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Complex ions
Transition metals form at least one cation
with vacant d orbitals.
Ligands
Vacant d orbitals on transition metal ions can
accept electron pairs from anions or
molecules with a lone pair.
As they are “tied” to the metal cation these
ions and molecules are referred to as
ligands.
Ligands form co-ordinate (dative) covalent
bonds with transition metal cations.
Eg; The commonest copper ion is Cu2+
Water has a lone pair; H2O:
This means that it can act as a ligand, donating
the lone pair to Cu.
Hydrated copper (ii) ions are therefore complex
ions;
Cu2+ + 6H2O → [Cu(H2O)6]2+
[ ] are used to represent a complex ion.
Some ligands form only one co-ordinate
bond with the metal cation.
They are referred to as monodentate (“onetoothed”).
Eg
H2O
CN-
Bidentate (“two toothed”),
ligands can make two coordinate bonds.
Bidentate
ligands
XX
XX
Eg; Ethylene diamine (en).
• Write out the structures of the following
ions and show how they can act as
bidentate ligands.
• Benzene 1,2 diol
• Ethandioate ion (C2O42-)
EDTA
How many co-ordinate bonds can
ethylene diamine tetra acetate form with
a transition metal ion?
It is
hexadentate.
Complexes with polydentate ligands are called chelates (after the Greek for
claw) and are very effective at removing transition metals from solutions.
Co-ordination number
The co-ordination number of a complex ion is
the number of co-ordinate bonds it contains.
The most common co-ordinate numbers are 2,
4 and 6.
Eg;
[Cu(H2O)6]2+
Each ion involves
six co-ordinate
bonds, therefore it
has a co-ordination
number of 6.
Shapes of complex ions.
Electrons in the dative bonds are all negative.
Like charges repel each other.
So the ligands will arrange themselves so that
they will be as far apart as possible.
So the shape of a complex ion depends on its
co-ordination number.
Co-ordination number of 2 eg; [Ag(NH3)2]+
The structure will be linear.
3HN:
+
Ag
:NH3
With bond angles of 18Oo
A co-ordination number of 4 has two different possible
structures.
a) Planar
b) Tetrahedral
Eg;
[Pt(NH3)2(Cl)2]2-
Four ligands in
the same plane.
All bond
angles are 90o.
b) Tetrahedral Complex eg [CoCl4]2-
Four ligands
form a
tetrahedron
Bond angles
are all 109.5 o
Complexes with a co-ordination number
of 6 are octahedral eg [Fe(H2O)6]3+
One above,
Four
ligands are
in the same
plane.
NB All bond
angles are 90o.
One below
• Predict the shape of the following
complex ions;
• 1) [Co(NH3)5Cl]2+
• 2) [Cr(H2O)4Cl2]+
• 3) [FeCl4]
Haemoglobin
Fe2+ ion
with a coordination
number of
6.
Four coordinate bonds
are made with
lone pairs on the
nitrogen of a
porphyrin ring
(haem)
A fifth bond is made
below the plane of
the ring with the
protein globin.
Water can also weakly bind.
The Fe2+/ O2 bond is weak
hence easily broken.
Oxygen molecules
act as ligands to
make the sixth coordinate bond.
• Other ligands can form
stronger bonds with Fe2+
, binding irreversibly.
• Eg; Carbon monoxide.