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

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.

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

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 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.

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 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.

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.

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.

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.

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.

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

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

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

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.

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.

In an ideal octahedral molecular geometry, the metal $d$ orbitals divide into lower $t_{2g}$ and higher $e_g$ sets. Their separation is ${\Delta }_o$. Whether electrons pair in the lower set depends on that separation relative to pairing costs; observed many-electron transition energies need not equal ${\Delta }_o$ exactly.