Ligand

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Cobalt complex HCo(CO)4 with five ligands HCo(CO)4-3D-balls.png
Cobalt complex HCo(CO)4 with five ligands

In coordination chemistry, a ligand [lower-alpha 1] is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electron pairs often through Lewis Bases. [1] The nature of metal–ligand bonding can range from covalent to ionic. Furthermore, the metal–ligand bond order can range from one to three. Ligands are viewed as Lewis bases, although rare cases are known to involve Lewis acidic "ligands". [2] [3]

Contents

Metals and metalloids are bound to ligands in almost all circumstances, although gaseous "naked" metal ions can be generated in a high vacuum. Ligands in a complex dictate the reactivity of the central atom, including ligand substitution rates, the reactivity of the ligands themselves, and redox. Ligand selection is a critical consideration in many practical areas, including bioinorganic and medicinal chemistry, homogeneous catalysis, and environmental chemistry.

Ligands are classified in many ways, including: charge, size (bulk), the identity of the coordinating atom(s), and the number of electrons donated to the metal (denticity or hapticity). The size of a ligand is indicated by its cone angle.

History

The composition of coordination complexes have been known since the early 1800s, such as Prussian blue and copper vitriol. The key breakthrough occurred when Alfred Werner reconciled formulas and isomers. He showed, among other things, that the formulas of many cobalt(III) and chromium(III) compounds can be understood if the metal has six ligands in an octahedral geometry. The first to use the term "ligand" were Alfred Werner and Carl Somiesky, in relation to silicon chemistry. The theory allows one to 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 called hexol into optical isomers, overthrowing the theory that chirality was necessarily associated with carbon compounds. [4] [5]

Strong field and weak field ligands

In general, ligands are viewed as electron donors and the metals as electron acceptors, i.e., respectively, Lewis bases and Lewis acids. This description has been semi-quantified in many ways, e.g. ECW model. Bonding is often described using the formalisms of molecular orbital theory. [6] [7]

Ligands and metal ions can be ordered in many ways; one ranking system focuses on ligand 'hardness' (see also hard/soft acid/base theory). Metal ions preferentially bind certain ligands. In general, 'hard' metal ions prefer weak field ligands, whereas 'soft' metal ions prefer strong field ligands. According to the molecular orbital theory, the HOMO (Highest Occupied Molecular Orbital) of the ligand should have an energy that overlaps with the LUMO (Lowest Unoccupied Molecular Orbital) of the metal preferential. Metal ions bound to strong-field ligands follow the Aufbau principle, whereas complexes bound to weak-field ligands follow Hund's rule.

Binding of the metal with the ligands results in a set of molecular orbitals, where the metal can be identified with a new HOMO and LUMO (the orbitals defining the properties and reactivity of the resulting complex) and a certain ordering of the 5 d-orbitals (which may be filled, or partially filled with electrons). In an octahedral environment, the 5 otherwise degenerate d-orbitals split in sets of 2 and 3 orbitals (for a more in depth explanation, see crystal field theory).

3 orbitals of low energy: dxy, dxz and dyz
2 of high energy: dz2 and dx2y2

The energy difference between these 2 sets of d-orbitals is called the splitting parameter, Δo. The magnitude of Δo is determined by the field-strength of the ligand: strong field ligands, by definition, increase Δo more than weak field ligands. Ligands can now be sorted according to the magnitude of Δo (see the table below). This ordering of ligands is almost invariable for all metal ions and is called spectrochemical series.

For complexes with a tetrahedral surrounding, the d-orbitals again split into two sets, but this time in reverse order.

2 orbitals of low energy: dz2 and dx2y2
3 orbitals of high energy: dxy, dxz and dyz

The energy difference between these 2 sets of d-orbitals is now called Δt. The magnitude of Δt is smaller than for Δo, because in a tetrahedral complex only 4 ligands influence the d-orbitals, whereas in an octahedral complex the d-orbitals are influenced by 6 ligands. When the coordination number is neither octahedral nor tetrahedral, the splitting becomes correspondingly more complex. For the purposes of ranking ligands, however, the properties of the octahedral complexes and the resulting Δo has been of primary interest.

