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Complex Ions and Coordination Compounds Nomenclature
1. A complex ion consists of a central metal cation (usually derived from a transition
metal) joined by coordinate covalent bonds to molecules or anions called ligands.
Complex ions can be cations or anions. The coordination number is the number of
places on the metal ion where ligands are bound. The bond between the metal ion
and the ligand, where the ligand supplies both electrons, is known as a coordinate
covalent bond. Most metal ions exist as water complexes in water solution. For
example, Cu+2 is not a bare ion in water: rather, it exists as the Cu(H2O)6+2 ion.
Complex Ion
Ag(NH3)2+
Cu(H2O)42+
Fe(CN)63-
Central Metal
Cation
Ag+
Cu2+
Fe3+
Ligands
2 NH3 molecules
4 H2O molecules
6 CN- ions
Coordination Number (#
of bonds)
2
4
6
2. Compounds composed of a complex ion and counter ions are coordinate compounds.
Complex ions are often enclosed in brackets. Coordinate compounds are usually
colored.
Coordinate
Complex Ion
Compound
[Cu(H2O)4]SO4
Cu(H2O)42+
Counter Ion
SO42-
[Ag(NH3)2]NO3
Ag(NH3)2+
NO3-
K3[Fe(CN)6]
Fe(CN)63-
K+
3. Ligands: ordinarily contain at least one unshared pair of electrons
Note: Ligands are Lewis Bases, Metals are Lewis Acids. If the ligand contains
two or more unshared pairs on different, nonadjacent atoms, it can act as a
chelating agent, forming more than one bond with the central metal atom.
Rhonda Alexander
Robert E. Lee High School, Tyler, TX
2
4. Nomenclature of Complex Ions and Coordination Compounds
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 one of
them is a complex ion.
The Rules for Naming Coordinate Compounds:
I. Cation name precedes anion name
II. Naming complex ion:
 Ligands are listed first then central metal ion.
 Negative ligand are often named by replacing the ion ending with an –o. (chloro,
bromo, nitrito, etc.). Examples of ligand names are chloro, hydroxo, cyano, aqua
(for H2O), ammine (for NH3), and thiosulfato (for S2O32-).
Ligands Names Not Ending in –o:
H2O - aqua
NH3 - ammine
CH3NH2 - methylamine
CO - carbonyl
NO - nitrosyl
 The number of each kind of ligand is specified by the usual Greek prefix. The
ligands are named in alphabetic order. Prefixes are not used to alphabetize.
Rhonda Alexander
Robert E. Lee High School, Tyler, TX
3


The oxidation state of the central metal ion is designated by a Roman numeral in
parentheses.
If the complex is the anions the complex name must end in –ate. When a Latin
symbol is used for the element, the element takes the Latin name in anions but
not in cations.
For example:
1. Central metal have a Latin symbol:
Cu(NH3)42+ is called the tetraamminecopper(II) ion but
Cu(CN)64- is called the hexacyanocuprate(II) ion.
2. Central metal does not have a Latin symbol:
Al(NH3)63+ is called the hexaamminealuminum(III) ion but
Al(OH)4- is called the tetrahydroxoaluminate(III) ion.
Latin Names Used for Metals in Anionic Complex Ions
Iron
Ferrate
Copper
Cuprate
Pb
Plumbate
Silver
Argentate
Gold
Aurate
Tin
Stannate
Example.
1.
2.
3.
4.
5.
6.
7.
8.
9.
[Co(NH3)4Cl2]Cl: dichlorotetramminecobalt(III) chloride.
[Co(NH3)5Cl]Cl2: monochloropentamminecobalt(III) chloride.
K3[Co(NO2)6]: potassium hexanitritocobaltate(III).
[Pt(NH3)3Cl]2[PtCl4: di(monochlorotriammineplatinum(II))
tetrachloroplatinate(II).
[Pt(NH3)4][PtCl4]: tetrammineplatinum(II) tetrachloroplatinate(II).
NH4[Cr(NH3)2(SCN)4]: ammonium tetrathiocyanatodiamminechromate(III).
[Pt(NH3)6]Cl4: hexammineplatinum(IV) chloride.
K[Pt(NH3)Cl3]: potassium trichloroammineplatinate(II).
K[Pt(NH3)Cl5]: potassium pentachloroammineplatinate(IV)
Rhonda Alexander
Robert E. Lee High School, Tyler, TX