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IUPAC Name and Formula of Ligands and Complex Ions Introductory Chemistry Tutorial

Key Concepts

  1. Anionic ligands have names ending in 'o'.(2)

    ⚛ 'halide' → 'halido', eg, chloride → chlorido

    ⚛ anion ending in 'ate' → 'ato', eg, sulfate → sulfato

    ⚛ anion ending in 'ite' → 'ito', eg, sulfite → sulfito

  2. Neutral ligands are named as the molecule without modification, with these notable exceptions:(3)

    ⚛ H2O → aqua

    ⚛ NH3 → ammine

    ⚛ CO → carbonyl

    ⚛ NO → nitrosyl

  3. The numbers of ligands in a complex are specified using the Greek prefixes:(4)

    ⚛ di for 2

    ⚛ tri for 3

    ⚛ tetra for 4

    ⚛ penta for 5

    ⚛ hexa for 6

  4. The name of a cationic complex ion ends in the name of the central metal ion with the oxidation state shown as a Roman numeral in parantheses at the end of the metal's name, eg, iron(III).(5)

  5. The name of an anionic complex ion ends in 'ate',
    ⚛ chromium(II) → chromate(II)
    ⚛ nickel(II) → nickelate(II)
    ⚛ platinum(II) → platinate(II)

    sometimes the latin name is used, eg,
    ⚛ silver(I) → argentate(I)
    ⚛ iron(II) → ferrate(II)
    ⚛ copper(I) → cuprate(I)
    ⚛ lead(II) → plumbate(II)
    ⚛ gold(I) → aurate(I)
    ⚛ tin(IV) → stannate(IV)

  6. Ligands are named before the central metal atom.(6)

  1. Identify the central metal ion

  2. Identify the oxidation state on the central metal ion (shown in Roman numerals parantheses)

  3. Identify the ligands

  4. Identify the number of ligands

  5. Calculate the total charge on the ligands

  6. Calculate the charge on the complex ion
        = oxidation state of metal ion + total charge on ligands

  7. Write the formula giving the central metal ion first followed by the ligands, and enclose everything except the charge in square brackets:(7)
    for example, [Mn(H2O)6]2+ and [Fe3(CO)12]

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Common Ligands

Ligands can be:

The table below lists some common polar molecules which can be present as ligands:

Polar Compound Name Formula Ligand Name Bonding Atom
water H2O aqua O
carbon monoxide CO carbonyl C
ammonia NH3 ammine N
nitrogen monoxide NO nitrosyl N

The table below lists some anions which can be present as ligands:

Ion Name Formula Ligand Name Bonding Atom
chloride Cl- chlorido Cl
fluoride F- fluorido F
bromide Br- bromido Br
iodide I- iodido I
hydride H- hydrido H
cyanide CN- cyanido C
hydroxide OH- hydroxido O
thiosulfate S2O32- thiosulfato O
carbonate CO32- carbonato O and O
oxalate C2O42- oxalato O and O

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Naming Complex Ions

Example: Name the complex ion with the forumla [Fe(CN)6]3-

  1. Anionic ligands have names ending in 'o'.
    CN- (cyanide ion) named as cyanido

  2. The numbers of ligands in a complex are specified using a Greek prefix:
    6 ligands = hexa → hexacyanido

  3. Oxidation state of the central metal atom is shown with a Roman numeral in parantheses at the end of the metal's name:
    Central metal ion is iron
        oxidation state of iron: 3- = x + (6 × 1-)
        3- = x -6
        x = 3+
    Central metal ion: iron(III)

  4. The complex ion is an anion, therefore the name will end in ferrate(III)

  5. Ligands are named before central metal ion: hexacyanidoferrate(III)

Examples of Names of Complex Ions

Formula Ligand
Name 		No. of Ligands
and prefix 		Central Ion
Name 		Complex Ion Name
[Ag(NH3)]2+ ammine 2 → di silver(I)
+1= x + 2(0)
x = +1
 diamminesilver(I) ion
(complex is a cation)
[Ag(CN)2]- cyanido 2 → di silver(I) → argentate(I)
-1 = × + 2(-1)
x = +1
dicyanidoargentate(I) ion
(complex is an anion)
[Cu(H2O)6]2+ aqua 6 → hexa copper(II)
+2 = x + 6(0)
x = +2
hexaaquacopper(II) ion
(complex is a cation)
[CuCl4]2- chlorido 4 → tetra copper(II) → cuprate(II)
-2 = x + 4(-1)
x = +2
tetrachloridocuprate(II) ion
(complex is an anion)

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Writing the Formula of Complex Ions

Example: write the formula for the complex ion tetraamminecopper(II)

