron Source in Oak Ridge, TN and J‐PARC in Tokyo, Japan. In addition, a number of neutron diffraction instruments and their sample requirements are illustrated.

In the past, neutron diffraction has been overlooked by most practicing chemists as an efficient analytical tool. However, with the newer more powerful facilities and instrumentation in development, chemical systems studied with neutrons should become more common.
Nuclear Magnetic Resonance (NMR) Spectroscopy of Inorganic/Organometallic Molecules
Authors: Iggo Jonathan A. , Liu Jianke , Khimyak Yaroslav Z.
Published: December 2011

NMR spectroscopy is an indispensable spectroscopic tool for the characterization of inorganic compounds. This contribution provides a guide to the application of recent advances in high‐resolution, solid‐state, and high‐pressure NMR spectroscopic methods to organometallic/inorganic molecules and solids. The topics covered include methods for the determination of structure, intermolecular interactions, solid‐state organization, reaction kinetics, and diffusion.
Nuclear Quadrupole Resonance (NQR) Spectroscopy
Authors: Wulfsberg Gary P.
Published: December 2011

The article concerns the usefulness of nuclear quadrupole resonance (NQR) spectroscopy in inorganic and organometallic chemistry. NQR‐active nuclei have spinI ≥ 1; those of high enough abundance to be commonly useful are found in the odd‐numbered groups of the periodic table. These are tabulated and examples given for each. The number of NQR frequencies observed in a solid‐state compound reflects the number of chemically different quadrupolar atoms in the compound and also the number of crystallographically inequivalent atoms; changes in the latter may indicate phase transitions or different polymorphs of a given compound. More commonly, one uses average NQR frequencies for chemically different atoms, and from these average frequencies, derives atomic parameters such as populations of valence orbitals, partial ionic charges, π‐bond orders, and covalent bond orders of the quadrupolar atom. The article mainly reviews selected applications of halogen (35Cl,79, 81Br,127I) NQR to specific bonding questions in inorganic chemistry, and divides these according to whether the halogen or other atoms are (a) single bonded, (b) single bonded but with secondary bonds with other atoms, or (c) bonded to several atoms. For singly bonded atoms, the applications reviewed include (a1) relativistic effects, (a2) effects of nearby ionic charges on haloacetate ions, (a3) the contrasting effects of nearby ionic charges on halometallate ions, (a4) effects of intramolecular partial charges on chlorines in organosilicon compounds, (a5) cis‐and trans‐ ligand effects in substituted halometallate complexes, and (a6) interpretations of multicentered bonding in trihalide ions. Secondary bonding of single‐bonded halogen atoms are reviewed particularly in the cases of (b1) halocarbons as ligands, (b2) chloroacetates and chlorophenolates as chlorine‐donor ligands, and (b3) in salts of weakly coordinated halogenated anions. Finally, some illustrations are given of NQR studies of central multicoordinated atoms (for example, transition‐metal atoms in complexes).
Nuclear Resonance Vibrational Spectroscopy (
NRVS
)
Authors: Zeng Weiqiao , Silvernail Nathan J. , Scheidt W. Robert , Sage J. Timothy
Published: December 2011

Nuclear Resonance Vibrational Spectroscopy (NRVS) utilizes the simultaneous excitation of the Mössbauer nucleus and vibrational quanta. Vibrational features appear as sidebands on the recoilless resonance. NRVS reveals the complete vibrational spectrum of the probe nucleus with the resolution almost equal to that of Raman or infrared. Major advantages include selectivity to vibrations of the Mössbauer isotope with quantitative information on vibrational amplitudes, as well as frequencies. Single crystal measurements are selective for motion along the exciting X‐ray beam. NRVS is equally applicable to all ligation and oxidation states. Questions that can be addressed include the strength of metal coordination, the energetic cost of probe displacement, discrimination of metal‐ligand vibrations from cofactor or peptide vibrations, and the local vs. global character of active site vibrations.
Photoelectron Spectroscopy
Authors: Gruhn Nadine E. , Lichtenberger Dennis L.
Published: December 2011