The arrangement of the d-orbitals on the central atom (as determined by the 'strength' of the ligand), has a strong effect on virtually all the properties of the resulting complexes. E.g., the energy differences in the d-orbitals has a strong effect in the optical absorption spectra of metal complexes. It turns out that valence electrons occupying orbitals with significant 3 d-orbital character absorb in the 400–800 nm region of the spectrum (UV–visible range). The absorption of light (what we perceive as the color) by these electrons (that is, excitation of electrons from one orbital to another orbital under influence of light) can be correlated to the ground state of the metal complex, which reflects the bonding properties of the ligands. The relative change in (relative) energy of the d-orbitals as a function of the field-strength of the ligands is described in Tanabe–Sugano diagrams.

In cases where the ligand has low energy LUMO, such orbitals also participate in the bonding. The metal–ligand bond can be further stabilised by a formal donation of electron density back to the ligand in a process known as back-bonding. In this case a filled, central-atom-based orbital donates density into the LUMO of the (coordinated) ligand. Carbon monoxide is the preeminent example a ligand that engages metals via back-donation. Complementarily, ligands with low-energy filled orbitals of pi-symmetry can serve as pi-donor.

Metal-EDTA complex, wherein the aminocarboxylate is a hexadentate (chelating) ligand. Metal-EDTA.svg
Metal–EDTA complex, wherein the aminocarboxylate is a hexadentate (chelating) ligand.
Cobalt(III) complex containing six ammonia ligands, which are monodentate. The chloride is not a ligand. CoA6Cl3.png
Cobalt(III) complex containing six ammonia ligands, which are monodentate. The chloride is not a ligand.

Classification of ligands as L and X

Especially in the area of organometallic chemistry, ligands are classified as L and X (or combinations of the two). The classification scheme – the "CBC Method" for Covalent Bond Classification – was popularized by M.L.H. Green and "is based on the notion that there are three basic types [of ligands]... represented by the symbols L, X, and Z, which correspond respectively to 2-electron, 1-electron and 0-electron neutral ligands." [8] [9] Another type of ligand worthy of consideration is the LX ligand which as expected from the used conventional representation will donate three electrons if NVE (Number of Valence Electrons) required. Example is alkoxy ligands( which is regularly known as X ligand too). L ligands are derived from charge-neutral precursors and are represented by amines, phosphines, CO, N2, and alkenes. X ligands typically are derived from anionic precursors such as chloride but includes ligands where salts of anion do not really exist such as hydride and alkyl. Thus, the complex IrCl(CO)(PPh3)2 is classified as an MXL3 complex, since CO and the two PPh3 ligands are classified as Ls. The oxidative addition of H2 to IrCl(CO)(PPh3)2 gives an 18e ML3X3 product, IrClH2(CO)(PPh3)2. EDTA 4− is classified as an L2X4 ligand, as it features four anions and two neutral donor sites. Cp is classified as an L2X ligand. [10]

Polydentate and polyhapto ligand motifs and nomenclature

Denticity

Denticity (represented by κ ) refers to the number of times a ligand bonds to a metal through noncontiguous donor sites. Many ligands are capable of binding metal ions through multiple sites, usually because the ligands have lone pairs on more than one atom. Ligands that bind via more than one atom are often termed chelating . A ligand that binds through two sites is classified as bidentate , and three sites as tridentate . The "bite angle" refers to the angle between the two bonds of a bidentate chelate. Chelating ligands are commonly formed by linking donor groups via organic linkers. A classic bidentate ligand is ethylenediamine, which is derived by the linking of two ammonia groups with an ethylene (−CH2CH2−) linker. A classic example of a polydentate ligand is the hexadentate chelating agent EDTA, which is able to bond through six sites, completely surrounding some metals. The number of times a polydentate ligand binds to a metal centre is symbolized by "κn", where n indicates the number of sites by which a ligand attaches to a metal. EDTA4−, when it is hexidentate, binds as a κ6-ligand, the amines and the carboxylate oxygen atoms are not contiguous. In practice, the n value of a ligand is not indicated explicitly but rather assumed. The binding affinity of a chelating system depends on the chelating angle or bite angle.

Complexes of polydentate ligands are called chelate complexes. They tend to be more stable than complexes derived from monodentate ligands. This enhanced stability, the chelate effect, is usually attributed to effects of entropy, which favors the displacement of many ligands by one polydentate ligand. When the chelating ligand forms a large ring that at least partially surrounds the central atom and bonds to it, leaving the central atom at the centre of a large ring.[ clarification needed ] The more rigid and the higher its denticity, the more inert will be the macrocyclic complex. Heme is a good example: the iron atom is at the centre of a porphyrin macrocycle, being bound to four nitrogen atoms of the tetrapyrrole macrocycle. The very stable dimethylglyoximate complex of nickel is a synthetic macrocycle derived from the anion of dimethylglyoxime.