  1. Identify the central metal ion :
    copper, Cu

  2. Identify the oxidation state of the central metal ion (shown in parantheses):
    2+

  3. Identify the ligands:
    ammine = NH3 (neutral species, charge on ligand = 0)

  4. Identify the number of ligands:
    tetra = 4

  5. Calculate the total charge on the ligands:
    = 4 × 0 = 0

  6. Calculate the charge on the complex ion
    = oxidation state of metal ion + total charge on ligands = 2+ + 0 = 2+

  7. Write the formula giving the central metal ion first followed by the ligands, everything enclosed in square brackets followed by the charge on the ion as a superscript:
    [Cu(NH3)4]2+

Examples of Formulae of Complex Ions

Name Central Ion
Formula 		Ligand
Formula 		No. of
Ligands 		Complex Ion
Formula
hexaaquacobalt(II) ion Co2+
(interpreting oxidation state as a charge in parentheses) 		H2O
(aqua = H2O)
 hexa = 6 [Co(H2O)6]2+
(6 × 0) +2 = 2+)
tetrachloridocobaltate(II) ion
(ate = anion) 		Co2+
(interpreting oxidation state as a charge in parentheses) 		Cl-
(chlorido = Cl-)
 tetra = 4 [CoCl4]2-
(4 × -1) + 2 = 2-
tetracarbonylnickel(II) ion Ni2+
(interpreting oxidation state as a charge in parentheses) 		CO
(carbonyl = CO)
 tetra = 4 [Ni(CO)4]2+
(4 × 0) + 2 = 2+
tetracyanidonickelate(II) ion
(ate = anion) 		Ni2+
(interpreting oxidation state as a charge in parentheses) 		CN-
(cyanido = CN-)
 tetra = 4 [Ni(CN)4]2-
(4 × -1) +2 = 2-)

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Shapes (Geometry) of Some Complex Ions

Examples of the Shapes of Some Complex Ions

The shape, or geometry, of a number of complex ions is given in the table below:

Complex Ion Formula No. of Ligands Coordination Number Shape
(Geometry)
[Ag(NH3)2]+ 2 2 linear
[CuCl2]- 2 2 linear
[Cr(NH3)6]3+ 6 6 octahedral
[Fe(CN)6]3- 6 6 octahedral

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Footnotes: reference "Nomenclature of Inorganic Chemistry: IUPAC Recommendations 2005" (Red Book)

(1) Historically, coordination compounds were considered to be formed by adding independently stable compounds to a simple central atom or ion. Naming of these coordination compounds was therefore based on an additive principle whereby the names of added compounds and the central atom were combined.
In this tutorial we will name the complex ion using the oxidation state for the central metal atom. This form of additive nomenclature has been in use for a very long time, but it has problems, notably it may not accurately reflect the distribution of electrons within the complex ion.
There are other, arguably better, systematic IUPAC ways to name a complex ion, for example, you could name the complex ion as an ion (see Naming cations and Naming Anions) in which case you enclose the charge on the ion in round brackets after the additive name (no need to try to determine the oxidation state of the metal and hence removes the problem of electron distribution).

(2) Previously, anionic ligands were named by removal of "ide" and replacement by "o", for example chloride became chloro, and, removal of "ate" and replacement with "o" for example sulfate became sulfo.
Note the changes: final "e" is removed and replaced by "o", chloride becomes chlorido and sulfate becomes sulfato.
Also note that hydride becomes hydrido when coordinating to all elements EXCEPT boron.
Coordinated cyanide is also named cyanido.

(3) The names of cationic ligands are also used without modification, even if the name ends in "ide", "ate" or "ite".
Note the change of name when water is a ligand from the previous "aquo" to the new "aqua".

(4) This is true for the names of simple ligands. For complex ligand names, the prefixes bis, tris, tetrakis etc are used with enclosing marks around the multiplicand in order to aviod ambiguity.

(5) If the metal has a negative oxidation state, then a minus sign is written to the left of the Roman numeral indicating its oxidation state, eg, Fe(-II) If the oxidation state is zero, then an arabic 0 is placed in parentheses.

(6) If more than one type of ligand is present, the ligands are named in alphabetical order (disregard any multiplicative prefixes when determining that order), eg, [CoCl(NH3)5]Cl2 is pentaaminechloridocobalt(2+) chloride.

(7) If more than one type of ligand is present, the symbols are given in alphabetical order, eg, if Cl- and NH3 both occur as ligands in the same coordination compound, then, because C occurs before N in the alphabet, so we write [CrCl2(NH3)4]+
Note that the ligand CO preceded the Cl ligand because single letter symbols preceded two letter symbols.
Note the change: previously we wrote the symbols for anionic ligands before the symbols for neutral ligands.

(8) This is NOT an exhaustive list of shapes, but these are the ones you are most likely to encounter in an introductory inorganic chemistry course.