Ligand in coordination chemistry

A ligand in coordination chemistry is an atom, ion or molecule attached to a central atom through one or more donor sites. This use is distinct from a protein-binding ligand. A neutral CO ligand and an anionic chloride ligand can both donate two electrons in ionic electron counting.

Two linked pyrimidine rings; the 2,2′-linked isomer has two N,N donor pockets. It can bind one metal as a bidentate ligand, or bridge two metals by using one pocket at each end. In the latter mode it is a bis-bidentate bridging ligand.

A coordination ligand assigned a negative formal charge in ionic electron counting. Distinguish the assigned charge from its electron donation and from the actual distributed charge in the complex.

An axially chiral binaphthyl diphosphine ligand used in asymmetric transition-metal catalysis.

A commonly anionic N–C–N ligand with delocalized bonding. Substituents and denticity influence both steric protection and the geometry of supported metal centres.

The ligand $[\mathrm{ArNCHNAr}{]}^{-}$ in a common diaryl form. Three such bridges can support a lantern-type dimetal unit.

A bound $\mathrm{C}{\mathrm{N}}^{-}$ ligand, usually C-bound in the strongly pi-accepting coordination mode used in elementary spectrochemical comparisons. It is an anionic two-electron donor in ionic electron counting.

A silicon-bound $\mathrm{Si}{\mathrm{R}}_3^{-}$ group, normally a two-electron X-type donor in ionic electron counting.

A carbon-bound group $-\mathrm{C}(\mathrm{O})\mathrm{R}$, formally anionic and normally a two-electron donor in ionic electron counting. It differs from a neutral carbonyl ligand $\mathrm{CO}$.

A ligand with three donor sites arranged to bind one metal in a meridional pattern. The framework may impose a preferred geometry and may itself undergo redox or proton-transfer changes.

An intact ${\mathrm{N}}_2$ group coordinated to a metal. End-on neutral dinitrogen normally counts as a two-electron L-type donor; metal-to-ligand pi backbonding can weaken its N–N bond without making it a fully cleaved nitride.

The anion formed by N–H deprotonation of pyrrole. It can bind through nitrogen as a two-electron ${\eta }^1$ donor, or through its aromatic ring as a six-electron ${\eta }^5$ donor. Specifying hapticity is essential for electron counting.

A coordinated oxide, formally ${\mathrm{O}}^{2-}$. A terminal $\mathrm{M}=\mathrm{O}$ unit is conventionally counted as an $X_2$ ligand giving four electrons in ionic electron counting. Additional oxygen-to-metal pi donation is an orbital-bonding issue and must not be silently mixed with another counting convention.

A carboxylate bound through one or both oxygen atoms. It can be monodentate, chelating or bridging; its charge is $-1$ regardless of whether one or two oxygen donors bind.

An ortho-dioxygen aromatic framework that can bind through both oxygen atoms in several redox states. The semiquinonate monoanion has a ligand-centred unpaired electron, whereas the catecholate dianion has a filled-shell configuration. non-innocent ligand behaviour means metal oxidation state cannot be inferred from an overall complex charge without specifying the ligand redox state.

A free radical monoanion intermediate in redox state between a quinone and a catecholate; an isolated ligand has spin $S=1/2$. Its spin may couple to a metal spin in a complex.

The dianionic, closed-shell O,O donor obtained by two deprotonations of catechol.

A bidentate ligand binds a metal through two donor atoms. Simultaneous attachment can produce a chelate ring; the two donor atoms need not be the same element.

The number of donor atoms from one ligand in coordination chemistry bonded to a particular metal centre. Unlike hapticity, it counts distinct donor sites rather than a contiguous bonded fragment.

A metal-bound methyl group, formally CH3-minus in ionic electron counting; it is commonly an X-type ligand.

The anion ${\mathrm{C}}_3{\mathrm{H}}_3{\mathrm{N}}_2^{-}$ formed from pyrazole. Bridging through both nitrogen atoms supplies four ligand electrons overall in ionic counting, or three in neutral counting.

Coordinated NO is non-innocent. Conventional neutral counting assigns three electrons to a terminal linear NO, one to a terminal bent NO, and three in total to a simple doubly bridging NO.

A bent M–N–O mode conventionally represented as $\mathrm{N}{\mathrm{O}}^{-}$, a two-electron ionic donor and a one-electron neutral donor. These are formal bookkeeping limits rather than measured atomic charges.

A near-linear M–N–O coordination mode conventionally represented as $\mathrm{N}{\mathrm{O}}^{+}$, a two-electron donor in ionic counting and a three-electron donor in neutral counting.

