REACTIONS OF INORGANIC COMPOUNDS

SUBSITUTION REACTIONSWhen 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 [M(NH3)6]2+ + H2O The overall reaction is:

[M(H2O)6]2+ + 6NH3 [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 [Co(H2O)4(OH)2] + 2NH4+

pink solution

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 [Co(NH3)6]2+ + 4H2O + 2OH- blue-green precipitate

straw 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.

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 [Cu(H2O)4(OH)2] + 2NH4+

blue solution

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(H2O)4(OH)2] + 4NH3 [Cu(NH3)4(H2O)2]2+ + 2H2O + 2OH- blue precipitate

deep blue solution

The overall reaction can be represented as:

[Cu(H2O)6]2+ + 4NH3 [Cu(NH3)4(H2O)2]2+ + 4H2O

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.

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 [M(H2NCH2CH2NH2)3]2+ + 2H2O

The overall reaction is:

[M(H2O)6]2+ + 3 H2NCH2CH2NH2 [M(H2NCH2CH2NH2)3]2+ + 6H2O

Example: [Ni(en)3]2+

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.

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 co-ordinate 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.

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+ + 4Cl- [CoCl4]2- + 6H2O

pink solution

deep blue solution

(octahedral)

(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+ + 4Cl- [CuCl4]2- + 6H2O

blue solution

yellow-green solution

(octahedral)

(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.

A similar complex forms in Fehlings solution, when 2,3-dihydroxybutanedioate ions are added to copper(II) ions.

5. Anionic Multidentate Ligands

When multidentate ligands such as haem (discussed in Topic 13.14) and EDTA4- 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 -TS, 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- 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.

2+

CH2

CH2

CH2

CH2

CH2

CH2

:

H2N

:

H2N

..

NH2

:

NH3

..

NH2

..

O

H

H

..

O

H

H

Cu

:

NH3

:

H3N

:

longer and weaker bonds

:

H3N

NH2

2+

:

NH2

Ni

shape: octahedral

no. of ligands = 3

co-ordination no. = 6

oxidation state of nickel = +2

This complex is chiral and can therefore exhibit optical activity.

H2C

H2C

CH2

CH2

CH2

CH2

:

Cl

:

Cl

..

NH2

..

NH2

:

NH2

+

:

NH2

Co

C

CH2

CH2

C

:

N

H2

:

H2

N

..

Cl

..

Cl

C

trans

+

cis

This complex exhibits geometrical isomerism.

Co

:

H2

N

:

N

H2

C

C

C

:

O

:

O

..

O

..

O

:

O

3-

:

O

Fe

O

O

O

O

O

O

NCH2CH2N

..

CH2COO-

CH2COO-

-OOCCH2

-OOCCH2

..

..

..

..

..

EDTA4- has six donor atoms; it is the anion of

bis[di(carboxymethyl)amino]ethane:

Some complexes contain both unidentate and bidentate ligands, for example [CoCl2(en)2]+.

TOPIC 13.21: SUBSTITUTION REACTIONS 1