Nomenclature of Complex Ions and Coordination Compounds
Complex ions are
ions which are made up of a single central atom or ion, usually a transition
metal ion, to which other atoms, molecules, or ions called
ligands can be attached. The naming of complex cations and
complex anions is similar, except that anions are always made to end in
-ate. Coordination compounds, like simpler inorganic compounds,
are named with the cation preceding the anion regardless of which (if either)
one of them is a complex ion.
The rules for naming complex ions or compounds are as follows:
To name any complex ion, list first the ligands, then the central atom.
The ligand names are made to end in -O if they are negative (chloro,
bromo, nitrito, etc.), and unless they are negative the ligand names must not
end in -O. Examples of ligand names are chloro, hydroxo, cyano, aqua (for
H2O), ammine (for NH3), and thiosulfato (for
Anions end in -ate and, when a Latin symbol is used for
the element, the element takes the Latin name in anions but
not in cations. For example,
Cu(NH3)42+ is called the
tetraamminecopper(II) ion but Cu(CN)64- is called the
hexacyanocuprate(II) ion. Likewise Al(NH3)63+
is called the hexaamminealuminum(III) ion but Al(OH)4-
is called the tetrahydroxoaluminate(III) ion.
The number of each kind of ligand is specified by the usual Greek prefix.
The ligands are named in alphabetic order. For example,
Cu(Cl2Br2I2)4- is the
dibromodichlorodiiodocuprate(II) ion while
Cu(Cl2Br2INH3)4- is the
The oxidation state of the central metal atom to which the ligands are
attached must be indicated, unless it is an element which has only one known
oxidation state such as sodium. The oxidation state is not always obvious; the
compound GaCl2 was named gallium dichloride until it was discovered
to be the dimeric Ga2Cl4. Its renaming as digallium
tetrachloride was not sufficient, since its actual structure is
Ga+[GaCl4]- and so its name really should be
gallium(I) tetrachlorogallate(III) in order to indicate the oxidation states.