Ligand
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In chemistry, a ligand is an atom, ion, or molecule (see also: functional group) that bond to a central metal, generally involving formal donation of one or more of its electrons. The metal-ligand bonding ranges from covalent bond to more ionic. Furthermore, the metal-ligand bond order can range from one to three. Ligands are viewed as a Lewis base although rare cases are known where Lewis acidic ligands serve as electron acceptor.
All metal and metalloids are bound to ligands in virutally all situations, although gaseous "naked" metal ions can be generated in high vacuum. Borane BH3 as ligand for the protection of phosphine PH3),≥ e resulting from the coordination of a ligand (or an array of ligands) to a central atom is termed a complex.
Factors that characterize the ligands are their charge, size (bulk), and of course the nature of the constituent atoms. Ligands in a complex dictate the reactivity of the central atom.
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Ligands in metal complexes
In coordination chemistry, the ligands that are directly bonded to the metal (that is, share electrons), are sometimes called "inner sphere" ligands. "Outer-sphere" ligands are not directly attached to the metal, but are bonded, generally weakly, to the first coordination shell, affecting the inner sphere in subtle ways. The complex of the metal with the inner sphere ligands is then called a coordination complex, which can be neutral, cationic, or anionic). The complex, along with its counter ions (if required), is called a coordination compound. The size of a ligand is indicated by its cone angle.
Donation and back-donation
In general, ligands are viewed as donating electrons to the central atom. Bonding is often described using the formalisms of molecular orbital theory. In general, electron pairs) occupy the HOMO of the ligands. In cases where the ligand has low energy LUMO, such orbitals also participate in the bonding and 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 preeminant example a ligand that engages metals via back-donation.
Strong field and weak field ligands
Template:Main 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. From a MO point of view, the HOMO of the ligand should have an energy that makes overlap with the LUMO 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 dx2−y2
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 dx2−y2
- 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 3d-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.
Ligand bonding motifs and nomenclature
Chelation
Many ligands are capable of binding metal ion through multiple sites, usually because they have lone pairs on more than one atom. Ligands that bind via more than one atom are 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 example is ethylene diamine, 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 atoms with which a polydentate ligand bind to the metal centre is called its denticity, symbolized κn, where n indicates the number non-contiguous donor sites by which a ligand attaches to a metal. 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.
Hapticity (η) or eta refers to the number of contiguous atoms in a ligand that are attached to a metal. Butadiene forms both η2 and η4 complexes depending on the number of of carbon atoms are bonded to the metal. Cyclopentadienyl (Cp) is typically bound in the η5 mode in which all five carbon atoms are coordinated to the metal. Cp can, however, ring slip to η3 or η1 mode in which only three- or one carbon atoms are coordinated to the metal. In this situation the Cp group shifts from the initial 6 electron donor to a 4 or 2 electron donor under the ionic model.
Bridging ligands link two or more metal centers. Polyatomic ligands such as CO22- are especially prone to bridge. The bonding is complicated because polyatomic ligands are ambidentate and thus the capacity for many different linkage isomers. Atoms that bridge metals are soemtimes indicated with prefix of "μ" (mu). Most inorganic solids, e.g. FeCl2, are polymers by virtue of the presence of multiple bridging ligands.
Ambidentate ligand or polyfunctional ligand can bond to a metal center through different ligand atoms to form various isomers.
Noninnocent ligand is a ligand that bonds with metals in such a manner that the distribution of electron density between the metal center and ligand is unclear. Describing the bonding of noninnocent ligands often involves writing multiple resonance forms which have partial contributions to the overall state.
Metal ligand multiple bonds 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 non-bonding 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.
Hapticity vs denticity
Hapticity (η) and denticity are often confused. Hapticity refers to contiguous atoms that are attached to a metal. Ethylene forms η2 complexes because two adjacent carbon atoms bind to the metal. Ethylenediamine forms κ2 complexes. Cyclopentadienyl is typically bonded in η5 mode because all five carbon atoms are bonded to the metal. EDTA4− on the other hand, when it is sexidentate, is κ6 mode, the amines and the carboxylate oxygen atoms are not connected directly. To simplify matters, ηn tends to refer to unsaturated hydrocarbons and κn tends to describe polydentate amine and carboxylate ligands.
Complexes of polydentate ligands are called chelate complexes. They tend to be more stable than complexes derived from monodentate ligands. This enhanced stability is attributed to the necessity to break all of the bonds to the central atom for the hexadentate ligand to be displaced. This increased stability or inertness is called the chelate effect. In terms of the enhanced thermodynamic stability of chelate complexes, entropy favors the displacement of many ligands by one polydentate ligand. The increase in the total number of molecules in solution is favorable.
Related to the chelate effect is the macrocyclic effect. A macrocyclic ligand is any large cyclic ligand which at least partially surrounds the central atom and bonds to it, leaving the central atom at the centre of a large ring. 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.
Unlike polydentate ligands, ambidentate ligands can attach to the central atom in two places but not both. A good example of this is thiocyanide, SCN−, which can attach at either the sulfur atom or the nitrogen atom. Such compounds give rise to linkage isomerism.
Common ligands
- See nomenclature.
