Ligand

Ligand:

 A molecule, ion, hormone or compound that binds to a specific receptor site that binds to another molecule.
The atoms or groups of atoms bound to the CENTRAL
ATOM (see COORDINATION). The root of the word
is sometimes converted into the verb to ligate, meaning
to coordinate as a ligand, and the derived participles,
ligating and ligated. This use should not be confused
with its use to describe the action of LIGASEs (a class of
ENZYMEs). The names for anionic ligands, whether
inorganic or organic, end in -o. In general, if the anion
name ends in -ide, or -ate, the final -e is replaced by -o,
giving -ido and -ato, respectively. Neutral and cationic
ligand names are used without modification. Ligands
bonded by a single carbon atom to metals are regarded
as radical substituents, their names being derived from
the parent hydrocarbon, from which one hydrogen atom has been removed. In general, the final letter -e of the name is replaced by -yl. In biochemistry the term ligand has been used more widely: if it is possible or convenient to regard part of a polyatomic molecular entity as central, then the atoms or groups or molecules bound to that part may be called ligands.