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Ligands and Complex Ions

Key Concepts

  • A ligand, or complexing agent, is a polar molecule or ion bonded to a central metal ion.

  • A complex ion is a polyatomic species consisting of a central metal ion surrounded by several ligands.

  • Naming Complex Ions:
  1. Anionic ligands have names ending in 'o'.
    1. 'ide' → 'o', eg, chloride → chloro
    2. 'ate' → 'ato', eg, sulfate → sulfato

  2. Neutral ligands are named as the molecule with these notable exceptions:
    1. H2O → aquo
    2. NH3 → ammine
    3. CO → carbonyl

  3. The numbers of ligands in a complex are specified using the Greek prefixes:
    1. di for 2
    2. tri for 3
    3. tetra for 4
    4. penta for 5
    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. The name of an anionic complex ion ends in 'ate',
    sometimes the latin name is used, eg,
        chromium (II) → chromate (II)
        nickel (II) → nickelate (II)
        platinum (II) → platinate (II)
        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.

  • Writing the Formula of a Complex:
  1. Identify the central metal ion

  2. Identify the charge on the central metal ion (shown in 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
        = charge on metal ion + total charge on ligands

  7. Write the formula giving the central metal ion first followed by the ligands, eg, Mn(H2O)62+

  8. If more than one type of ligand is present, anion ligands are given before neutral ligands, eg, CrCl2(NH3)4+
  • The coordination number of the complex ion is the number of bonds formed between the central metal ion and its ligands.

  • The shape (geometry) of a complex ion is related to its coordination number.

Common Ligands

Ligands can be:
  • polar molecules (neutral species)

  • anions (negatively charged species)
Compound Name Formula Ligand Name Bonding Atom
Polar Molecules
water H2O aquo O
carbon monoxide CO carbonyl C
ammonia NH3 ammine N
Anions
chloride Cl- chloro Cl
fluoride F- fluoro F
bromide Br- bromo Br
iodide I- iodo I
cyanide CN- cyano C
hydroxide OH- hydroxo O
thiosulfate S2O32- thiosulfato O
carbonate CO32- carbonato O and O
oxalate C2O42- oxalato O and O

Naming Complex Ions

Example: Name the complex ion with the forumla Fe(CN)63-
  1. Anionic ligands have names ending in 'o'.
    CN- named as cyano

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

  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
    Charge on iron: 3- = x + (6 x 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: hexacyanoferrate (III)

Examples

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- cyano 2 → di silver (I) → argentate (I)
(-1= x + 2(-1), x = +1)
dicyanoargentate (I) ion
(complex is an anion)
Cu(H2O)62+ aquo 6 → hexa copper (II)
(+2= x + 6(0), x = +2)
hexaaquocopper (II) ion
(complex is a cation)
CuCl42- chloro 4 → tetra copper (II) → cuprate (II)
(-2= x + 4(-1), x = +2)
tetrachlorocuprate (II) ion
(complex is an anion)

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 charge on the central metal ion (shown in parantheses): 2+

  3. Identify the ligands: ammine = NH3 (neutral species)

  4. Identify the number of ligands: tetra = 4

  5. Calculate the total charge on the ligands = 4 x 0 = 0

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

  7. Write the formula giving the central metal ion first followed by the ligands : Cu(NH3)42+

Examples

Name Central Ion
Formula
Ligand
Formula
No. of
Ligands
Complex Ion
Formula
hexaaquocobalt (II) ion Co2+
(charge in parentheses)
H2O
(aquo = H2O)
hexa = 6 Co(H2O)62+
(4 x 0 +2 = +2)
tetrachlorocobaltate (II) ion
(ate = anion)
Co2+
(charge in parentheses)
Cl-
(chloro = Cl-)
tetra = 4 CoCl42-
(4 x -1 + 2 = -2)
tetracarbonylnickel (II) ion Ni2+
(charge in parentheses)
CO
(carbonyl = CO)
tetra = 4 Ni(CO)42+
(4 x 0 + 2 = +2)
tetracyanonickelate (II) ion
(ate = anion)
Ni2+
(charge in parentheses)
CN-
(cyano = CN-)
tetra = 4 Ni(CN)42-
(4 x -1 +2 = -2)

Shapes (Geometry) of Some Complex Ions

  • Coordination number = number of ligands = 2 → linear

  • Coordination number = number of ligands = 4 → tetrahedral or square-planar

  • Coordination number = number of ligands = 6 → octahedral
    (octahedral geometry is most common for transition metal complexes)

Examples

Complex Ion Formula No. of Ligands Coordination Number Shape
Ag(NH3)2+ 2 2 linear
CuCl2- 2 2 linear
Cr(NH3)63+ 6 6 octahedral
Fe(CN)63- 6 6 octahedral


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