When one reactant containshydrogen atoms, a reaction can take place by exchanging protons inacid-base chemistry. In a more general definition, any chemical species capable of binding to electron pairs is called aLewis acid; conversely any molecule that tends to donate an electron pair is referred to as aLewis base.[5] As a refinement of acid-base interactions, theHSAB theory takes into accountpolarizability and size of ions.
materials chemistry andsolid state chemistry, extended (i.e. polymeric) solids exhibiting properties not seen for simple molecules. Many practical themes are associated with these areas, includingceramics.
Inorganic chemistry is a highly practical area of science. Traditionally, the scale of a nation's economy could be evaluated by their productivity ofsulfuric acid.
Descriptive inorganic chemistry focuses on the classification of compounds based on their properties. Partly the classification focuses on the position in the periodic table of the heaviest element (the element with the highest atomic weight) in the compound, partly by grouping compounds by their structural similarities
Classical coordination compounds feature metals bound to "lone pairs" of electrons residing on the main group atoms of ligands such as H2O, NH3,Cl−, andCN−. In modern coordination compounds almost all organic and inorganic compounds can be used as ligands. The "metal" usually is a metal from the groups 3–13, as well as thetrans-lanthanides andtrans-actinides, but from a certain perspective, all chemical compounds can be described as coordination complexes.
The stereochemistry of coordination complexes can be quite rich, as hinted at by Werner's separation of twoenantiomers of[Co((OH)2Co(NH3)4)3]6+, an early demonstration that chirality is not inherent to organic compounds. A topical theme within this specialization is supramolecular coordination chemistry.[9]
Coordination compounds show a rich diversity of structures, varying from tetrahedral for titanium (e.g., TiCl4) to square planar for some nickel complexes to octahedral for coordination complexes of cobalt. A range of transition metals can be found in biologically important compounds, such as iron in hemoglobin.
These species feature elements fromgroups I, II, III, IV, V, VI, VII, 0 (excluding hydrogen) of the periodic table. Due to their often similar reactivity, the elements in group 3 (Sc,Y, andLa) and group 12 (Zn,Cd, andHg) are also generally included, and thelanthanides andactinides are sometimes included as well.[10]
Main group compounds have been known since the beginnings of chemistry, e.g., elementalsulfur and the distillable whitephosphorus. Experiments on oxygen,O2, byLavoisier andPriestley not only identified an importantdiatomic gas, but opened the way for describing compounds and reactions according tostoichiometric ratios. The discovery of a practical synthesis ofammonia using iron catalysts byCarl Bosch andFritz Haber in the early 1900s deeply impacted mankind, demonstrating the significance of inorganic chemical synthesis.Typical main group compounds are SiO2, SnCl4, and N2O. Many main group compounds can also be classed as "organometallic", as they contain organic groups, e.g., B(CH3)3). Main group compounds also occur in nature, e.g.,phosphate inDNA, and therefore may be classed as bioinorganic. Conversely, organic compounds lacking (many) hydrogen ligands can be classed as "inorganic", such as the fullerenes,buckytubes and binary carbon oxides.
Usually, organometallic compounds are considered to contain the M-C-H group.[11] The metal (M) in these species can either be a main group element or a transition metal. Operationally, the definition of an organometallic compound is more relaxed to include also highlylipophilic complexes such asmetal carbonyls and even metalalkoxides.
Organometallic compounds are mainly considered a special category because organic ligands are often sensitive to hydrolysis or oxidation, necessitating that organometallic chemistry employs 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 weakly coordinating 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.
Clusters can be found in all classes ofchemical compounds. According to the commonly accepted definition, a cluster consists minimally of a triangular set of atoms that are directly bonded to each other. But metal–metal bonded dimetallic complexes are highly relevant to the area. 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 ornanotechnology and specifically arise from the study ofquantum size effects incadmium selenide clusters. Thus, large clusters can be described as an array of bound atoms intermediate in character between a molecule and a solid.
By definition, these compounds occur in nature, but the subfield includes anthropogenic species, such as pollutants (e.g.,methylmercury) and drugs (e.g.,Cisplatin).[12] The field, which incorporates many aspects of biochemistry, includes many kinds of compounds, e.g., the phosphates in DNA, and also metal complexes containing ligands that range from biological macromolecules, commonlypeptides, to ill-defined species such ashumic acid, and towater (e.g., coordinated togadolinium complexes employed forMRI). Traditionally bioinorganic chemistry focuses on electron- and energy-transfer in proteins relevant to respiration. Medicinal inorganic chemistry includes the study of both non-essential andessential elements with applications to diagnosis and therapies.
This important area focuses onstructure,[13] bonding, and the physical properties of materials. In practice, solid state inorganic chemistry uses techniques such ascrystallography to gain an understanding of the properties that result from collective interactions between the subunits of the solid. Included in solid state chemistry are metals and theiralloys or intermetallic derivatives. Related fields arecondensed matter physics,mineralogy, andmaterials science.
