Inorganic chemistry

Inorganic chemistry is the branch of chemistry concerned with the properties and behavior of inorganic compounds. This field covers all chemical compounds except the myriad organic compounds (compounds containing C-H bonds), which are the subjects of organic chemistry. The distinction between the two disciplines is far from absolute, and there is much overlap, most importantly in the sub-discipline of organometallic chemistry.

Descriptive inorganic chemistry

Descriptive inorganic chemistry focuses on a classification and properties of inorganic compounds. Classification is often based on the position of the heaviest element in the compound. A more common and more successful classification scheme focuses on structural families:

Main group compounds

These species feature elements of groups 13-18 and the alkali and alkaline earth metals. Elements in group 3 (Sc, Y, and La) and group 12 (Zn, Cd, and Hg) are generally included. Of course, many main group compounds are “organometallic” as they contain organic groups (example B(CH₃)₃). Furthermore, many main group compounds occur in nature, e.g. phosphate in DNA. Finally, carbon compounds lacking (many) hydrogen ligands are “inorganic” such as the fullerenes, buckytubes, binary carbon oxides.

Examples: S₄N₄, C₆₀, B₂H₆, silicones.

• History. Main group compounds have been known since the beginnings of chemistry, e.g. elemental sulfur and the distillable white phosphorus. Experiments on oxygen, O₂, by Lavoisier and Priestley not only identified an important diatomic gas, but opened the way for describing compounds and reactions according to stoichiometric ratios. The discovery of a practical synthesis of ammonia using iron catalysts by Carl Bosch and Fritz Haber in the early 1900’s deeply impacted mankind, demonstrating the significance of chemical synthesis.

Coordination compounds (or Werner-type complexes)

Classical coordination compounds feature metals bound to "lone pairs" of electrons residing on the main group atoms of ligands such as H₂O, NH₃, Cl^-, and CN^-. In this context, "metal" usually means groups 3-13, as well as the trans-lanthanides and trans-actinides. Of course, organometallic compounds are also coordination complexes. In fact, from a certain perspective, all chemical compounds are coordination complexes. The stereochemistry of coordination complexes can be quite rich, as hinted at by Werner's separation of two enantiomers of [Pt(S₅)₃]^2-, an early example that chirality is not inherent to organic compounds. A topical theme within this specialization is supramolecular coordination chemistry.

Examples: [Co(EDTA)]^-, [Co(NH₃)₆]³⁺. TiCl₄(THF)₂.

• History. Coordination complexes were known - although not understood in any sense - since the beginning of chemistry, e.g. Prussian blue and copper vitriol. The key breakthrough occurred when Alfred Werner proposed, inter alia, that Co(III) bears six ligands in an octahedral geometry. The theory allows one understand the difference between coordinated and ionic chloride in the cobalt ammine chlorides and to explain many of the previously inexplicable isomers. He resolved the first coordination complex into optical isomers, overthrowing the theory that chirality was inimically associated with carbon compounds.

Organometallic compounds

Usually, organometallic compounds are considered to contain the M-C-H group. Operationally, the definition of organometallic compound is more relaxed to include highly lipophilic complexes such as metal carbonyls and even metal alkoxides. In terms of electronic and atomic structures, organometallic compounds are simply coordination compounds. Differences arise because organic ligands are often sensitive to hydrolysis or oxidation, necessitating that organometallic chemistry employ more specialized preparative methods than was traditional in Werner-type complexes. Synthetic methodology, especially the ability to manipulate complexes in solvents of low coordinating power, enabled the exploration of very weak ligands such as hydrocarbons, H₂, and N₂. Because the ligands are petrochemicals in some sense, the area of organometallic chemistry has greatly benefited from its relevance to industry.

Examples: (C₅H₅)Fe(CO)₂CH₃, Fe(C₅H₅)₂. Mo(CO)₆, B₂H₆, and Pd[P(C₆H₅)₃]₄.

• History. Early developments: Cadet’s synthesis of methyl arsenic compounds related to the cacodylic acid, Zeise's platinum-ethylene complex, Edward Frankland’s discovery of dimethyl zinc, Ludwig Mond’s discovery of Ni(CO)₄, and Victor Grignard’s organomagnesium compounds. The abundant and diverse products from coal and petroleum led to Ziegler-Natta, Fischer-Tropsch, hydroformylation catalysis which employ CO, H₂, and alkenes as feedstocks and ligands. Recognition of organometallic chemistry as a distinct subfield culminated in the Nobel Prizes to Fischer and Wilkinson for work on metallocenes. In 2005, Chauvin, Grubbs, and Schrock shared the Nobel Prizes for metal-catalyzed alkene metathesis.