Hapticity

Hapticity (represented by η ) refers to the number of contiguous atoms that comprise a donor site and attach to a metal center. Butadiene forms both η2 and η4 complexes depending on the number of carbon atoms that are bonded to the metal. [10]

Ligand motifs

Trans-spanning ligands

Trans-spanning ligands are bidentate ligands that can span coordination positions on opposite sides of a coordination complex. [11]

Ambidentate ligand

Unlike polydentate ligands, ambidentate ligands can attach to the central atom in two places. A good example of this is thiocyanate, SCN, which can attach at either the sulfur atom or the nitrogen atom. Such compounds give rise to linkage isomerism. Polyfunctional ligands, see especially proteins, can bond to a metal center through different ligand atoms to form various isomers.

Bridging ligand

A bridging ligand links two or more metal centers. Virtually all inorganic solids with simple formulas are coordination polymers, consisting of metal ion centres linked by bridging ligands. This group of materials includes all anhydrous binary metal ion halides and pseudohalides. Bridging ligands also persist in solution. Polyatomic ligands such as carbonate are ambidentate and thus are found to often bind to two or three metals simultaneously. Atoms that bridge metals are sometimes indicated with the prefix "μ". Most inorganic solids are polymers by virtue of the presence of multiple bridging ligands. Bridging ligands, capable of coordinating multiple metal ions, have been attracting considerable interest because of their potential use as building blocks for the fabrication of functional multimetallic assemblies. [12]

Binucleating ligand

Binucleating ligands bind two metal ions. [13] Usually binucleating ligands feature bridging ligands, such as phenoxide, pyrazolate, or pyrazine, as well as other donor groups that bind to only one of the two metal ions.

Metal–ligand multiple bond

Some ligands can bond to a metal center through the same atom but with a different number of lone pairs. The bond order of the metal ligand bond can be in part distinguished through the metal ligand bond angle (M−X−R). This bond angle is often referred to as being linear or bent with further discussion concerning the degree to which the angle is bent. For example, an imido ligand in the ionic form has three lone pairs. One lone pair is used as a sigma X donor, the other two lone pairs are available as L-type pi donors. If both lone pairs are used in pi bonds then the M−N−R geometry is linear. However, if one or both these lone pairs is nonbonding then the M−N−R bond is bent and the extent of the bend speaks to how much pi bonding there may be. η1-Nitric oxide can coordinate to a metal center in linear or bent manner.

Spectator ligand

A spectator ligand is a tightly coordinating polydentate ligand that does not participate in chemical reactions but removes active sites on a metal. Spectator ligands influence the reactivity of the metal center to which they are bound.

Bulky ligands

Bulky ligands are used to control the steric properties of a metal center. They are used for many reasons, both practical and academic. On the practical side, they influence the selectivity of metal catalysts, e.g., in hydroformylation. Of academic interest, bulky ligands stabilize unusual coordination sites, e.g., reactive coligands or low coordination numbers. Often bulky ligands are employed to simulate the steric protection afforded by proteins to metal-containing active sites. Of course excessive steric bulk can prevent the coordination of certain ligands.

The N-heterocyclic carbene ligand called IMes is a bulky ligand by virtue of the pair of mesityl groups. 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (aka IMes).png
The N-heterocyclic carbene ligand called IMes is a bulky ligand by virtue of the pair of mesityl groups.

Chiral ligands

Chiral ligands are useful for inducing asymmetry within the coordination sphere. Often the ligand is employed as an optically pure group. In some cases, such as secondary amines, the asymmetry arises upon coordination. Chiral ligands are used in homogeneous catalysis, such as asymmetric hydrogenation.

Hemilabile ligands

Hemilabile ligands contain at least two electronically different coordinating groups and form complexes where one of these is easily displaced from the metal center while the other remains firmly bound, a behaviour which has been found to increase the reactivity of catalysts when compared to the use of more traditional ligands.

Non-innocent ligand

Non-innocent ligands bond with metals in such a manner that the distribution of electron density between the metal center and ligand is unclear. Describing the bonding of non-innocent ligands often involves writing multiple resonance forms that have partial contributions to the overall state.

