In chemistry, a ligand is either an atom, ion, or molecule (see also: functional group) that bonds to a central metal, generally involving formal donation of one or more of its electrons. The metal-ligand bonding ranges from covalent to more ionic. Furthermore, the metal-ligand bond order can range from one to three. Ligands are viewed as Lewis bases, although rare cases are known involving Lewis acidic "ligands.
Metal and metalloids are bound to ligands in virtually all circumstances, although gaseous "naked" metal ions can be generated in 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: their charge, size (bulk), the identity of the coordinating atom(s), and their denticity. The size of a ligand is indicated by its cone angle.
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 overlaps 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).
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:
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.
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.
Related to but distinct to from denticity is hapticity, symbolized η or eta. Hapticity 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 carbon atoms are bonded to the metal. To simplify matters, ηn usually refers to unsaturated hydrocarbons and κn usually 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, 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. 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.
A diphosphane linked with pentamethylene was claimed to span across a square planare complex. This early attempt was followed by ligands with more rigid backbones. "TRANSPHOS" was the first trans-spanning diphosphane ligand that usually coordinates to palladium(II) and platinum(I1) in a trans manner. TRANSPHOS features benzo[c]phenanthrene substituted by diphenylphosphinomethyl (Ph2PCH2) groups at the 1 and 11 positions. The polycyclic framework suffers sterically clashing hydrogen centers. XANTHOS is a more reliable trans-spanning ligand. without the steric problems associated with TRANSPHOS. SPANPHOS is comparable to XANTHOS.
Subsequent to the reports on SPANPHOS and related ligands was a genuine trans-spanning ligand reported, one that would form neither bimetallic nor oligomeric complexes with certain transition metals, and strictly function as a trans-chelator This ligand, TRANSDIP, represented the first trans-spanning ligand to give exclusively chelating complexes, even when reacted with d8 metal ion halides. TRANSDIP is based on a α-cyclodextrin.
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.
|Ligand||formula (bonding atom(s) in bold)||Charge||Most common denticity||Remark(s)|
|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, below)|
|Chloride chlorido||Cl−||monoanionic||monodentate||also found bridging|
|Hydroxide hydroxo||O-H−||monoanionic||monodentate||often found as a bridging ligand|
|Isothiocyanate isothiocyanato||N=C=S−||monoanionic||monodentate||ambidentate (see also thiocyanate, above)|
|2,2'-Bipyridine||bipy||neutral||bidentate||easily reduced to its (radical) anion or even to its dianion|
|Nitrite nitro||N-O2−||monoanionic||monodentate||ambidentate (see also nitrito)|
|Nitrite nitrito||O-N-O−||monoanionic||monodentate||ambidentate (see also nitro)|
|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).
|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|
|1,1-Bis(diphenylphosphino)methane (dppm)||C25H22P2||neutral||Can bond to 2 metal atoms at once, forming dimers|
|Crown ethers||neutral||primarily for alkali and alkaline earth metal cations|
|2,2,2-crypt||hexadentate||primarily for alkali and alkaline earth metal cations|
|Diethylenetriamine (dien)||C4H13N3||neutral||tridentate||related to TACN, but not constrained to facial complexation|
|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)|
|Nitrosyl||NO+||cationic||bent (1e) and linear (3e) bonding mode|
|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|