Cluster compounds

Clusters can be found in all classes of chemical compounds. According to the commonly accepted definition, a cluster consists minimally of a triangular set of atoms that are directly bonded to each other. Clusters occur in "pure" inorganic systems, organometallic chemistry, main group chemistry, and bioinorganic chemistry. The distinction between very large clusters and bulk solids is increasingly blurred. This interface is the chemical basis of nanoscience or nanotechnology and specifically arise from the study of quantum size effects in cadmium selenide clusters. Thus, clusters can be described an array of bound atoms intermediate in character between a molecule and a solid.

Examples: Fe₃(CO)₁₂, B₁₀H₁₄, [Mo₆Cl₁₄]^2-, and 4Fe-4S clusters.

• History. The development of metal carbonyl compounds led quickly to the isolation of Fe₂(CO)₉ and Fe₃(CO)₁₂. Linus Pauling characterized “MoCl₂” to contain Mo₆ octahedra. Rundle and Dahl discovered that Mn₂(CO)₁₀ featured an “unsupported” Mn-Mn bond, thereby verifying the ability of metals to bond to one another in molecules. F. Albert Cotton established that ReCl₃ was in fact the cluster Re₃Cl₉, which could be converted to a host of adducts without breaking the Re-Re bonds. Contemporaneously with the development of metal cluster compounds, numerous boron hydrides were discovered by Alfred Stock and his successors who popularized the use of vacuum-lines for the manipulation of volatile, air-sensitive materials. In the 1970's, ferredoxin was demonstrated to contain Fe₄S₄ clusters and later nitrogenase was shown to contain a strikingly distinctive MoFe₇S₉ active site.

Bioinorganic chemistry

These coordination (and occasionally organometallic) complexes occur in nature, by definition but the subfield includes anthropogenic species, such as pollutants and drugs such as cis-platin. The ligands range from biological macromolecules, commonly peptides, to ill-defined species such as humic acid and even water when considering the gadolinium complexes employed for MRI.

Examples: hemoglobin. methylmercury, carboxypeptidase.

• History. Paul Ehrlich used organoarsenic (“arsenicals”) for the treatment of syphilis, demonstrating the relevance of metals, or at least metalloids, to medicine, that blossomed with Rosenberg’s discovery of the anti-cancer activity of “cisplatin (cis-PtCl₂(NH₃)₂). The first protein ever crystallized (see James B. Sumner) was urease, which was later shown to contain nickel at its active site. Vitamin B₁₂, the cure for pernicious anemia was shown crystallographically by Dorothy Crowfoot Hodgkin to consist of a cobalt in a corrin macrocycle. The Watson-Crick structure for DNA demonstrated the key structural role played by phosphate-containing polymers.

Solid state compounds (including minerals)

This important area focuses on structures, bonding, and the physical properties of materials. Of course, all compounds can be made into solids, thus the field in principle touches on all matter. In practice, solid state inorganic chemistry emphasizes crystallography and properties that result from collective interactions between the subunits of the solid. Included in solid state chemistry are metals and their alloys or intermetallic derivatives. Related fields are condensed matter physics and materials science.

Examples; silicon chips, zeolites, YBa₂Cu₃O₇.

• History. Because of its direct relevance to products of commerce, solid state inorganic chemistry has been strongly driven by technology well in advance of atomic-level descriptions or academic studies. 20th century landmarks included zeolite- and platinum-based catalysts for petroleum processing in the 1950’s, high-purity silicon as a core component of microelectronic devices in the 1960’s, and “high temperature” superconductivity in the 1980’s. The invention of X-ray crystallography in the early 1900's by the Braggs was enabling.

Theoretical inorganic chemistry

An alternative perspective on the area of inorganic chemistry begins with the Bohr model of the atom and expands into bonding in simple and then more complex molecules. Most tools and models of theoretical chemistry are applied to analyze inorganic molecules and their properties. Exact descriptions of bonding by quantum mechanics is extremely difficult for multielectron species, which is the province of inorganic chemistry. This challenge has spawn many semi-quantitative or semi-empirical approaches including molecular orbital theory and ligand field theory, In parallel with these theoretical descriptions, approximate methodologies are employed, including density functional theory.

Exceptions to theories, qualitative and quantitative, are extremely important in the development of the field. For example, Cu^II₂(OAc)₄(H₂O)₂ is almost diamagnetic below room temperature whereas Crystal Field Theory predicts that the molecule would have two unpaired electrons. The disagreement between qualitative theory (paramagnetic) and observation (diamagnetic) led to the development of models for "magnetic coupling." These improved models led to the development of new magnetic materials and new technologies.

