Ligand
An ion or molecule that donates one or more electron pairs to a central metal atom.
Unit
Dimensionless
In depth
A ligand must possess at least one lone pair, so it is by definition a Lewis base. Ligands are classified by how many donor atoms they use and by their position in the spectrochemical series, which determines how strongly they split the metal's d orbitals.
Examples in the real world
H₂O, NH₃, Cl⁻, CN⁻ and CO as simple ligands; EDTA as a six-donor ligand.