Inorganic chemistry
[[Image:Inorganic-montage.png|thumb|right|150px|
Inorganic compounds show rich variety:A: {{Diborane}} features {{Three-center two-electron bond|unusual bonding}}
B: {{Caesium chloride}} has an archetypal {{crystal structure}}
C: {{Cyclopentadienyliron dicarbonyl dimer|Fp
2}} is an {{organometallic}} {{Category:Iron compounds|iron compound}}
D: {{Polydimethylsiloxane|Silicone}}'s uses range from {{breast implant}}s to {{Silly Putty}}
E: {{Grubbs' catalyst}} won the {{Nobel Prize in Chemistry|2005 Nobel Prize}} for {{Robert H. Grubbs|its discoverer}}
F: {{Zeolite}}s find extensive use as {{molecular sieve}}s
G: {{Copper(II) acetate}} surprised {{Theoretical chemistry|theoreticians}} with its {{diamagnetism}}]]
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.
The bulk of
inorganic compounds occur as
salts, the combination of
cations and
anions joined by
ionic bonding. Examples of cations are
sodium Na
+,
magnesium and Mg
2+ and examples of anions are
oxygen O
2âˆ' and
chlorine Cl
âˆ'. As the salt is neutral these ions form compounds such as
sodium oxide Na
2O or
magnesium chloride MgCl
2. The ions are described by their
oxidation state and their ease of formation can be inferred from the
ionization potential (for cations) or from the
electron affinity (anions) of the parent elements.
Important classes of inorganic compounds are the
oxides, the
carbonates, the
sulfates and the
halides. Many inorganic compounds are characterized by high
melting points. Inorganic salts typically are poor
conductors in the solid state. Another important feature is their solubility in e.g. water (see:
solubility chart), and ease of
crystallization). Where some salts (e.g.
NaCl) are very soluble in water, others (e.g.
SiO2) are not.
The simplest
inorganic reaction is
double displacement when in mixing of two salts the ions are swapped without a change in oxidation state. In
redox reactions one reactant, the
oxidant, lowers its oxidation state and another reactant, the
reductant, has its oxidation state increased. The net result is an exchange of
electrons. Electron exchange can occur indirectly as well, e.g. in
batteries, a key concept in
electrochemistry.
When one reactant contains hydrogen atoms, a reaction can take place by exchanging protons in
acid-base chemistry. In a more general definition, an acid can be any chemical species capable of binding to electron pairs is called a
Lewis acid; conversely any molecule that tends to donate an electron pair is referred to as a
Lewis base. As a refinement of acid-base interactions, the
HSAB theory takes into account polarizability and size of ions.
Inorganic compounds are found in nature as
minerals. Dig in the ground and you might find iron sulfide as
pyrite or calcium sulfate as
gypsum. Inorganic compounds are also found multitasking as
biomolecules: as electrolytes (
sodium chloride), in energy storage (
ATP) or in construction (the
polyphosphate backbone in
DNA).
The first important man-made inorganic compound was
ammonium nitrite for soil fertilization through the
Haber process. Inorganic compounds are synthesized for use as
catalysts such as
vanadium(V) oxide and
titanium(III) chloride, or as
reagents in
organic chemistry such as
lithium aluminium hydride.
Subdivisions of inorganic chemistry are
organometallic chemistry,
cluster chemistry and
bioinorganic chemistry. These fields are active areas research in inorganic chemistry, aimed toward new
catalysts,
superconductors, and
therapies.
Industrial inorganic chemistry
Inorganic chemistry is a highly practical area of science. Traditionally, the scale of a nation's economy could be evaluated by their productivity of sulfuric acid. The top 20 inorganic chemicals manufactured in Canada, China, Europe, Japan, and the US (2005 data):
["Facts & Figures Of The Chemical Industry" Chemical and Engineering News, July 10, 2006.]aluminium sulfate,
ammonia,
ammonium nitrate,
ammonium sulfate,
carbon black,
chlorine,
hydrochloric acid,
hydrogen,
hydrogen peroxide,
nitric acid,
nitrogen,
oxygen,
phosphoric acid,
sodium carbonate,
sodium chlorate,
sodium hydroxide,
Sodium silicate,
Sodium sulfate,
sulfuric acid, and
titanium dioxide,
Descriptive inorganic chemistry focuses on the classification of compounds based on their properties. Partly the classification focusses 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. When studying inorganic compounds, one often encounters parts of the different classes of inorganic chemistry (an organometallic compound is characterized by its coordination chemistry, and may show interesting solid state properties).