A nitrosyl shared between metal centres. In the simple mu2 counting model it contributes three electrons overall in the neutral method, or can be represented as an $\mathrm{N}{\mathrm{O}}^{-}$ donor of four electrons overall in ionic counting.

An oxygen-bound anionic ligand derived formally by removing a proton from an alcohol; it is normally X-type in electron counting.

An alkyne bound through its pi system. The neutral two-electron donor picture and a reduced metallacyclic picture emphasize different limits of the same bonding interaction.

A coordinated aryne. Strong metal-to-aryne electron transfer can make a dianionic metallacyclopropene description more useful than an unreduced neutral alkyne description.

A ligand attached through an alkene carbon, commonly an X-type ligand in electron counting.

A ligand attached to a metal through an aromatic carbon, commonly counted as an X-type ligand with charge minus one in ionic bookkeeping.

A bromide ligand is coordinated $\mathrm{B}{\mathrm{r}}^{-}$, conventionally an anionic two-electron donor in ionic electron counting.

A boryl ligand attaches through boron as an X-type ligand. In ionic electron counting it is formally anionic and contributes two electrons. It is distinct from a neutral borylene and from attachment to an entire borazine pi face.

An alkene ligand commonly binds through both double-bond carbons, ${\eta }^2$. The balance of donation and backbonding is described by the Dewar-Chatt-Duncanson model.

A phosphite ligand such as $\mathrm{P}(\mathrm{OR}{)}_3$ binds through a phosphorus lone pair and can accept pi backbonding. Its donation and acceptance differ from those of both phosphine ligands and carbonyl ligands.

Hapticity, written ${\eta }^n$, specifies the number of contiguous ligand atoms involved in attachment to a metal. It differs from denticity, which counts donor sites in a chelate.

A change in the number of adjacent ring atoms coordinating a metal, such as ${\eta }^5\to {\eta }^3$. It changes the coordination ligand donation and can open a site without loss of the entire ring coordination ligand.

An allyl ligand can bind as ${\eta }^3$ ${\mathrm{C}}_3{\mathrm{H}}_5^{-}$, donating four electrons in ionic electron counting. Protonation can release an alkene and make coordination sites available.

The cyclopentadienyl ligand is based on ${\mathrm{C}}_5{\mathrm{H}}_5^{-}$. In its common ${\eta }^5$ mode it binds through five ring carbons and donates six electrons in ionic electron counting. Ring substitution can create distinct NMR environments.

The five ring p orbitals form one nondegenerate and two doubly degenerate symmetry-adapted sets. About the ring normal, these match metal $d_{z^2}$, $(d_{xz},d_{yz})$, and $(d_{xy},d_{x^2-y^2})$ by sigma, pi and delta symmetry respectively. Matching symmetry permits interaction, but spatial overlap and energy separation determine its strength.

The pentamethylcyclopentadienyl ligand is the anion ${\mathrm{C}}_5\mathrm{M}{\mathrm{e}}_5^{-}$, normally a six-electron ${\eta }^5$ donor in ionic electron counting. Its methyl substituents change steric hindrance and electron donation relative to an unsubstituted cyclopentadienyl ligand.

An amido ligand contains an anionic nitrogen donor, such as $\mathrm{N}{\mathrm{R}}_2^{-}$. A neutral secondary amine retaining its N–H bond is a different donor type; confusing them changes formal metal oxidation state.

A phenoxide ligand binds through the negatively charged oxygen of deprotonated phenol or a substituted phenol. It is distinguished from a neutral phenol donor and from an ordinary neutral ether.

A chloride ligand is coordinated $\mathrm{C}{\mathrm{l}}^{-}$. It is an anionic two-electron donor in ionic electron counting, with occupied orbitals that can contribute pi donation.

A carbonyl ligand is coordinated carbon monoxide, usually attached through carbon. Sigma donation and pi backbonding cooperate in the metal–carbon bond. Terminal and bridging forms have different electron-counting descriptions.

For a series of comparable metal carbonyls, greater metal-to-CO pi backbonding populates CO antibonding orbitals, weakens its internal bond and lowers its infrared stretching frequency. Coupled normal modes and changing geometry must be considered when comparing observed bands.

A carbonyl attached primarily to one metal but with a secondary interaction with another. Formula alone does not determine whether terminal, bridging or semibridging CO is present.