In contrast to mostorganic compounds, many inorganic compounds are magnetic and/or colored. These properties provide information on the bonding and structure. The magnetism of inorganic compounds can be comlex. For example, most copper(II) compounds are paramagnetic butCuII2(OAc)4(H2O)2 is almost diamagnetic below room temperature. The explanation is due tomagnetic coupling between pairs of Cu(II) sites in the acetate.
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, thestructures of main group compounds, such as an explanation for whyNH3 is pyramidal whereasClF3 is T-shaped. For the transition metals,crystal field theory allows one to understand the magnetism of many simple complexes, such as why[FeIII(CN)6]3− has only one unpaired electron, whereas [FeIII(H2O)6]3+ has five. A particularly powerful qualitative approach to assessing the structure and reactivity begins with classifying molecules according toelectron counting, focusing on the numbers ofvalence electrons, usually at the central atom in a molecule.[citation needed]
A construct in chemistry ismolecular symmetry, as embodied inGroup theory. Inorganic compounds display a particularly diverse symmetries, so it is logical that Group Theory is intimately associated with inorganic chemistry.[14] Group theory provides the language to describe the shapes of molecules according to theirpoint 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. Knowledge of the symmetry properties of the ground and excited states allows one to predict the numbers and intensities of absorptions in vibrational and electronic spectra. 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.
Group theory highlights commonalities and differences in the bonding of otherwise disparate species. For example, the metal-based orbitals transform identically forWF6 andW(CO)6, but the energies and populations of these orbitals differ significantly. A similar relationship existsCO2 and molecularberyllium difluoride.
An alternative quantitative approach to inorganic chemistry focuses on energies of reactions. This approach is highly traditional andempirical, but it is also useful. Broad concepts that are couched in thermodynamic terms includeredox potential,acidity,phase changes. A classic concept in inorganic thermodynamics is theBorn–Haber cycle, which is used for assessing the energies of elementary processes such aselectron affinity, some of which cannot be observed directly.
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 theoctet rule, as explained in the article onhypervalent molecules. The mechanisms of their reactions differ from organic compounds for this reason. Elements lighter thancarbon (B,Be,Li) as well asAl andMg often form electron-deficient structures that are electronically akin tocarbocations. Such electron-deficient species tend to react via associative pathways. The chemistry of the lanthanides mirrors many aspects of chemistry seen for aluminium.
Transition metal and main group compounds often react differently.[15] 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 oncoordination chemistry andligand. 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 exchange 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* denotesisotopically 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 are prevalent for the transition elements. Two classes of redox reaction are considered: atom-transfer reactions, such as oxidative addition/reductive elimination, andelectron-transfer. A fundamental redox reaction is "self-exchange", which involves thedegenerate reaction between an oxidant and a reductant. For example,permanganate and its one-electron reduced relativemanganate exchange one electron:
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 ofcatalysis hinges on the ability of metals to modify the reactivity of organic ligands.Homogeneous catalysis occurs in solution andheterogeneous catalysis occurs whengaseous ordissolved substrates interact with surfaces of solids. Traditionallyhomogeneous catalysis is considered part of organometallic chemistry andheterogeneous catalysis is discussed in the context ofsurface 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. Ligands can also undergo ligand transfer reactions such astransmetalation.
Because of the diverse range of elements and the correspondingly diverse properties of the resulting derivatives, inorganic chemistry is closely associated with many methods of analysis. Older methods tended to examine bulk properties such as the electrical conductivity of solutions,melting points,solubility, andacidity. With the advent ofquantum theory and the corresponding expansion of electronic apparatus, new tools have been introduced to probe the electronic properties of inorganic molecules and solids. Often these measurements provide insights relevant to theoretical models. Commonly encountered techniques are:
Ultraviolet-visible spectroscopy: Historically, this has been an important tool, since many inorganic compounds are strongly colored
NMR spectroscopy: Besides1H and13C many other NMR-active nuclei (e.g.,11B,19F,31P, and195Pt) can give important information on compound properties and structure. The NMR of paramagnetic species can provide important structural information. Proton (1H) NMR is also important because the light hydrogen nucleus is not easily detected by X-ray crystallography.
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 to the volatility or solubility of the component reactants.[16] Soluble inorganic compounds are prepared using methods oforganic synthesis. For metal-containing compounds that are reactive toward air,Schlenk line andglove box techniques are followed. Volatile compounds and gases are manipulated in "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 usingliquid nitrogen (b.p. 78K) or othercryogens. Solids are typically prepared using tube furnaces, the reactants and products being sealed in containers, often made of fused silica (amorphous SiO2) but sometimes more specialized materials such as welded Ta tubes or Pt "boats". Products and reactants are transported between temperature zones to drive reactions.
^Girolami, G.S.; Rauchfuss, T.B.; Angelici, R.J. (1999).Synthesis and Technique in Inorganic Chemistry (3rd ed.). Mill Valley, CA: University Science Books.ISBN978-0-935702-48-4.
