Coordination complex

The complex chemistry (Latin complexum, " surrounded ", " hugging ", " embrace" ) is an area of ​​inorganic chemistry. The term coordination chemistry is used synonymously in general. The complex chemistry is concerned with complexes or coordination compounds, which are composed of one or more central particle and one or more ligands. The central particles are usually atoms of transition metals, which may be charged or uncharged.

In contrast to conventional covalent bonds in complexes usually control the ligand to bind at all electrons, instead of each reaction partner contributes with one electron to an electron pair bond; but there is still more complex covalent nature. The formation of complexes can thus be understood as an acid -base reaction in the sense of the Lewis definition, in which the ligands (bases ) act as electron pair donors and the central particles (acid) through gaps in its electron configuration as the acceptor.

Complex compounds play in different areas an important role in the art ( for example as a catalyst, see Figure of the Grubbs catalyst right), in biology (hemoglobin and chlorophyll ), in medicine ( chelation therapy ) or in analytical chemistry. Compounds with organic ligands are also the subject of organometallic chemistry. Since complexes of transition metals are sometimes very colorful, this is also used as dyes or pigments ( Prussian blue ) are. Particularly intense stainings show the charge-transfer complexes, such as permanganate.

For a long time chemists had no understanding of the structure of coordinative compounds which are designated as " higher order compounds ". In addition, many behaviors of complexes with the former theories could not explain how, for example, the stability of cobalt (III ) chloride in aqueous solution upon addition of ammonia. For the correct interpretation of the structure and formation conditions in complexes of the Swiss Alfred Werner was awarded in 1913 as the first and for decades the only inorganic chemist awarded the Nobel Prize for Chemistry.

  • 3.1 central particle
  • 3.2 ligands 3.2.1 Anionic ligands
  • 3.2.2 Neutral ligands
  • 3.2.3 Cationic ligands
  • 3.2.4 Organic ligands
  • 4.1 coordination number and coordination polyhedra
  • 4.2 symmetry
  • 5.1 linkage isomerism
  • 5.2 Geometrical isomerism 5.2.1 cis-/trans-Isomerie
  • 5.2.2 fac - and mer - arrangement
  • 7.1 VB theory
  • 7.2 Crystal and ligand field theory
  • 7.3 MO theory
  • 9.1 sandwich complexes
  • 9.2 Polynuclear complexes
  • 9.3 Macrocyclic metal complexes

History

The systematic study of the structure of complex compounds began in the late 19th century and is primarily characterized by Sophus Mads Jørgensen and the name Alfred Werner. Jørgensen made ​​Although the synthesis of many new complexes a big name, but was a supporter of the regime established by Christian W. Blomstrand " chain theory." According to this theory, the ligand rows one behind the other and thus form chains, which acts nowadays makes little sense, but nevertheless took into account the valence of the metal.

Werner, who was for decades as an opponent Jørgensen, formulated in 1892, however, a theory that is in principle still valid today, and was first published in the following year. The derivable statements about about possible stereoisomerism could be experimentally confirmed and consolidated the theory over the years. A few years after being awarded the Nobel Prize in Chemistry in 1913 Werner died.

In the following decades resulted in the development of new theories of chemical bonding and new technologies such as X-ray crystallography to great advances in coordination chemistry. In addition, it has since appeared more chemistry Nobel prizes in the field of coordination chemistry, for example, 1973 for a study of so-called sandwich complexes by Ernst Otto Fischer and Geoffrey Wilkinson, or 2005 for the description of the olefin and the development of suitable ( complex ) catalysts by Yves Chauvin, Robert Grubbs and Richard R. Schrock.

Nomenclature

Complex formulas

The oxidation number is determined by considering the original charge of the central atom, as if all ligands were removed under entrainment of the common electron pairs. The sum of the charge contributions of the ligands and the oxidation number of / the central particle must equal the charge of the complex.

Complex names

In the systematic naming of complex salts are added to the first cation and the anion, whether complex or not, respectively. The naming of the components of a coordination unit occurs in the following order:

The full name of the coordination unit is written as one word. Except for the names of the ligands aqua, ammine nitrosyl and the names of all neutral ligands are enclosed in parentheses. The names of inorganic anionic ligands are then placed in parentheses if they already contain numeric prefixes or if this ambiguity can be avoided. In the name of complex salts, a hyphen is written between the name of the cation and the anion.