Common ligands

Virtually every molecule and every ion can serve as a ligand for (or "coordinate to") metals. Monodentate ligands include virtually all anions and all simple Lewis bases. Thus, the halides and pseudohalides are important anionic ligands whereas ammonia, carbon monoxide, and water are particularly common charge-neutral ligands. Simple organic species are also very common, be they anionic (RO and RCO
2
) or neutral (R2O, R2S, R3−xNHx, and R3P). The steric properties of some ligands are evaluated in terms of their cone angles.

Beyond the classical Lewis bases and anions, all unsaturated molecules are also ligands, utilizing their pi electrons in forming the coordinate bond. Also, metals can bind to the σ bonds in for example silanes, hydrocarbons, and dihydrogen (see also: Agostic interaction).

In complexes of non-innocent ligands, the ligand is bonded to metals via conventional bonds, but the ligand is also redox-active.

Examples of common ligands (by field strength)

In the following table the ligands are sorted by field strength (weak field ligands first):

Ligandformula (bonding atom(s) in bold)ChargeMost common denticityRemark(s)
Iodide (iodo)I monoanionic monodentate
Bromide (bromido)Brmonoanionicmonodentate
Sulfide (thio or less commonly "bridging thiolate")S2−dianionicmonodentate (M=S), or bidentate bridging (M−S−M')
Thiocyanate (S-thiocyanato)S−CNmonoanionicmonodentateambidentate (see also isothiocyanate, below)
Chloride (chlorido)Clmonoanionicmonodentatealso found bridging
Nitrate (nitrato)ONO
2
monoanionicmonodentate
Azide (azido)NN
2
monoanionicmonodentateVery Toxic
Fluoride (fluoro)Fmonoanionicmonodentate
Hydroxide (hydroxido)O−Hmonoanionicmonodentateoften found as a bridging ligand
Oxalate (oxalato)[O−CO−CO−O]2−dianionicbidentate
Water (aqua)O−H2neutralmonodentate
Nitrite (nitrito)O−N−Omonoanionicmonodentateambidentate (see also nitro)
Isothiocyanate (isothiocyanato)N=C=Smonoanionicmonodentateambidentate (see also thiocyanate, above)
Acetonitrile (acetonitrilo)CH3CNneutralmonodentate
Pyridine (py)C5H5Nneutralmonodentate
Ammonia (ammine or less commonly "ammino")NH3neutralmonodentate
Ethylenediamine (en)NH2−CH2−CH2NH2neutralbidentate
2,2'-Bipyridine (bipy)NC5H4−C5H4Nneutralbidentateeasily reduced to its (radical) anion or even to its dianion
1,10-Phenanthroline (phen)C12H8N2neutralbidentate
Nitrite (nitro)NO
2
monoanionicmonodentateambidentate (see also nitrito)
Triphenylphosphine P−(C6H5)3neutralmonodentate
Cyanide (cyano)C≡N
N≡C
monoanionicmonodentatecan bridge between metals (both metals bound to C, or one to C and one to N)
Carbon monoxide (carbonyl)CO, othersneutralmonodentatecan bridge between metals (both metals bound to C)

The entries in the table are sorted by field strength, binding through the stated atom (i.e. as a terminal ligand). The 'strength' of the ligand changes when the ligand binds in an alternative binding mode (e.g., when it bridges between metals) or when the conformation of the ligand gets distorted (e.g., a linear ligand that is forced through steric interactions to bind in a nonlinear fashion).

Other generally encountered ligands (alphabetical)

In this table other common ligands are listed in alphabetical order.