A phosphine ligand such as $\mathrm{P}{\mathrm{R}}_3$ donates a phosphorus lone pair. It can also accept pi backbonding into orbitals with substantial P–R antibonding character; phosphorus $3d$ orbitals need not be essential acceptors. Alkyl substitution generally increases sigma donation relative to electron-withdrawing substituents.

The diphosphine $\mathrm{P}{\mathrm{h}}_2\mathrm{P}(\mathrm{C}{\mathrm{H}}_2{)}_3\mathrm{PP}{\mathrm{h}}_2$. Chelation to one metal makes a six-membered ring.

The diphosphine $\mathrm{P}{\mathrm{h}}_2\mathrm{PC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{PP}{\mathrm{h}}_2$. Chelation to one metal makes a five-membered ring.

The diphosphine $\mathrm{P}{\mathrm{h}}_2\mathrm{PC}{\mathrm{H}}_2\mathrm{PP}{\mathrm{h}}_2$. Chelation to one metal makes a four-membered metal–P–C–P ring.

1,2-Bis(dimethylphosphino)ethane has the connectivity $\mathrm{M}{\mathrm{e}}_2\mathrm{PC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{PM}{\mathrm{e}}_2$. Chelation through both phosphorus donors contributes four electrons in ionic electron counting.

The Tolman electronic parameter compares ligand electronic influence using the symmetric CO stretching band of a matched metal-carbonyl probe, classically $\mathrm{Ni}(\mathrm{CO}{)}_3\mathrm{L}$. Lower frequency commonly indicates greater net electron release into the remaining CO ligands. It combines donation and acceptance and is not a direct measurement of octahedral splitting.

A hydroxo ligand is coordinated $\mathrm{O}{\mathrm{H}}^{-}$. Deprotonation of coordinated water can make it without first breaking the metal–oxygen bond.

A nitrito ligand is O-bound nitrite, $\mathrm{M}-\mathrm{O}-\mathrm{N}=\mathrm{O}$. Its oxygen donor provides sigma donation and can provide pi donation; an N-bound nitro ligand is its linkage isomer.

A nitro ligand is N-bound nitrite, $\mathrm{M}-\mathrm{N}(\mathrm{O}{)}_2$, normally treated as a sigma-donor ligand and pi-acceptor ligand. The word here concerns coordinated nitrite, not an independently introduced organic nitro substituent.

A pi-acceptor ligand has a suitable unoccupied orbital for pi backbonding. Stabilization of metal $t_{2g}$-derived levels increases the usual octahedral splitting.

A pi-donor ligand has an occupied orbital that can overlap sideways with a metal orbital. In the standard octahedral splitting picture it raises the metal $t_{2g}$-derived levels, tending to reduce their separation from $e_g$.

A sigma-donor ligand supplies an occupied orbital for sigma donation. Additional occupied or empty orbitals can make it a pi-donor ligand or a pi-acceptor ligand as well.

A bridging ligand attaches to two or more metal centers. The symbol $\mu$ records bridging; its electron donation must be apportioned consistently between the metals in electron counting.

A pair of metal centres connected through one or more thiolate sulfur donors. A ${\mathrm{M}}_2(\mu -\mathrm{SR}{)}_2$ diamond can coexist with a direct metal–metal bond; each bridging thiolate supplies a donor interaction at both metals.

A ligand interacting with the vertices of a cluster face. In an Mo6Cl8 core, each inner chloride caps a triangular face of the metal octahedron.

An alkynyl-derived group connecting metal centres. Sigma attachment and alkyne pi donation can both contribute; the exact orbital distribution is not established by one averaged NMR resonance.

An alkyl group attached across metal centres. A bridging methyl can form a three-centre two-electron bond, rather than two full independent two-electron metal–carbon bonds.

A chloride-derived ligand connecting more than one centre. chlorine lone-pair donation permits a different electron count from an electron-deficient alkyl bridge.

A coordination chain in which halide ligands connect successive metal sites. Asymmetric short and long metal–halide contacts can accompany mixed valence.

A bridging carbonyl ligand spans more than one metal. A conventional two-metal bridging CO contributes one electron to each metal in neutral electron counting. Its stretching band often lies below terminal-carbonyl bands because of stronger C–O weakening, but assignment should also use composition and structural evidence.

In bidentate coordination, two donor atoms from one ligand bind the same metal. Attachment through both oxygens of nitrite is distinct from choosing between its N and O monodentate modes.

An ambidentate ligand has alternative donor atoms through which it can attach, producing different binding modes. Nitrite can bind through N or O; changing that attachment gives linkage isomerism.