Structure and properties of complexes

Central particle

As a central particles are mainly transition metals into consideration that have sufficient free d- orbitals with which the ligands can join. Just as often occur complexes even with lanthanides and actinides. The number of valence electrons influenced type, stability, and structure of the complexes formed. Explanations for this can be found in corresponding theories will be discussed later. Examples of central particle are:

  • Cationic central ions: Cu2 , Mg2 , Fe2 , Fe3 , Ni2
  • Neutral central particle: Fe0, Cr0, Ni0

Usually, the metal in a complex have a positive oxidation state, but also compounds with metal atoms of the oxidation state of zero can be prepared by the reaction of metal or metal vapor with the corresponding ligand. An example is the reaction of nickel with carbon monoxide to tetracarbonyl ( Mond process ) or of iron to iron pentacarbonyl. Such reactions may also be used for purification of the corresponding metals ( Chemical Transport ). By the reaction of metals with ligands from the gas phase, for example, carbonyl, phosphine, olefin, aromatic and cyclopentadienyl be prepared in which the formation is connected especially in the latter but with a redox reaction.

Ligands

Complexes can consist of identical or different ligands. A complex contains exclusively similar ligands, it is referred to homoleptically, otherwise heteroleptic. Since complexes in the number of partners is often independent of the oxidation state of the central particle in a bond with the central particle, there is also for complexes originating from the 19th century term " compounds of higher order."

Ligands may contribute different numbers of electrons in the bond, for example bring Cl and PPh3 two electrons with one, η5 -Cp and η6 - C6H6 six electrons, unbridged μ1 -CO ( see bridge (chemistry) ) two electrons bridged μ2 -CO an electron ( see hapticity ). Ligands can not only bring different number of electrons, but coordinate with appropriate size with multiple locations simultaneously on the same central particle. The number of bonds this possible is called denticity. The usual simple ligands such as aqua, ammine (NH3), chlorido (Cl - ) or Cyanido (CN- ) are all monodentate or monodentate, and bind eg H3N -M.

Ligands that have multiple coordination sites for the same metal, are referred to as chelating ligands (Greek chele, " crab claw "). The chelate complexes thus formed possess both thermodynamically and kinetically greater stability. High thermodynamic stability due to the increase in the entropy of the system, since, for example, to form a octahedral complex with a bidentate ligand ( ligand having two coordination sites ) runs the following reaction, in aqueous solution:

Here are four free particles ( on the left ) seven free particles ( on the right ). The kinetic stability based on the fact that ( according to the kinetic theory of gases ) have to make fewer particles to form the complex and all bonds of a ligand to the central particle must be opened simultaneously in the dissociation.

Include the common chelating bidentate ethylenediamine (en ), the tetradentate nitrilotriacetic acid (NTA ), and the hexadentate ethylenediaminetetraacetate (EDTA). The latter is used medically for example to bind toxic heavy metals such as lead or mercury in the body and then excrete ( chelation therapy ).

Anionic ligands

  • H-( hydrido)
  • F ( fluorido ); Cl ( chlorido ); Br ( bromido ); I- ( iodido )
  • O2 - ( oxido ) - O22 ( peroxido ); OH ( hydroxido )
  • Oxalate (ox)
  • Acetylacetonate (acac)
  • S2 - (thio, sulfido ); SO42 - ( sulfato ); S2O32 ( thiosulfato )
  • SCN- ( thiocyanato and isothiocyanato with coordination through N)
  • CN ( cyanido, isocyanido in coordination via N or cyanido -C and cyanido -N)
  • NO2 ( nitrito, nitro with coordination through N and nitrito -N and nitrito -O); NO3- ( nitrato )

Neutral ligands

  • NH3 ( ammine)
  • Ethylenediamine (en )
  • Ethylenediaminetetraacetate (EDTA)
  • Porphine / porphyrins ( important in biochemistry )
  • Nitrilotriacetic acid ( NTA)
  • H2O ( aqua, aquo outdated )
  • CO ( carbonyl)
  • NO ( nitrosyl )

Cationic ligands

  • NO ( nitrosyl )

Organic ligands

  • Cyclopentadienyl (Cp)

Geometry

Coordination numbers and coordination polyhedra

The coordination number (CN ) indicates how many monodentate ligands surrounding a central particle. Depending on the coordination number to arrange the ligands in certain arrangements around the center, which often, but not always, coincide with the predictions of the VSEPR model. If we imagine between the ligands connecting lines, we obtain the coordination polyhedra with which the structure of complexes is usually described. Accessible are coordination numbers 2-9, beyond numbers can only be achieved with particularly large central particle and chelating ligands. Most commonly, however, the coordination numbers 4 and 6 after the VSEPR model, the following polyhedra are accepted:

  • KZ 2: a linear complex, eg [ AuCl2 ] -
  • KZ 3 is a trigonal planar or trigonal aplanare structure ( the central particle is not exactly in the middle of the triangle, but slightly above )
  • KZ 4: a tetrahedron or a square-planar structure
  • KZ 5: a square-pyramidal or trigonal- bipyramidal structure; both can be converted by the Berry pseudorotation each other and are at an appropriate temperature in equilibrium
  • KZ 6: usually an octahedron, in part, a trigonal antiprism or ( less frequently) a trigonal prism
  • KZ 7 (very rare): a pentagonal bipyramid, an easy überkapptes octahedron or a trigonal prism simply überkapptes
  • KZ 8: a square antiprism, trigonal dodecahedron, a doubly überkapptes trigonal prism or, more rarely, a hexahedron (cube )
  • KZ 9: a triple überkapptes trigonal prism, for example, [ ReH9 ] 2 -

Noteworthy is otherwise still the coordination number 12, which results in an icosahedron or cuboctahedron.

Symmetry

Since the central particle and ligand of a stable complex as are the ions occupy geometrically ordered structures within crystal lattices, they are assigned certain point groups. The marking is usually done after the Schoenflies symbolism.

Isomerism

When a plurality of distinguishable compounds of one and the same summation formula exist, it is called (as in -organic compounds ) of isomerism. For complexes of the geometrical, optical and bonding isomerism are relevant.

Linkage isomerism

The linkage isomerism can occur when a ligand can coordinate with different atoms. Thus, for example, the ligand SCN both bind with the sulfur atom ( thiocyanato ) as well as with the nitrogen atom ( isothiocyanato ).

Geometric isomerism

Depending on the coordination number and composition of a complex compound, occur cis-/trans-Isomerie or facial or meridional arrangement.

Cis-/trans-Isomerie

The cis-/trans-Isomerie occurs in square planar ( coordination number CN = ​​4 ) and octahedral ( CN = ​​6 ) complexes. Shown in the adjacent figure, the cis and trans isomers are one octahedral compound of general formula MA4BL, where L is either another ligand of the type B, and may represent a ligand of the third kind C. For the description is only relevant if the two "special" ligand ( B twice or once B and C once ) to each other ( cis) or order apart ( trans). Accordingly, the four ligands of type A is a rocker ( cis ) or a square plane ( trans).

When the two A ligands that do not lie on the equatorial plane, mentally removed in the sketch, you get a square planar complex with the general formula MA2BL. As with the octahedral complex is immaterial whether B and L to each other ( cis) or away from (trans ) have. When the square planar complex present four different types of ligands, the number of possible isomers increases to three.

Fac - and mer - arrangement

The facial (fac ) and meridional (mer ) assembly occurs for octahedral complexes of the general formula MA3B3. Show the ligands of the type A and the clearly separate from type B by a plane containing the central particle M, each have three ligands of fac - arrangement in one direction as two oppositely facing faces (german faces ). If the ligands of the two species do not separate from each other, but only on orthogonal planes of pure grade A or B can be split, one speaks of the mer arrangement, as always need are three identical ligands on the meridian of the sphere whose surface all six ligands involves.

Optical isomerism

The requirement for optical isomerism is chirality, wherein a metal complex may also include a chiral metal center without a ligand molecule itself is chiral. Exist depending on the type of coordination different requirements that must be met for optical isomerism occurs:

  • Tetrahedral ( CN = ​​4 ) metal complexes are chiral, with four different ligands are bound to the metal center.
  • Square planar ( CN = ​​4 ) metal complexes are chiral, as soon as sterically demanding ligands interfere with each other, and thus prevent the rotation around a ligand -metal bond.
  • Octahedral ( CN = ​​6 ) Metal complexes often have chiral centers, if they contain chelating ligands.

Formation of complexes

The complex formation reaction is a classic acid-base reaction, according to the theory of Gilbert Newton Lewis. Herein, the central particle is the Lewis acid ( electron pair acceptors ); the ligand is a Lewis base, which is a molecule or an ion which may provide at least one lone pair (electron pair donor ) to form a bond.