Different classifications are:
Coordination compounds
Classical coordination compounds feature metals bound to "
lone pairs" of electrons residing on the main group atoms of ligands such as H
2O, NH
3,
Clâˆ', and
CNâˆ'. 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 the
trans-
lanthanides and
trans-
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 two
enantiomers of
[Co((OH)2Co(NH3)4)3]6+, an early demonstrtion that chirality is not inherent to organic compounds. A topical theme within this specialization is supramolecular coordination chemistry.
[Lehn, J. M., Supramolecular Chemistry: Concepts and Perspectives, VCH: Weinhiem, 1995] *Examples: [Co(
EDTA)]
âˆ',
[Co(NH3)6]3+,
TiCl4(
THF)
2.
Main group compounds
These species feature elements from
groups 1, 2 and 13-18 (excluding hydrogen) of the periodic table. Due to their often similar reactivity, the elements in group 3 (
Sc,
Y, and
La) and group 12 (
Zn,
Cd, and
Hg) are also generally included.
[Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.]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 inorganic chemical synthesis.Typical main group compounds are SiO
2, SnCl
4, and N
2O. 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 in
DNA, 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.
*Examples:
Tetrasulfur tetranitride S
4N
4,
Diborane B
2H
6,
Silicones,
Buckminsterfullerene C
60.
Transition metal compounds
Compounds containing metals from group 4 to group 11 are considered transition metal compounds. Compounds with a metal from group 3 or from group 12 are sometimes also incorporated into this group, but also often classified as main group compounds.
Transition metal compounds show a rich coordination chemistry, varying from tetrahedral for titanium (e.g. TiCl
4) 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 haemoglobin.
*Examples:
iron pentacarbonyl,
titanium tetrachloride,
cisplatinOrganometallic compounds
Usually, organometallic compounds are considered to contain the M-C-H group.
[C. Elschenbroich, A. Salzer "Organometallics : A Concise Introduction" (2nd Ed) (1992); Wiley-VCH: Weinheim. ISBN 3527281657] 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 highly
lipophilic complexes such as
metal carbonyls and even metal
alkoxides.
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, H
2, and N
2. Because the ligands are petrochemicals in some sense, the area of organometallic chemistry has greatly benefited from its relevance to industry.
*Examples:
Cyclopentadienyliron dicarbonyl dimer (C
5H
5)Fe(CO)
2CH
3,
Ferrocene Fe(C
5H
5)
2,
Molybdenum hexacarbonyl Mo(CO)
6,
Diborane B
2H
6,
Tetrakis(triphenylphosphine)palladium(0) Pd[P(C
6H
5)
3]
4Cluster 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. 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 or
nanotechnology and specifically arise from the study of
quantum size effects in
cadmium selenide clusters. Thus, large clusters can be described an array of bound atoms intermediate in character between a molecule and a solid.
*Examples:
Fe3(CO)12,
B10H14, [Mo
6Cl
14]
2âˆ',
4Fe-4SBioinorganic compounds
These compounds occur (by definition) in nature, but the subfield includes anthropogenic species, such as pollutants and drugs (e.g.
cis-platin).
[S. J. Lippard, J. M. Berg "Principles of Bioinorganic Chemistry" University Science Books: Mill Valley, CA; 1994. ISBN 0-935702-73-3.] The field includes many compounds, e.g. the phosphates in DNA, but also metal complexes containing ligands that range from biological macromolecules, commonly
peptides, to ill-defined species such as
humic acid, and to
water (e.g. coordinated to
gadolinium complexes employed for
MRI).
