Complex Ions
A ligand is a species that can donate one or
more lone pairs of electrons to form a
coordinate bond with a central metal ion.
The coordination number of a
complex is the coordinate bonds
formed to the central metal cation in
the complex ion. So, for
hexaquacopper(II), it is 6.
This is the
hexaquacopper(II)
complex ion.
A central metal ion
surrounded by
ligands.
The water molecules are
acting as ligands – specifically
unidentate ligands, as each
water is donating one lone
pair of electrons in order to
form a coordinate bond.
Ligand
No. coordinate
bonds
Examples
Unidentate
1
H2O, NH3, Cl-, CN-
Bidentate
2
1,2 diaminoethane
NH2CH2CH2NH2
Ethandioate C2O42-
Hexadentate
6
EDTA4-
A hexadentate ligand donates 6 lone pairs of electrons to
form 6 coordinate bonds with a central metal ion.
The molecule shape above is
octahedral, with a bond
angle of 90 degrees.
Octahedral molecules have a
coordination number of 6.
A bidentate ligand donates 2
lone pairs of electrons to form
2 coordinate bonds with a
central metal ion.
Unidentate Ligand
Complex
Shape & Bond
Angle
Coordination
Number
Water
[Cu(H20)6]2+
Octahedral 90
6
Ammonia
[Co(NH3)4(H20)2]2+
Octahedral 90
6
Chloride ion
[Cu(Cl)4]2-
Tetrahedral 109.5
4
Ammonia/Chloride
[Pt(NH3)2(Cl)2]
Square Planar 90
4
Cyanide-
[Cu(CN)4]2-
Tetrahedral 109.5
4
Cyanide, CN-
[Ni(CN)4]2-
Square Planar 90
4
Examples of Bidentate Ligands
Ethanedioate
(oxalate) C2O42-
Ethanedioate is able to form 2
coordinate bonds with the metal ion
by donating 2 lone pairs of electrons
(from oxygen atoms).
!,2-diaminoethane
(ethylenediamine)
NH2CH2CH2NH2
Etylenediamine is able to form 2
coordinate bonds with the metal
ion by donating 2 lone pairs of
electrons (form nitrogen atoms).
Stereoisomerism
Stereoisomers have the same structural
formula but they have a different spatial
arrangement of their atoms.
Cis
Cis isomer: unidentate ligands are 90°
apart. Trans isomer: unidentate ligands are
180 ° apart.
This is an example of stereoisomerism in a square planar
complex. In the cis-isomer, the two Cl unidentate ligands
are separated by 90. In the trans-isomer, the Cl
unidentate ligands are separated by 180.
Trans
Cis platin is used as an anti-cancer drug. It prevents the division of cancer cells by binding to
the DNA, and ultimately causes apoptosis – programmed cell death.
Optical Isomerism
Optical isomers are stereoisomers
that are non-superimposable mirror
images of each other (also called
enantiomers).
Ligand Substitution Reactions
[Cu(H20)6]2+ + 4Cl-  [Cu(Cl)4]2- + 6H2O
 Change in colour from blue to
yellow/green
 Change in coordination number
from 6 to 4: shape from
octahedral to tetrahedral
 Change in charge as chloride
ions are negative.
[Cu(H20)6]2+ + 4NH3  [Cu(NH3)4 (H20)2]2+ + 4H2O
 Change in colour from blue to
dark blue
 No change in coordination
number
 No change in charge
[Co(H20)6]2+ + 4Cl-  [Co(Cl)4]2- + 6H2O
 Change in colour from pink to
blue
 Change in coordination number
from 6 to 4: shape from
octahedral to tetrahedral
 Change in charge as chloride
ions are negative.
Haemoglobin
The haem molecule
consists of a
porphyrin ring that
forms four coordinate
bonds with an iron (II)
ion. The globin
protein forms a fifth
coordinate bond with
the iron (II) ion.
Carbon monoxide, CO, bonds to
haemoglobin with a higher affinity than
oxygen. If a person inhales CO, a ligand
exchange reaction occurs and the CO
prevents the transport of O2. This leads to
drowsiness, lack of consciousness and can
be fatal.
Oxygen molecules bind to the Fe2+
ion in the vacant 6th position of the
octahedral complex. The binding of
the O2 is weak enough to allow it to
be removed easily.
Globin Protein
O2