An example of a typical complex formation is the addition of water to copper ( II) sulfate. The colorless salt reacts with water to form a blue complex:

Here, the Cu2 as a Lewis acid and the water reacts with its lone pairs as a Lewis base and there is a Hexaaquakomplex. This reaction is often used because of the highly visible effect in chemistry lessons at school as proof of water.

The type of chemical bond, resulting from the formation reaction is carried out (even - outdated - as a dative bond or a donor - acceptor bond ) as a coordinate bond denoted and thus of the other forms of chemical bond ( covalent bond, ionic bond, metallic bond) distinguished. This ( controversial ) distinction is justified by the fact that the bonding electron pair originally mostly alone originates from the ligand, and not (as in a covalent bond ) one electron from each binding partner. In old textbooks, the binding is still partially marked by an arrow in the direction of the acceptor, but these representations are outdated. A coordinate bond is now drawn in analogy to the covalent bond as a line ( see, e.g., adjacent drawing), since a complex is not be regarded generally as a Lewis acid-base adduct, coordination may, however, as in the for homogeneous catalysis, also be carried out by oxidative addition, a portion of the bonding electrons are contributed by the metallic central atom, which can remain in this during reductive elimination again.

A typical example of a coordinate bond is the case shown in the adjacent sketch. Ammonia ( NH3) has a free electron pair which represents a coordinate bond to form the molecule H3N - BF3 available. Formally transmits the nitrogen atom to the boron atom in this case an electron, thereby the former (in general: the donor) a formal positive, the latter (generally the acceptor ) is replaced by a formal negative charge. Note that these formal charges have nothing to do with the actual charge distribution: As nitrogen has a considerably higher electronegativity as Boron (3.0 versus 2.0 ), the bond to the nitrogen through polarized (and the oxidation states remain unchanged).

In addition, there are complexes whose bonding is really worth only by sophisticated concepts (such as the molecular orbital theory ) adequately describe, such as metal clusters, sandwich complexes ( eg, bis ( benzene) chromium, Cylopentadienylmangan and ferrocene), the related half-sandwich compounds, but also olefin (such as Zeise's salt).

Bonding

The bond between central particles and ligands and the stability of complexes can be described in more detail by different models, which allow to make statements about properties such as color or magnetism.

VB theory

The earliest explanation provided the Valenzstrukturtheorie ( valence bond theory, VB theory ). This assumes that occupied ligand orbitals overlap with unoccupied orbitals of the central particle and thus form a bond. In order to explain the spatial structure of complexes, it is assumed that the formation of hybrid orbitals in the metal. Thus, the VB theory explains, although the geometry and magnetic properties, but not for example the color. With the 18- electron rule can also, in certain cases, the stability of transition metal complexes can be estimated, with the scope of the rule, however, is severely limited.

Crystal and ligand field theory

As a further applies the crystal field theory based on pure electrostatic interactions between the ligand and the central particle and can also explain the color of the complexes. The model is often mixed in the parlance of the ligand field theory, which extends the crystal field theory and examines the influence of the point-like ligands on the energies of the d orbitals of the central metal. This approach is due to their ability to be able to explain many features despite its simplicity, is still very common.

The energies of the d orbitals of the metal are initially degenerate ( same energy ). Now approaching one spherically symmetric ligand field, the energy of all the d- orbitals increases to the same extent. The ligand field theory now considered the spatial shape of these orbitals and the geometry of the ligand field in the form of point charges. If you look, for example, an octahedral complex, so it is found that in an octahedral ligand field the dz2 and dx2 - y2 orbitals are energetically located less favorable due to the geometry, and the orbitals dxy, dyz and dxz turn more favorable. The sum of these energy differences is the ligand field stabilization energy called ( LFSE ) and referred to in an octahedral complex with ΔO. The absolute value of this splitting can be experimentally determined by spectroscopy and is both the center as well as the ligand -dependent, but generally specified relative as 10 Dq. The influence of the centers and ligands on the splitting can be read on the spectrochemical series.

The increase in energy makes up 3 /5 of the LFSE and the reduction corresponding to 2 / 5th If all orbitals occupied, one arrives at the sum to 0 Dq, since the energy center of gravity of the d orbitals may not change (dashed line in the figure). The energetically higher orbitals are called eg orbitals ( e for two times "degenerate" ) and the lowered orbitals t2g orbitals (t for " triplet - degenerate" ). In a complex, these orbitals are now occupied according to the dog control with electrons. Deviation was that observed at high field splitting and the formation of electron pairs instead of the occupation with unpaired electrons, provided the LFSE is higher than the spin pairing energy. Such complexes are low-spin complexes called, in contrast to the usual high-spin complexes.

