SUBSITUTION REACTIONS
When the M-O co-ordinate bond is broken, the water ligand is lost and is replaced by
another ligand. This is a ligand substitution reaction (nucleophilic substitution). The
ligands can be neutral molecules or negative ions and may be unidentate, bidentate or
multidentate.
1. Neutral unidentate ligands
The water molecules in an octahedral hexaaqua metal ion can be replaced sequentially
by ammonia molecules. Water and ammonia are neutral molecules of similar size, and
therefore no change in shape or co-ordination number takes place during ligand
substitution.
[M(H2O)6]2+ + NH3
[M(H2O)5(NH3)]2+ + H2O
[M(H2O)5(NH3)]2+ + NH3
[M(H2O)4(NH3)2]2+ + H2O
[M(H2O)4(NH3)2]2+ + NH3
[M(H2O)3(NH3)3]2+ + H2O
[M(H2O)3(NH3)3]2+ + NH3
[M(H2O)2(NH3)4]2+ + H2O
[M(H2O)2(NH3)4]2+ + NH3
[M(H2O)(NH3)5]2+ + H2O
[M(H2O)(NH3)5]2+ + NH3
The overall reaction is:
[M(H2O)6]2+ + 6NH3
[M(NH3)6]2+ + H2O
[M(NH3)6]2+ + 6H2O
In these reactions, ammonia is acting as a Lewis base (ligand / nucleophile).
Reaction with [Co(H2O)6]2+
When ammonia is added to a solution containing hexaaquacobalt(II) ions, a sequence
of reactions takes place. Initially a green-blue precipitate of cobalt(II) hydroxide is
formed. Here ammonia acts as a Bronsted-Lowry base and accepts protons in the
acidity reaction of the cobalt(II) ions.
[Co(H2O)6]2+ + 2NH3
pink solution
[Co(H2O)4(OH)2] + 2NH4+
blue-green precipitate
When an excess of concentrated ammonia is added, the precipitate re-dissolves to give
a pale straw-coloured solution. This is a ligand substitution reaction:
[Co(H2O)4(OH)2] + 6NH3
blue-green precipitate
[Co(NH3)6]2+ + 4H2O + 2OHstraw coloured solution
The overall reaction can be represented as:
[Co(H2O)6]2+ + 6NH3
[Co(NH3)6]2+ + 6H2O
If air is allowed to enter the reaction vessel, the hexaamminecobalt(II) ion is oxidised to
the yellow hexaamminecobalt(III) ion (and other ammines). The reaction mixture turns
from a pale straw colour to dark brown.
TOPIC 13.21: SUBSTITUTION REACTIONS 1
Reaction with [Cu(H2O)6]2+
When concentrated aqueous ammonia is added to a solution containing
hexaaquacopper(II) ions, only four of the six water molecules are replaced by ammonia.
Initially a blue precipitate of copper(II) hydroxide is formed. Here ammonia acts as a
Bronsted-Lowry base and accepts protons in the acidity reaction of the copper(II) ions.
[Cu(H2O)6]2+ + 2NH3
blue solution
[Cu(H2O)4(OH)2] + 2NH4+
blue precipitate
When an excess of concentrated ammonia is added, the precipitate re-dissolves to give
a deep blue solution containing tetraamminebisaquacopper(II) ions. This is a ligand
substitution reaction:
[Cu(NH3)4(H2O)2]2+ + 2H2O + 2OHdeep blue solution
[Cu(H2O)4(OH)2] + 4NH3
blue precipitate
The overall reaction can be represented as:
[Cu(H2O)6]2+ + 4NH3
H
[Cu(NH3)4(H2O)2]2+
+ 4H2O
H
..O
2+
H3N:
: NH3
longer and
weaker bonds
Cu
H3N:
: NH3
..
O
H
H
Further substitution of water molecules is possible if the concentration of ammonia is
increased, by for example using liquid ammonia instead of a concentrated aqueous
solution.
TOPIC 13.21: SUBSTITUTION REACTIONS 2
2. Neutral bidentate ligands
1,2-diaminoethane (en) is a bidentate ligand which has two donor nitrogen atoms. When
this ligand is added to a solution containing hexaaqua metal ions, the water molecules
are sequentially replaced, each 1,2-diaminoethane molecule replacing two water
molecules. The octahedral geometry is retained.
[M(H2O)6]2+ + H2NCH2CH2NH2
[M(H2O)4(H2NCH2CH2NH2)]2+ + 2H2O
[M(H2O)4(H2NCH2CH2NH2)]2+ + H2NCH2CH2NH2
[M(H2O)2(H2NCH2CH2NH2)2]2+ + 2H2O
[M(H2O)2(H2NCH2CH2NH2)2]2+ + H2NCH2CH2NH2
The overall reaction is:
[M(H2O)6]2+ + 3 H2NCH2CH2NH2
[M(H2NCH2CH2NH2)3]2+ + 2H2O
[M(H2NCH2CH2NH2)3]2+ + 6H2O
Example: [Ni(en)3]2+
CH2
NH2
..
CH2
2+ shape: octahedral
CH2
:NH2
H2N:
CH2
Ni
H2N:
no. of ligands = 3
co-ordination no. = 6
oxidation state of nickel = +2
:NH2
..
CH2
NH2
CH2
This complex is chiral and can
therefore
exhibit
optical
activity.
The positive entropy change which takes place during this reaction drives the
equilibrium well to the right hand side. Equilibrium constants for the formation of
metal(II)-tris(1,2-diaminoethane) complexes are of the order of 1020.