*Examples:
hemoglobin,
methylmercury,
carboxypeptidaseSolid state compounds
This important area focuses on
structures,
[Wells, A.F. (1984). Structural Inorganic Chemistry, Oxford: Clarendon Press.] bonding, and the physical properties of materials. 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,
mineralogy, and
materials science.
*Examples:
silicon chips,
zeolites,
YBa2Cu3O7An alternative perspective on the area of inorganic chemistry begins with the
Bohr model of the atom and, using the tools and models of
theoretical chemistry and
computational chemistry, expands into bonding in simple and then more complex molecules. Precise quantum mechanical descriptions for multielectron species, the province of inorganic chemistry, is difficult. This challenge has spawned 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,
CuII2(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
has only one unpaired electron]]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 NH
3 is pyramidal whereas ClF
3 is T-shaped? For the transition metals,
crystal field theory allows one to understand the magnetism of many simple complexes, for example why does
[FeIII(CN)6]3âˆ' have only one unpaired electron, whereas [Fe
III(H
2O)
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.
[Cotton, F. A., Chemical Applications of Group Theory, John Wiley & Sons: New York, 1990] 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. 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.
As an instructional tool, Group Theory highlights commonalities and differences in the bonding otherwise disparate species, such as
WF6 and
Mo(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.
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.
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.
[R. G. Wilkins "Kinetics and Mechanism of Reactions of Transition Metal Complexes" Wiley-VCH Verlag; 2nd, 1991) ISBN 3527283897] 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(H
2O)
6]
n+::[M(H
2O)
6]
n+ + 6 H
2O* â†' [M(H
2O*)
6]
n+ + 6 H
2O:where H
2O* denotes
isotopically enriched water, e.g. H
217OThe 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. A fundamental redox reaction is "self-exchange", which involves the
degenerate reaction between an oxidant and a reductant. For example,
permanganate and its one-electron reduced relative
manganate exchange one electron::[MnO
4]
- + [Mn*O
4]
2- â†' [MnO
4]
2- + [Mn*O
4]
-Reactions at ligands
Coordinated ligands display reactivity distinct from the free ligands. For example, the acidity of the ammonia ligands in [Co(NH
3)
6]
3+ is elevated relative to NH
3 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, H
2, O
2, and C
2H
4. The industrial significance of these feedstocks drives the active area of catalysis.
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, and
acidity. With the advent of
quantum 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. For example, measurements on the
photoelectron spectrum of
methane demonstrated that describing the bonding by the two-center, two-electron bonds predicted between the carbon and hydrogen using
Valence Bond Theory is not appropriate for describing ionisation processes in a simple way. Such insights led to the popularization of
molecular orbital theory as fully delocalised orbitals are a more appropriate simple description of electron removal and electron excitation.
Commonly encountered techniques are:
*
X-ray crystallography: This technique allows for the 3D determination of
molecular structures.
*
Spectroscopy: Historically,
UV-
vis spectroscopy has been an important tool, since many inorganic compounds are strongly colored.
*
Electron-spin resonance: ESR (or EPR) allows for the measurement of the environment of
paramagnetic metal centra.
*
Electrochemistry:
Cyclic voltammetry and related techniques probe the redox characteristics of compounds.
*
NMR spectroscopy: Besides
1H and
13C many other "good" NMR nuclei (e.g. 11B, 19F, 31P, and 195Pt) give important information on compound properties and structure. Also the NMR of paramagnetic species can result in important structural information.
*Electron-nuclear double resonance (ENDOR) spectroscopy
*Mössbauer spectroscopyAlthough 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 or solubility of the component reactants.[Girolami, G. S.; Rauchfuss, T. B. and Angelici, R. J., Synthesis and Technique in Inorganic Chemistry, University Science Books: Mill Valley, CA, 1999] Soluble inorganic compounds are prepared using methods of organic synthesis. For metal-containing compounds that are reactive toward air, Schlenk and glove 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 using liquid nitrogen (b.p. 78K) or other cryogens. 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.