Similar considerations can also be set up for other coordination polyhedra as the octahedron. Thus, the ligand field theory provide simple explanations, among other things for the magnetic behavior of complexes by the pairing of electrons is considered, or even for the color, which can be explained by electron transitions between the orbitals. The geometrical distortion caused by the Jahn -Teller effect can be explained by the electron configuration here.

MO theory

However, the best results are achieved by molecular orbital theory of the ligand field theory is only an excerpt. It deals with both the central particle and the ligand quantum mechanically and is therefore the most accurate, but also at the most demanding.

Stability

To estimate or explanation of the stability of complex compounds into account several considerations can be drawn.

Spectrochemical series

The experimentally established spectrochemical series assigns ligands and metal particles according to the strength of the damage they have ligand field splitting. When the ligand is thus results in the following order:

In this series of Iodido ligand causes the smallest splitting of the carbonyl ligand and the largest. This list is basically the same base strength according to the Lewis concept. Similarly, you can also sort the metal:

From this, the rule of thumb, it is clear that higher ionic charges cause a higher decomposition. The farther to the right, a particle is, the more likely it is therefore also a low-spin configuration in a corresponding complex.

HSAB concept

The principle of hard and soft acids and bases by Ralph G. Pearson states that hard acids prefer to react with hard bases and soft acids with soft bases accordingly. As hard particles are denoted by a high charge density and soft as those with low charge density, which are easily polarizable. This concept can also be applied to the stability of complex compounds.

Return bond

The MO theory gives further insight into the stability, for example, of metal carbonyls. Thus, in complexes all serve as ligands σ -donors, but strong ligands such as CO are also strong π - acceptors. The σ - Hinbindung done through the HOMO of CO, which overlaps with an empty d- orbital of the metal. In addition, however, the LUMO of the CO overlap with a filled d orbital of the metal of suitable symmetry and thus accomplishes a π back-donation, which helps the complex to particular strength. In addition, this back-bonding reinforced again the σ - Hinbindung why this is called synergy.

Application of the mass action law

For a quantitative description of the stability of complexes, the equilibrium constants can be set up as the Lewis acid -base reactions of complex formation reactions are equilibrium reactions, for which the law of mass action can be established. The overall reaction can be divided into steps (so called elementary reactions ), i.e., respectively for the attachment of a ligand. The product of the equilibrium constants of the individual elementary reaction for complex formation then gives the equilibrium constant for the overall reaction.

The resulting constant is called the complex formation constant. This constant indicates also how stable the complex is or whether he tends to dissociate. Therefore, the complex formation constant is also called complex stability constant or complex association constant KA. Its reciprocal is referred to as KD Komplexdissoziationskonstante so KD = KA -1. The higher the complex formation constant K is, the more stable the complex, the smaller, the more easily is the dissociation.

Special complex compounds

Sandwich complexes

In sandwich complexes, the metal centers are of two planar and cyclic organic ligands included as two buns, which is why they gave the name said this type of connection. The sandwich complexes include the metallocenes and the " piano-stool complexes ". The most common ligand is the cyclopentadienyl anion (Cp ) having six π - electrons, and thus aromatic (see Hückel rule). But there are also corresponding complexes with benzene as ligand possible, for example, bis ( benzene) chromium, or uranocene with cyclooctatetraenyl ligands.

The first sandwich complex was synthesized in 1951 the ferrocene, but its possible structure for some time raised puzzles. Ernst Otto Fischer, Geoffrey Wilkinson and Robert B. Woodward clarified the correct structure finally independently on what the first two in 1973 received the Nobel Prize in Chemistry. Ferrocene was discovered in research on catalysts randomly and fell by the unusual stability of its orange -colored crystals on. By meeting the 18- electron rule, it is more stable than similar compounds with other metals, such as cobaltocene or nickelocene.

The organic ligands bind in this type of compounds with their π - electrons to the metal center. Since this does not necessarily happen with the whole ring, but also other bound states are possible, there is to describe these conditions the concept of hapticity. The hapticity η when ferrocene is for example 5, since each ligand binds with five atoms. This is also reflected in the notation of [Fe ( η5 -C5H5 ) 2] for the complex.

