Formation of Complexes
Complex compounds consist of a central metal atom surrounded by ligands.
A ligand is an electron pair donor.
A ligand may be an atom, an ion or a molecule.
N.B. Ligands, nucleophiles and Lewis bases are all defined as electron pair donors.
Since the ligand donates both of the electrons it shares with the central metal ion, a
coordinate bond is formed.
The co-ordination number is the number of atoms bonded to the metal ion.
In a complex, the co-ordination number of the metal differs from its oxidation state.
Unidentate Ligands
Some ligands have only one atom which can donate a pair of electrons, they therefore
form only one bond with the metal ion. These are described as unidentate ligands;
examples of unidentate ligands are:
CN-
NH3
OH-
H2O
F-
Cl-
Br-
For a complex which is formed from a metal ion and unidentate ligands only, the number
of ligands is equal to the co-ordination number.
e.g.
hexaaquairon(II) ion, [Fe(H2O)6]2+
H
H
2+
..
O
H
:O
O:
H
H
H
Fe
O:
:O
H
H
H
H
..
shape: octahedral
no. of ligands = 6
co-ordination no. = 6
oxidation state of iron = +2
O
H
H
The Fe-O and O-H bonds within this complex ion are covalent bonds.
TOPIC 13.18: TRANSITION METALS COMPLEXES 1
Bidentate Ligands
Some ligands contain two atoms which can donate a pair of electrons, they are therefore
able to form two bonds with the metal ion. These are described as bidentate ligands;
examples of bidentate ligands are:
O:
NH2
..
-
O
CH2
C
CH2
C
..
..N
O
-
O:
NH2
1,2-diaminoethane (en)
e.g
..N
ethanedioate ion
bipyridyl (bipy)
[Ni(en)3]2+
CH2
NH2
..
CH2
2+
CH2
shape: octahedral
:NH2
H2N:
CH2
no. of ligands = 3
Ni
H2N:
co-ordination no. = 6
:NH2
..
oxidation state of nickel = +2
CH2
NH2
CH2
The number of ligands is no longer equal to the co-ordination number.
Multidentate Ligands
Some ligands contain many atoms which can donate a pair of electrons, they are
therefore able to form many bonds with the metal ion. These are described as
multidentate ligands. The most important multidentate ligand is EDTA4-, which has six
donor atoms. It is the anion of bis[di(carboxymethyl)amino]ethane:
..
..
-
-OOCCH
2
..
..
NCH2CH2N
OOCCH2
CH2COO
..-
..
CH2COO-
EDTA forms 1:1 octahedral complexes with M2+ ions, for example:
[Cu(H2O)6]2+ + EDTA4[Cu(EDTA)]2- + 6H2O
TOPIC 13.18: TRANSITION METALS COMPLEXES 2
The red pigment haem, which
is a complex of iron(II), is found
in blood. The iron is bonded to
the four nitrogen atoms of a
planar porphyrin molecule. The
porphyrin
molecule
is
a
quadridentate ligand.
N
N
Fe
N
N
In haemoglobin, the iron has a
coordination number of 6, with
octahedral
geometry.
In
addition to its four bonds to the
porphyrin molecule, iron bonds
to a fifth nitrogen atom, from
the protein globin, and to either
an
oxygen
molecule
(in
oxygenated blood) or a water
molecule (in deoxygenated
blood). Changing the ligand
from O2 to H2O changes the
colour of the complex from
bright red to red-purple.
Strong field ligands, such as CO and CN- are able to bond more strongly to haem than
oxygen can; they can therefore prevent the blood from carrying oxygen. This explains
the high toxicity of carbon monoxide and cyanides.
Shapes of Complex Ions
Co-ordination number 2
Silver ions and copper(I) ions, both of which have a d 10 configuration, are unusual in
forming linear complexes by bonding to only two ligands. The bond angle is 180 o.
Examples:
[CuCl2]This complex ion, which is one of the few stable complexes of copper(I), is
a dark olive brown in colour. When it is poured into cold water, it is hydrolysed to
copper(I) chloride, which forms as a white precipitate.
[Ag(NH3)2]+ This complex ion, which is formed when either AgCl or Ag 2O dissolves in
excess aqueous ammonia, is colourless because of the d 10 configuration of silver(I). It is
the ion present in Tollen’s reagent. Tollen’s reagent is used to distinguish aldehydes
from ketones: when aldehydes are heated with Tollen’s reagent, the complex is reduced
to metallic silver, which is deposited on the test tube walls as a mirror. There is no
reaction with ketones.
RCHO + 2[Ag(NH3)2]+ + 3OH-
TOPIC 13.18: TRANSITION METALS COMPLEXES 3
RCOO- + 2Ag +4NH3 +2H2O
[Ag(S2O3)2]3- A photographic film contains an emulsion of AgBr. Where a film has
been exposed to light, AgBr is converted to finely divided silver during the development
process; this appears black on the negative. In order to render the film insensitive to
further exposure to light, any unreacted AgBr must be removed. This is done during the
fixing process by adding sodium thiosulphate solution. The water-soluble linear complex
ion [Ag(S2O3)2]3- is formed.
AgBr + 2S2O32-
[Ag(S2O3)2]3- + Br-
[Ag(CN)2]- This complex ion, which is formed in solution when silver salts dissolve in
an aqueous solution of potassium cyanide, is used as the electrolyte in silver plating.
Co-ordination number 4
Where a ligand is large and negatively charged, such as Cl -, Br-, I-, there is not usually
enough space to fit six ligands around the central metal atom, and four ligands bond
instead. Complexes with a co-ordination number of four are usually tetrahedral in shape.
2-
Examples:
2-
Cl
Cl
Cu
Co
Cl
Cl
Cl
[CuCl4]2yellow-green
Cl
Cl
Cl
[CoCl4]2blue
Another possible shape for four ligands around a central metal ion is square planar.
Although generally rare, this geometry occurs in some nickel complexes, such as
[Ni(CN)4]2-, and is common among complexes of platinum. An important example is
cisplatin, [PtCl2(NH3)2].
Cl
H3N
Pt
Cl
NH3
Cisplatin is an anti-cancer drug. It is used in
chemotherapy for certain types of cancer, such as
testicular cancer, and has had a major success rate.
Co-ordination number 6
The most common shape for complex ions is octahedral; it is found with a wide variety
of ligands and metals. For example:
[Cu(H2O)6]2+
[Ni(NH3)6]2+
[Fe(CN)6]3[Cr(OH)6]3- [PtCl6]2- [Co(NO2)6]3All the transition metals of the first row transition series form hexaaqua ions which are
octahedral.
TOPIC 13.18: TRANSITION METALS COMPLEXES 4