LigandFormula (bonding atom(s) in bold)ChargeMost common denticityRemark(s)
Acetylacetonate (acac)CH3−CO−CH2−CO−CH3monoanionicbidentateIn general bidentate, bound through both oxygens, but sometimes bound through the central carbon only,
see also analogous ketimine analogues
Alkenes R2C=CR2neutralcompounds with a C−C double bond
Aminopolycarboxylic acids (APCAs)    
BAPTA (1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid)    
Benzene C6H6neutraland other arenes
1,2-Bis(diphenylphosphino)ethane (dppe)(C6H5)2P−C2H4P(C6H5)2neutralbidentate
1,1-Bis(diphenylphosphino)methane (dppm)(C6H5)2P−CH2P(C6H5)2neutralCan bond to two metal atoms at once, forming dimers
Corroles tetradentate
Crown ethers neutralprimarily for alkali and alkaline earth metal cations
2,2,2-cryptand hexadentateprimarily for alkali and alkaline earth metal cations
Cryptates neutral
Cyclopentadienyl (Cp)C
5
H
5
monoanionicAlthough monoanionic, by the nature of its occupied molecular orbitals, it is capable of acting as a tridentate ligand.
Diethylenetriamine (dien)C4H13N3neutraltridentaterelated to TACN, but not constrained to facial complexation
Dimethylglyoximate (dmgH)monoanionic
1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)    
Diethylenetriaminepentaacetic acid (DTPA) (pentetic acid)    
Ethylenediaminetetraacetic acid (EDTA) (edta4−)(OOC−CH2)2N−C2H4N(CH2-COO)2tetraanionichexadentate
EthylenediaminetriacetateOOC−CH2NH−C2H4N(CH2-COO)2trianionicpentadentate
Ethyleneglycolbis(oxyethylenenitrilo)tetraacetate (egta4−)(OOC−CH2)2N−C2H4O−C2H4O−C2H4N(CH2−COO)2tetraanionicoctodentate
Fura-2     
Glycinate (glycinato)NH2CH2COOmonoanionicbidentateother α-amino acid anions are comparable (but chiral)
Heme dianionictetradentatemacrocyclic ligand
Iminodiacetic acid (IDA)  tridentateUsed extensively to make radiotracers for scintigraphy by complexing the metastable radionuclide technetium-99m. For example, in cholescintigraphy, HIDA, BrIDA, PIPIDA, and DISIDA are used
Nicotianamine   Ubiquitous in higher plants
Nitrosyl NO+cationicbent (1e) and linear (3e) bonding mode
Nitrilotriacetic acid (NTA)    
Oxo O2−dianionmonodentatesometimes bridging
Pyrazine N2C4H4neutralditopicsometimes bridging
Scorpionate ligand tridentate
Sulfite OSO2−
2

SO2−
3
monoanionicmonodentateambidentate
2,2';6',2″-Terpyridine (terpy)NC5H4−C5H3N−C5H4Nneutraltridentatemeridional bonding only
Triazacyclononane (tacn)(C2H4)3(NR)3neutraltridentatemacrocyclic ligand
see also the N,N′,N″-trimethylated analogue
Tricyclohexylphosphine P(C6H11)3 or PCy3neutralmonodentate
Triethylenetetramine (trien)C6H18N4neutraltetradentate
Trimethylphosphine P(CH3)3neutralmonodentate
Tris(o-tolyl)phosphine P(o-tolyl)3neutralmonodentate
Tris(2-aminoethyl)amine (tren)(NH2CH2CH2)3Nneutraltetradentate
Tris(2-diphenylphosphineethyl)amine (np3)neutraltetradentate
Tropylium C
7
H+
7
cationic
Carbon dioxide CO2, othersneutralsee metal carbon dioxide complex
Phosphorus trifluoride (trifluorophosphorus)PF3neutral

Ligand exchange

A ligand exchange (also ligand substitution) is a type of chemical reaction in which a ligand in a compound is replaced by another. One type of pathway for substitution is the ligand dependent pathway. In organometallic chemistry this can take place via associative substitution or by dissociative substitution. [14]

Ligand–protein binding database

BioLiP [15] is a comprehensive ligand–protein interaction database, with the 3D structure of the ligand–protein interactions taken from the Protein Data Bank. MANORAA is a webserver for analyzing conserved and differential molecular interaction of the ligand in complex with protein structure homologs from the Protein Data Bank. It provides the linkage to protein targets such as its location in the biochemical pathways, SNPs and protein/RNA baseline expression in target organ. [16]

See also

Explanatory notes

  1. The word ligand comes from Latin ligare , to bind/tie. It is pronounced either /ˈlɡənd/ or /ˈlɪɡənd/ ; both are very common.

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Coordination complex Molecule or ion containing ligands datively bonded to a central metallic atom

A coordination complex consists of a central atom or ion, which is usually metallic and is called the coordination centre, and a surrounding array of bound molecules or ions, that are in turn known as ligands or complexing agents. Many metal-containing compounds, especially those that include transition metals, are coordination complexes.