A similar reaction occurs with hexaaqua metal(III) ions:
[M(H2O)6]3+ + 3 H2NCH2CH2NH2
[M(H2NCH2CH2NH2)3]3+ + 6H2O
Metal(III)-tris(1,2-diaminoethane) complexes even more stable, with equilibrium
constants of the order of 1030.
When bidentate or multidentate ligands bond to one metal ion, the complex is known as
a chelate; the extra stability which this produces is known as the chelate effect.
TOPIC 13.21: SUBSTITUTION REACTIONS 3
CH2
Some
complexes
contain
both
unidentate and bidentate ligands, for
example [CoCl2(en)2]+.
NH2
..
+
CH2
:NH2
Cl:
Co
This complex exhibits
geometrical isomerism.
Cl:
:NH2
..
CH2
NH2
cis
CH2
+
Cl
..
H2
:N
H2
N:
H2C
CH2
Co
H2C
N:
H2
:N
..
CH2
H2
Cl
trans
3. Anionic Unidentate Ligands
All negative ions are able to acts as ligands. The one most commonly encountered is
the chloride ion Cl-. It has four electron pairs, but only one of these will form a coordinate bond with a metal. The usual reagent used as a source of chloride ions is
concentrated hydrochloric acid; because of its great solubility (conc. HCl is
approximately 11M) it produces a much higher concentration of chloride ions than ionic
chlorides such as NaCl.
Chloride ions are larger than water molecules and are also negatively-charged. As the
size of the ligand increases, there is increased mutual repulsion between neighbouring
ligands. As a result, the octahedral structure becomes less favourable, and the
tetrahedral structure, in which the ligands are further apart in space, is preferred. The
reaction of metal(II)-aqua ions with chloride ions is represented by the equation:
[M(H2O)6]2+ + 4Cl-
[MCl4]2-
+ 6H2O

Titanium has larger ions than other transition metals in the first transition series
and is able to form hexachloro complexes.

The smaller fluoride ion readily forms hexafluoro complexes.
TOPIC 13.21: SUBSTITUTION REACTIONS 4
Reaction with [Co(H2O)6]2+
When excess concentrated hydrochloric acid is added to a solution containing
hexaaquacobalt(II) ions, the pink colour is discharged and a deep blue solution
containing the tetrachlorocobaltate(II) ion is formed.
[Co(H2O)6]2+ + 4Clpink solution
(octahedral)
[CoCl4]2- + 6H2O
deep blue solution
(tetrahedral)
Water is a better ligand than chloride, so the equilibrium is only driven to the right hand
side by the high concentration of chloride ions. When the deep blue solution is diluted
with water, the equilibrium moves back to the left hand side, restoring the pink colour.
Reaction with [Cu(H2O)6]2+
When excess concentrated hydrochloric acid is added to a solution containing
hexaaquacopper(II) ions, the blue colour is discharged and a yellow-green solution
containing the tetrachlorocuprate(II) ion is formed.
[Cu(H2O)6]2+ + 4Clblue solution
(octahedral)
[CuCl4]2- + 6H2O
yellow-green solution
(tetrahedral)
When the yellow-green solution is diluted with water, the equilibrium moves back to the
left hand side and the blue colour is restored
4. Anionic Bidentate Ligands
The ethanedioate anion C2O42- forms a very stable complex with iron(III) ions,
[Fe(C2O4)3]3-. This complex is chiral and can exhibit optical activity.
O
O
C
O
..
C
O
O
C
:O
O:
C
3-
Fe
O:
:O
..
C
O
O
C
O
A similar complex forms in Fehling’s solution, when 2,3-dihydroxybutanedioate ions are
added to copper(II) ions.
TOPIC 13.21: SUBSTITUTION REACTIONS 5
5. Anionic Multidentate Ligands
When multidentate ligands such as haem (discussed in Topic 13.14) and EDTA 4- form
complexes, the increase in entropy is even greater than with bidentate ligands. The
result is that the complexes are even more stable (the Chelate Effect).
The enthalpy change for these reactions is small because the number of bonds broken
and formed is the same, and the type of bond is similar. The entropy change, however,
is large and positive; in the case of EDTA4- two reactant particles give seven product
particles.
[M(H2O)6]2+ + EDTA4[M(EDTA)]2- + 6H2O
The entropy change greatly outweighs the enthalpy change, even when H is positive,
and the reaction is therefore entropy driven.
Since G = H -TS, a large positive entropy change (S) will result in a large negative
value for G. Since G is always negative, the reaction is always feasible.
EDTA4- has six donor atoms; it is the
anion of
bis[di(carboxymethyl)amino]ethane:
..
..
-
-OOCCH
2
..
..
NCH2CH2N
OOCCH2
CH2COO
..-
..
CH2COO-
EDTA4- forms 1:1 octahedral complexes with M2+ ions, for example:
[Cu(H2O)6]2+ + EDTA4-
[Cu(EDTA)]2- + 6H2O
Since the equilibrium lies completely over to the right hand side, there are virtually no
hexaaqua metal ions present. Therefore, reactions of the hexaaqua metal ion are not
possible in the presence of an excess of EDTA4-. Metal ions can therefore be held in
solution (sequestered) even under conditions which would normally cause
precipitation.
For example, when OH- or CO32- ions are added to hexaaquacopper(II) ions in solution,
a precipitate of the hydroxide or the carbonate forms. If however, the copper(II) ions are
first sequestered by the addition of excess EDTA4-, no precipitate is formed when OH- or
CO32- ions are added.
The sequestering of calcium ions is used in water softening.
Sequestered metal ions are used in trace element fertilisers.
TOPIC 13.21: SUBSTITUTION REACTIONS 6