Polynuclear complexes

Polynuclear complexes containing more than one central particle. They are connected via a bridging ligand such as oxygen (O2, OH, H2O, CO) or chlorine. However, there are also complex compounds with (partly non-integer ) metal-metal multiple bonds, eg [ Tc2X9 ] 3 -, X = Cl, Br

Macrocyclic metal complexes

Certain natural antibiotics belonging to the class of cyclic peptides (e.g., valinomycin ), are capable of selectively binding potassium ions, and to transport. 1967 synthesized Charles Pedersen first crown ethers, which belong to the type of the macrocyclic polyether and which are able, in particular alkali metal and alkaline earth metal ions to complex and transport. Based on these macrocyclic polyethers, the group of Jean -Marie Lehn in 1969 for the first time synthesized a macrobicyclic ligands ( Azopolyether ), which was designated as cryptand. This also complexed in its cavity alkali and alkaline earth metal ions, the corresponding complexes were designated as cryptates. Subsequently, various Kryptande were prepared having different sized cavities and are therefore adapted to the size of the alkali metal or alkaline earth metal ions. The stability constants of the corresponding cryptates are relatively high, the complexes have good selectivity for the ions and are therefore suitable for the selective removal of ions from solutions. Also succeeded in the sequence to produce macrotricyclic Kryptande and other such heteroatoms. Many macrocyclic metal complexes also have a biological significance. Other examples are complexes with phthalocyanine as a ligand, as in the dye copper phthalocyanine.

Application

Biological Significance

In biology complexes play an important role. It can be either catalytically active proteins ( enzymes) or catalytically inactive proteins. Many enzymes contain complexes in their active sites. This topic is one of the focus areas of bioinorganic chemistry. In general, in this case there is a complexing metal atom which is not completely complexed by amino acid side chains as ligands. A ligand site acts as an active center for the implementation or temporary binding of the substrate. Most complex centers are iron, copper, zinc, calcium, magnesium and manganese. But there are also more unusual elements such as vanadium. Especially calcium, as well as zinc complexes have a structural importance (for example, zinc fingers in the DNA sequence recognition).

For non- catalytically active proteins, for example, porphyrin find as heme in hemoglobin and cytochromes, or the chlorophyll (each chelate ). Coordination compounds are thus responsible for ensuring that blood is red and a green leaf of a plant.

Technology

Complexes found in many chemical reactions used as catalysts. For example, carbene complexes are used in the above-mentioned olefin metathesis, for which there were a Nobel Prize in 2005, with ruthenium or molybdenum used (see Grubbs catalyst). Wilkinson's catalyst is a square planar rhodium ( I) complex, which is suitable for various applications such as the hydrogenation of olefins. Also worth mentioning is the industrial production of acetic acid from methanol and carbon monoxide with a rhodium catalyst in Monsanto's process.

Various complexing agents are used as food additives, as an additive in detergents and cleaners industry, in electroplating and printed circuit board industries and in chemical analysis.

Phthalocyanine complexes are used in CDs as a storage medium.

In analogue photography the remaining unexposed, hardly soluble in water silver is achieved by fixing salt solution ( ammonium or sodium thiosulfate ) from the layer after development: see fixing ( Photography ).

Research

It is generally a problem to fix short-lived and unstable molecules, which occur as intermediates in the reactions. One method is the fixation of complex formation. The fixed molecules, however, have this other chemical properties, in this way, but can be studied binding and structural relations. Examples include complexes with carbenes, cyclobutadiene, diimines and carbene analogues silylenes. After release from the complexes, the molecules are highly reactive again. Be used metal complexes with chromium, nickel, iron and manganese. As a starting complex Metallcarbonylkomplex is often used. Examples: tricarbonyl cyclobutadieneisen, methoxyphenyl - carbene pentacarbonylchromium, tetrachloro -bis ( tetramethylcyclobutadiene ) nickel.

Potassium permanganate Robert H. Grubbs Non-innocent Ligand Chemical transport reaction Fluorine Bromine Nitrite Nitrate Prism (geometry) Metallic bonding Cluster chemistry Bis(benzene)chromium 18-Electron rule Hund's rules Jahn–Teller effect Ralph Pearson HOMO/LUMO Cyclooctatetraene Robert Burns Woodward Jean-Marie Lehn Grubbs' catalyst Alkene Chemical structure Non-coordinating anion Henry Taube Digital Object Identifier
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