Qualitative theories

Inorganic chemistry has greatly benefited from qualitative theories. Such theories are easier to learn as they require little background in quantum theory. Within main group compounds, VSEPR theory powerfully predicts, or at least rationalizes, the structures of main group compounds, such as an explanation for why NH₃ is pyramidal whereas ClF₃ is T-shaped? For the transition metals, crystal field theory allows one to understand the magnetism of many simple complexes, for example why does [Fe^III(CN)₆]^3- have only one unpaired electron, whereas [Fe^III(H₂O)₆]³⁺ has five? A particularly powerful qualitative approach to assessing the structure and reactivity begins with classifying molecules according to electron counting, focusing on the numbers of valence electrons, usually at the central atom in a molecule.

Group Theory

A central construct in inorganic chemistry is Group Theory. Group Theory provides the language to describe the shapes of molecules according to their "point group symmetry". Group Theory also enables factoring and simplification of theoretical calculations.

Spectroscopic features are analyzed and described with respect to the symmetry properties of the, inter alia, vibrational or electronic states. Knoweldge of the symmetry properties of the ground and excited states allows one to predict the numbers and intensities of aborptions. A classic application of Group Theory is the prediction of the number of C-O vibrations in substituted metal carbonyl complexes. The most common applications of symmetry to spectroscopy involve vibrational and electronic spectra.

As an instructional tool, Group Theory highlights commonalities and differences in the bonding otherwise disparate species, such as WF₆ and W(CO)₆ or CO₂ and NO₂.

Reaction pathways

The theory of chemical reactions is more challenging than the theory for a static molecule. Marcus theory provides a powerful linkage between bonding, mechanism, and reactivity. The relative strengths of metal-ligand bonds, which can be calculated theoretically, anticipates the kinetically accessible pathways.

Thermodynamics and inorganic chemistry

An alternative quantitive approach to inorganic chemistry focuses on energies of reactions. This approach is highly traditional and empirical, but it is also useful. Broad concepts that are couched in thermodynamic terms include redox potential, acidity, phase changes. A classic concept in inorganic thermodynamics is the Born-Haber cycle, which is used for assessing the energies of elementary processes such as electron affinity, some of which cannot be observed directly.

Mechanistic inorganic chemistry

An important and increasingly popular aspect of inorganic chemistry focuses on reaction pathways. The mechanisms of reactions are discussed differently for different classes of compounds.

Main group elements and lanthanides

The mechanisms of main group compounds of groups 13-18 are usually discussed in the context of organic chemistry (organic compounds are main group compounds, after all). Elements heavier than C, N, O, and F often form compounds with more electrons than predicted by the octet rule, as explained in the article on hypervalent molecules. The mechanisms of their reactions differ from organic compounds for this reason. Elements lighter than carbon (B, Be, Li) as well as Al and Mg often form electron-deficient structures that are electronically akin to carbocations. Such electron-deficient species tend ro react via associative pathways. The chemistry of the lanthanides mirrors many aspects of chemistry seen for aluminium.

Transition metal complexes

Mechanisms for reactions transition metals are discussed differently from main group compounds. The important role of d-orbitals in bonding strongly influences the pathways and rates of ligand substitution and dissociation. These themes are covered in articles on coordination chemistry and ligand. Both associative and dissociative pathways are observed.

An overarching aspect of mechanistic transition metal chemistry is the kinetic lability of the complex illustrated by the exhange of free and bound water in the prototypical complexes [M(H₂O)₆]ⁿ⁺:

[M(H₂O)₆]ⁿ⁺ + 6 H₂O* → [M(H₂O*)₆]ⁿ⁺ + 6 H₂O

where H₂O* denotes isotopically enriched water, e.g. H₂¹⁷O

The rates of water exchange varies by 20 orders of magnitude across the periodic table, with lanthanide complexes at one extreme and Ir(III) species being the slowest.

Redox reactions

Redox reactions are prevalent for the transition elements. Two classes of redox reaction are considered: atom-transfer reactions, such as oxidative addition/reductive elimination, and electron-transfer.

Reactions at ligands

Coordinated ligands display reactivity distinct from the free ligands. For example, the acidity of the ammonia ligands in [Co(NH₃)₆]³⁺ is elevated relative to NH₃ itself. Alkenes bound to metal cations are reactive toward nucleophiles whereas alkenes normally are not. The large and industrially important area of catalysis hinges on the ability of metals to modify the reactivity of organic ligands. Homogeneous catalysis occurs in solution and heterogeneous catalysis occurs when gaseous or dissolved substrates interact with surfaces of solids. Traditionally homogeneous catalysis is considered part of organometallic chemistry and heterogeneous catalysis is discussed in the context of surface science, a subfield of solid state chemistry. But the basic inorganic chemical principles are the same. Transition metals, almost uniquely, react with small molecules such as CO, H₂, O₂, and C₂H₄. The industrial significance of these feedstocks drives the active area of catalysis.