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 RCO2−) 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 π-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):
| Ligand | formula (bonding atom(s) in bold) | Charge | Most common denticity | Remark(s) |
|---|---|---|---|---|
| Iodide iodo | I− | monoanionic | monodentate | |
| Bromide bromo | Br− | monoanionic | monodentate | |
| Sulfide thio or bridging thiolate | S2− | dianionic | monodentate (M=S), or bidentate bridging (M-S-M') | |
| Thiocyanate thiocyanato | S-CN− | monoanionic | monodentate | ambidentate (see also isothiocyanate, vide infra) |
| Chloride chloro | Cl− | monoanionic | monodentate | also found bridging |
| Nitrate | O-NO2− | monoanionic | monodentate | |
| Azide | N-N2− | monoanionic | monodentate | |
| Fluoride fluoro | F− | monoanionic | monodentate | |
| Hydroxide hydroxo | O-H− | monoanionic | monodentate | often found as a bridging ligand |
| Oxalate | [O-C(=O)-C(=O)-O]2− | dianionic | bidentate | |
| Water aqua | H-O-H | neutral | monodentate | monodentate |
| Isothiocyanate isothiocyanato | N=C=S− | monoanionic | monodentate | ambidentate (see also thiocyanate, vide supra) |
| Acetonitrile | CH3CN | neutral | monodentate | |
| Pyridine | C5H5N | neutral | monodentate | |
| Ammonia ammine | NH3 | neutral | monodentate | |
| Ethylenediamine | en | neutral | bidentate | |
| 2,2'-Bipyridine | bipy | neutral | bidentate | easily reduced to its (radical) anion or even to its dianion |
| 1,10-Phenanthroline | phen | neutral | bidentate | |
| Nitrite nitro or nitrito | O-N-O− | monoanionic | monodentate | ambidentate |
| Triphenylphosphine | PPh3 | neutral | monodentate | |
| Cyanide cyano | CN− | monoanionic | monodentate | can bridge between metals (both metals bound to C, or one to C and one to N) |
| Carbon monoxide carbonyl | CO | neutral | monodentate | can bridge between metals (both metals bound to C) |
Note: 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 non-linear fashion).
Other general encountered ligands (alphabetical)
In this table other common ligands are listed in alphabetical order.
| Ligand | formula (bonding atom(s) in bold) | Charge | Most common denticity | Remark(s) |
|---|---|---|---|---|
| Acetylacetonate (Acac) | CH3-C(O)-CH-C(O)-CH3 | monoanionic | bidentate | In general bidentate, bound through both oxygens, but sometimes bound through the central carbon only, see also analogous ketimine analogues |
| Alkenes | R2C=CR2 | neutral | compounds with a C-C double bond | |
| Benzene | C6H6 | neutral | and other arenes | |
| 1,2-Bis(diphenylphosphino)ethane (dppe) | Ph2PC2H4PPh2 | neutral | bidentate | |
| Corroles | tetradentate | |||
| Crown ethers | neutral | primarily for alkali and alkaline earth metal cations | ||
| 2,2,2-crypt | hexadentate | primarily for alkali and alkaline earth metal cations | ||
| Cryptates | neutral | |||
| Cyclopentadienyl | [C5H5]− | monoanionic | ||
| Diethylenetriamine (dien) | neutral | tridentate | related to TACN, but not constrained to facial complexation | |
| Dimethylglyoximate (dmgH−) | monoanionic | |||
| Ethylenediaminetetraacetate (EDTA) | tetra-anionic | hexadentate | actual ligand is the tetra-anion | |
| Ethylenediaminetriacetate | trianionic | pentadentate | actual ligand is the trianion | |
| glycinate | bidentate | other α-amino acid anions are comparable (but chiral) | ||
| Heme | dianionic | tetradentate | macrocyclic ligand | |
| Nitrosyl | NO+ | cationic | bent (1e) and linear (3e) bonding mode | |
| Scorpionate ligand | tridentate | |||
| Sulfite | monoanionic | monodentate | ambidentate | |
| 2,2',5',2-Terpyridine (terpy) | neutral | tridentate | meridional bonding only | |
| Thiocyanate | monoanionic | monodentate | ambidentate, sometimes bridging | |
| Triazacyclononane (tacn) | (C2H4)3(NR)3 | neutral | tridentate | macrocyclic ligand see also the N,N',N"-trimethylated analogue |
| Tricyclohexylphosphine | (C6H11)3P or (PCy3) | neutral | monodentate | |
| Triethylenetetramine (trien) | neutral | tetradentate | ||
| Tri(o-tolyl)phosphine | P(o-tolyl)3 | neutral | monodentate | |
| Tris(2-aminoethyl)amine (tren) | neutral | tetradentate | ||
| Tris(2-diphenylphosphineethyl)amine (np3) | neutral | tetradentate | ||
| Terpyridine | neutral | tridentate |
See also
- 2-Mercaptoindole
- Crystal field theory
- Ligand field theory
- Coordination chemistry
- Inorganic chemistry
- Radioligand
- Tanabe-Sugano diagram
- Spectrochemical series
- Scatchard equationaf:Ligand
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