Inorganic chemistry Field of chemistry

Inorganic chemistry deals with synthesis and behavior of inorganic and organometallic compounds. This field covers chemical compounds that are not carbon-based, which are the subjects of organic chemistry. The distinction between the two disciplines is far from absolute, as there is much overlap in the subdiscipline of organometallic chemistry. It has applications in every aspect of the chemical industry, including catalysis, materials science, pigments, surfactants, coatings, medications, fuels, and agriculture.

Lone pair Pair of valence electrons which are not shared with another atom in a covalent bond

In chemistry, a lone pair refers to a pair of valence electrons that are not shared with another atom in a covalent bond and is sometimes called an unshared pair or non-bonding pair. Lone pairs are found in the outermost electron shell of atoms. They can be identified by using a Lewis structure. Electron pairs are therefore considered lone pairs if two electrons are paired but are not used in chemical bonding. Thus, the number of electrons in lone pairs plus the number of electrons in bonds equals the number of valence electrons around an atom.

VSEPR theory Model for predicting molecular geometry

Valence shell electron pair repulsion (VSEPR) theory, is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. It is also named the Gillespie-Nyholm theory after its two main developers, Ronald Gillespie and Ronald Nyholm.

Ligand field theory (LFT) describes the bonding, orbital arrangement, and other characteristics of coordination complexes. It represents an application of molecular orbital theory to transition metal complexes. A transition metal ion has nine valence atomic orbitals - consisting of five nd, one (n+1)s, and three (n+1)p orbitals. These orbitals are of appropriate energy to form bonding interaction with ligands. The LFT analysis is highly dependent on the geometry of the complex, but most explanations begin by describing octahedral complexes, where six ligands coordinate to the metal. Other complexes can be described by reference to crystal field theory.

Cyanate

Cyanate is an anion with the structural formula [O=C=N], usually written OCN. It also refers to any salt containing it, such as ammonium cyanate.

Octahedral molecular geometry Molecular geometry

In chemistry, octahedral molecular geometry describes the shape of compounds with six atoms or groups of atoms or ligands symmetrically arranged around a central atom, defining the vertices of an octahedron. The octahedron has eight faces, hence the prefix octa. The octahedron is one of the Platonic solids, although octahedral molecules typically have an atom in their centre and no bonds between the ligand atoms. A perfect octahedron belongs to the point group Oh. Examples of octahedral compounds are sulfur hexafluoride SF6 and molybdenum hexacarbonyl Mo(CO)6. The term "octahedral" is used somewhat loosely by chemists, focusing on the geometry of the bonds to the central atom and not considering differences among the ligands themselves. For example, [Co(NH3)6]3+, which is not octahedral in the mathematical sense due to the orientation of the N−H bonds, is referred to as octahedral.

Copper proteins are proteins that contain one or more copper ions as prosthetic groups. Copper proteins are found in all forms of air-breathing life. These proteins are usually associated with electron-transfer with or without the involvement of oxygen (O2). Some organisms even use copper proteins to carry oxygen instead of iron proteins. A prominent copper proteins in humans is in cytochrome c oxidase (cco). The enzyme cco mediates the controlled combustion that produces ATP.

Coordination polymer Polymer consisting of repeating units of a coordination complex

A coordination polymer is an inorganic or organometallic polymer structure containing metal cation centers linked by ligands. More formally a coordination polymer is a coordination compound with repeating coordination entities extending in 1, 2, or 3 dimensions.

Coordination sphere

In coordination chemistry, the first coordination sphere refers to the array of molecules and ions directly attached to the central metal atom. The second coordination sphere consists of molecules and ions that attached in various ways to the first coordination sphere.

The 18-electron rule is a chemical rule of thumb used primarily for predicting and rationalizing formulas for stable transition metal complexes, especially organometallic compounds. The rule is based on the fact that the valence orbitals in the electron configuration of transition metals consist of five (n−1)d orbitals, one ns orbital, and three np orbitals, where n is the principal quantum number. These orbitals can collectively accommodate 18 electrons as either bonding or nonbonding electron pairs. This means that the combination of these nine atomic orbitals with ligand orbitals creates nine molecular orbitals that are either metal-ligand bonding or non-bonding. When a metal complex has 18 valence electrons, it is said to have achieved the same electron configuration as the noble gas in the period. The rule is not helpful for complexes of metals that are not transition metals, and interesting or useful transition metal complexes will violate the rule because of the consequences deviating from the rule bears on reactivity. The rule was first proposed by American chemist Irving Langmuir in 1921.