Physical methods in inorganic chemistry

Because of the diverse range of elements and the correspondingly diverse properties, inorganic chemistry is closely associated with many measurement tools. Older tools tended to examine bulk properties such as the electrical conductivity of solutions to test for ionization, melting points, solubility, and acidity. With the advent of quantum theory and the corresponding expansion of electronic apparatus, inorganic chemists employ tools to probe the electronic properties of inorganic molecules and solids. Often these measurements provide insights relevant to theoretical models. For example, measurements on the photoelectron spectrum of methane demonstrated that the bonding between the carbon and hydrogen is not described by the two-center, two-electron bonds predicted by Valence Bond Theory. Such insights led to the popularization of molecular orbital theory.

X-ray crystallography

Probably the single most powerful tool in inorganic chemistry is X-ray crystallography. Small crystals, typically 0.1 mm on edge, are required. The scattered X-rays are analyzed to give a 3-dimensional description of the locations of the atomic nuclei. The barrier to the application of this technique is the availability of high quality crystals. Consequently inorganic chemists dedicate substantial effort to growing crystals.

Optical spectroscopy

Historically, the most important spectroscopic tool in inorganic chemistry is UV-vis spectroscopy. The reason for the early development of this technique are easy to see, literally: many inorganic compounds are colored. Also the instrumentation, the spectrophotometers, are relatively cheap to build. Also called optical spectroscopy, this technique records the absorption of light over the range ca. 200-800 nm. The positions and the intensities of the absorptions give information on bonding. Although this technique continues to be important, the actual interpretation of the spectra is extraordinarily difficult, at least to the extent that one seeks insights into the structure or bonding in relatively complex chromophores. Variations on this technique include the use of circularly polarized light, sometimes in the presence of an external magnetic field. The goal of these "hyphenated techniques" is to increase the fine structure in the UV-vis spectrum by imposing new factors that affect the ability of the molecule to absorb light, the so-called selection rules.

Magnetism and electron-spin resonance

In contrast to the situation in organic chemistry, paramagnetism is common in inorganic chemistry. At the most basic level, the magnetic moment of compounds are measured to evaluate the number of unpaired electrons and their environment. Often the magnetism is recorded as a function of temperature and sometimes as a function of magnetic field strength. Paramagnetic compounds often often are amenable to Eletron spin resonance, which again gives insights into the environment of the unpaired spin.

Electrochemistry

Many metal complexes can lose or accept electrons at mild potentials, i.e. they are "redox-active." This fact has led to the widespread use of cyclic voltammetry and related electrochemical techniques to probe the redox characteristics of metal complexes and, in some cases, to generate the species using electrolysis, either in "bulk" or in a spectroscopic cell.

Heteronuclear NMR spectroscopy

Commonly NMR spectroscopy focuses on hydrogen-containing species because ¹H is relatively highly sensitive nucleus and is highly abundant. ¹H-NMR spectroscopy is also extremely important in coordination and especially organometallic chemistry. Many other nuclei are also good NMR nuclei, due to their natural abundance and magnetic moment, and nuclear spin. ¹¹B, ¹⁹F, ³¹P, and ¹⁹⁵Pt are typical spin-1/2 nuclei that inorganic chemists commonly examine by NMR spectroscopy.

Other important techniques

• Electron-nuclear double resonance spectroscopy
• Mössbauer spectroscopy

Synthetic inorganic chemistry

Although some inorganic species can be obtained in pure form from nature, most are synthesized in chemical plants and in the laboratory.

Inorganic synthetic methods can be classified roughly according the volatility of the components. Soluble inorganic compounds are prepared using methods of organic synthesis. For metal-containing compounds that are reactive toward air, Schlenk techniques are followed. Volatile compounds and gases are manipulated on “vacuum manifolds” consisting of glass piping interconnected through valves, the entirety of which can be evacuated to 0.001 mm Hg or less. Compounds are condensed using liquid nitrogen (b.p. 78K) or other cryogens. Solids are typically prepared using furnaces, the reactants and products being contained in fused silica (amorphous SiO₂) containers, but sometimes more elaborate materials such as welded Ta tubes or Pt “boats”. Products and reactants are transported between temperature zones to drive reactions.