Photoelectron spectroscopy measures the energies for emission of electrons from molecules or materials through application of the photoelectric effect. For neutral molecules this is a direct measure of the ionization energies. The ionizations are explicitly defined in terms of transitions between the ground state of a molecule and accessible states of the molecular positive ion, but in a first order approximation, each ionization of a molecule can be described as removal of an electron from an individual orbital. The ionization energies can then be considered as measures of orbital energies, and shifts can be interpreted in terms of orbital stabilizations or destabilizations due to changes in electron distributions and bonding. The intensities and profiles of the ionization bands in the photoelectron spectra provide additional information on the characters and bonding natures of the orbitals. All orbitals can be probed, from valence orbitals such as lone pairs and bonds to orbitals in the cores of heavy atoms. Photoelectron spectroscopy is a large field of study that has many subfields and new techniques continue to be introduced. These different subfields all have different instrumental and sample constraints, but taken as a whole photoelectron spectroscopic experiments can be performed on almost any system of interest in inorganic chemistry. This article summarizes the basic physical principles of photoelectron emission caused by one or more photons, the relationship of photoelectron spectra to the electronic structure of neutral and anionic molecules, and the additional considerations for photoionization of bulk samples. Auger electron spectroscopy (AES) and Penning ionization electron spectroscopy (PIES), techniques which are closely related to photoelectron spectroscopy, are also discussed. Case studies are given for applications of photoelectron spectroscopy to study of bioinorganic model complexes, including iron‐sulfur and molybdenum‐sulfur bonding and covalency, electronic structure of porphyrins and metalloporphyrins, negative‐ion photoelectron spectroscopy of phthalocyanines, and the use of the technique to measure inner‐sphere reorganization energies.
ResonanceRaman Spectroscopy
Authors: Czernuszewicz Roman S. , Zaczek Marzena B.
Published: December 2011

Resonance Raman (RR) spectroscopy is an invaluable and widely used probe of structure and bonding for chromophoric transition metal centers in proteins and enzymes. The spectacular increase in detection sensitivity and selectivity of RR spectroscopy for vibrations that are localized in the absorbing center(s) of the molecule has revolutionized the vibrational spectroscopic field of bioinorganic systems. In this article, we explain the characteristics of RR spectra, emphasizing the kind of information that can be obtained and the rules for optimal resonance enhancement. We then discuss the applications of RR scattering to the study of structural dynamics of two different mononuclear metal‐cysteinate sites, type‐1 copper in cupredoxins and Fe(S‐Cys)4center in rubredoxins, using a combination of site‐directed mutagenesis, production of isotopically labeled proteins, and cryogenic‐temperature RR methods. These chromophores, involved in biological electron transport, have been very suitable subjects for RR studies because they exhibit strong absorption bands in the visible region due to (Cys)S → Cu(II) (∼625 nm) and (Cys)S → Fe(III) (∼500–570 nm) charge‐transfer electronic transitions that facilitate the observation of resonantly enhanced Cu‐and Fe‐thiolate vibrations.
X‐Ray Absorption Spectroscopy
Authors: Bencze Krisztina Z. , Kondapalli Kalyan C. , Stemmler Timothy L.
Published: December 2011

X‐ray absorption spectroscopy (XAS), often referred to as X‐ray absorption fine structure (XAFS) spectroscopy, is a powerful technique available for determining the local coordination site structure and electronic properties of specific atoms in molecules. XAS can be performed on molecules in the gaseous, solution, or solid states, and generally every element is spectroscopically active. This technique utilizes high intensity X rays supplied by synchrotron radiation sources to selectively excite core electron transitions for a specific element of choice. XAS spectra are divided into two spectral regions, the X‐ray absorption near edge structure (XANES) region and the extended X‐ray absorption fine structure (EXAFS) region. XANES spectra can be utilized to provide: (1) qualitative information regarding the ligands coordinated to the absorbing atom, (2) semi‐quantitative information regarding the redox state of the absorbing atom, and (3) metal‐ligand coordination geometry. Simulations of the EXAFS data can provide information regarding absorber‐scatterer bond length values to extremely high accuracy (ca. ±0.02 Å), as well as coordination numbers and ligand identity at reduced accuracies. The high accuracy in bond lengths achieved from XAS makes this technique extremely sensitive for measuring subtle structural perturbations within molecules under perturbed or turnover conditions. When utilized with other techniques, XAS can supply the metrical parameters required to understand the chemical properties of specific elements in a molecule.