Metal nitrosyl complex Complex of a transition metal bonded to nitric oxide: Me–NO

Metal nitrosyl complexes are complexes that contain nitric oxide, NO, bonded to a transition metal. Many kinds of nitrosyl complexes are known, which vary both in structure and coligand.

In chemistry, crystallography, and materials science, the coordination number, also called ligancy, of a central atom in a molecule or crystal is the number of atoms, molecules or ions bonded to it. The ion/molecule/atom surrounding the central ion/molecule/atom is called a ligand. This number is determined somewhat differently for molecules than for crystals.

Bite angle

In coordination chemistry the bite angle is the ligand–metal–ligand bond angle of coordination complex containing a bidentate ligand. This geometric parameter is used to classify chelating ligands, including those in organometallic complexes. It is most often discussed in terms of catalysis, as changes in bite angle can affect not just the activity and selectivity of a catalytic reaction but even allow alternative reaction pathways to become accessible.

Diphosphines

Diphosphines, sometimes called bisphosphanes, are organophosphorus compounds most commonly used as bidentate phosphine ligands in inorganic and organometallic chemistry. They are identified by the presence of two phosphino groups linked by a backbone, and are usually chelating. A wide variety of diphosphines have been synthesized with different linkers and R-groups. Alteration of the linker and R-groups alters the electronic and steric properties of the ligands which can result in different coordination geometries and catalytic behavior in homogeneous catalysts.

Denticity Number of atoms in a ligand that bond to the central atom of a coordination complex

In coordination chemistry, denticity refers to the number of donor groups in a given ligand that bind to the central metal atom in a coordination complex. In many cases, only one atom in the ligand binds to the metal, so the denticity equals one, and the ligand is said to be monodentate. Ligands with more than one bonded atom are called polydentate or multidentate. The denticity of a ligand is described with the Greek letter κ ('kappa'). For example, κ6-EDTA describes an EDTA ligand that coordinates through 6 non-contiguous atoms.

In chemistry, π-effects or π-interactions are a type of non-covalent interaction that involves π systems. Just like in an electrostatic interaction where a region of negative charge interacts with a positive charge, the electron-rich π system can interact with a metal, an anion, another molecule and even another π system. Non-covalent interactions involving π systems are pivotal to biological events such as protein-ligand recognition.

Tetradentate ligands are ligands that bind four donor atoms to a central atom to form a coordination complex. This number of donor atoms that bind is called denticity and is a way to classify ligands. Tetradentate ligands are common in nature in the form of chlorophyll which has a core ligand called chlorin, and heme with a core ligand called porphyrin. They add much of the colour seen in plants and humans. Phthalocyanine is an artificial macrocyclic tetradentate ligand that is used to make blue and green pigments.

Triboracyclopropenyl

The triboracyclopropenyl fragment is a cyclic structural motif in boron chemistry, named for its geometric similarity to cyclopropene. In contrast to nonplanar borane clusters that exhibit higher coordination numbers at boron (e.g., through 3-center 2-electron bonds to bridging hydrides or cations), triboracyclopropenyl-type structures are rings of three boron atoms where substituents at each boron are also coplanar to the ring. Triboracyclopropenyl-containing compounds are extreme cases of inorganic aromaticity. They are the lightest and smallest cyclic structures known to display the bonding and magnetic properties that originate from fully delocalized electrons in orbitals of σ and π symmetry. Although three-membered rings of boron are frequently so highly strained as to be experimentally inaccessible, academic interest in their distinctive aromaticity and possible role as intermediates of borane pyrolysis motivated extensive computational studies by theoretical chemists. Beginning in the late 1980s with mass spectrometry work by Anderson et al. on all-boron clusters, experimental studies of triboracyclopropenyls were for decades exclusively limited to gas-phase investigations of the simplest rings (ions of B3). However, more recent work has stabilized the triboracyclopropenyl moiety via coordination to donor ligands or transition metals, dramatically expanding the scope of its chemistry.

Nontrigonal pnictogen compounds

Nontrigonal pnictogen compounds refer to tricoordinate trivalent pnictogen compounds that are not of typical trigonal pyramidal molecular geometry. By virtue of their geometric constraint, these compounds exhibit distinct electronic structures and reactivities, which bestow on them potential to provide unique nonmetal platforms for bond cleavage reactions.

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