Ligand Exchange Reactions

A ligand exchange reaction (also called a ligand substitution reaction) is a reaction in which one or more ligands in a transition metal complex ion are replaced by different ligands. If you have already met the basic ideas of ligands and complex ions, you know that a ligand is a molecule or ion that donates a lone pair of electrons to a central metal ion to form a coordinate (dative covalent) bond. In a ligand exchange reaction this coordinate bond breaks and a new one forms with a different donor species, while the oxidation state of the metal ion itself does not change.

Because transition metals belong to the d-block, they readily form complex ions with a range of coordination numbers and shapes; the general background to this bonding is covered in Introduction to transition metals and the d-block elements. Ligand exchange reactions are one of the main reasons transition metal chemistry produces such striking colour changes, since swapping one ligand for another alters the splitting of the d-orbitals and so the wavelength of light absorbed.

Ligand exchange can be complete or partial, depending on the size, charge and bonding strength of the incoming ligand.

Full substitution: every ligand around the metal is replaced. For example, hexaaquacopper(II) reacts with excess chloride ions:

\( [Cu(H_2O)_6]^{2+} + 4Cl^- \)→\( [CuCl_4]^{2-} + 6H_2O \)

Here the coordination number changes from 6 to 4 and the shape changes from octahedral to tetrahedral, because the larger chloride ions cannot fit six around the small copper ion. The colour changes from pale blue to yellow.

Partial substitution: only some of the original ligands are replaced. Aqueous ammonia added a little at a time to hexaaquacopper(II) only displaces four of the six water ligands:

\( [Cu(H_2O)_6]^{2+} + 4NH_3 \)→\( [Cu(NH_3)_4(H_2O)_2]^{2+} + 4H_2O \)

This gives a deep blue solution. The remaining two water ligands, which sit opposite each other in the axial positions, are much harder to displace, so complete substitution to \( [Cu(NH_3)_6]^{2+} \) does not normally happen in aqueous solution.

Most of the ligand exchange reactions you meet at this level involve octahedral complexes with six ligands, usually starting from a hexaaqua ion such as \( [Cu(H_2O)_6]^{2+} \), \( [Co(H_2O)_6]^{2+} \) or \( [Cr(H_2O)_6]^{3+} \). Adding excess concentrated hydrochloric acid to cobalt(II) converts the pink octahedral hexaaqua ion into a blue tetrahedral chloro complex:

\( [Co(H_2O)_6]^{2+} + 4Cl^- \)→\( [CoCl_4]^{2-} + 6H_2O \)

Similar colour and coordination number changes occur across the transition series; several further worked examples, including vanadium's well-known colour ladder, are covered in Reactions of vanadium and other transition metals.

Some \( d^8 \) metal ions, such as platinum(II), nickel(II) and palladium(II), form four-coordinate square planar complexes instead of six-coordinate octahedral ones. Ligand exchange in square planar complexes usually proceeds by an associative mechanism: the incoming ligand attaches to the metal from above or below the plane before the leaving ligand departs, since there is open space perpendicular to the square plane. The outcome of these substitutions often depends on which ligand is trans to the leaving group, an effect exploited in the synthesis of the anticancer drug cisplatin, \( [Pt(NH_3)_2Cl_2] \), where the two chloride ligands must end up cis to one another for the drug to work.

Multidentate ligands, which bond to the metal through two or more donor atoms at once, form noticeably more stable complexes than the same number of donor atoms provided by separate monodentate ligands. This is called the chelate effect. The classic example is \( EDTA^{4-} \), a hexadentate ligand that replaces all six water molecules from a hexaaqua ion in a single step:

\( [M(H_2O)_6]^{2+} + EDTA^{4-} \)→\( [M(EDTA)]^{2-} + 6H_2O \)

Although the enthalpy change for this reaction is similar to that of six separate monodentate substitutions, the reaction is far more thermodynamically favourable. That is because one EDTA ion and one metal ion (2 particles) become one chelate complex plus six free water molecules (7 particles): the large increase in the number of free particles produces a large positive entropy change, \( \Delta S \), which makes \( \Delta G = \Delta H - T\Delta S \) more negative. The larger stability constant, \( K_{stab} \), of the chelate compared with the equivalent monodentate complex is a direct measure of this extra stability.

Question: Write the equation for the reaction of hexaaquachromium(III), \( [Cr(H_2O)_6]^{3+} \), with excess concentrated hydrochloric acid, and state the colour and shape change involved.

Answer: Concentrated hydrochloric acid supplies a large excess of chloride ions, which are big enough to force a change in coordination number from six to four:

\( [Cr(H_2O)_6]^{3+} + 4Cl^- \)→\( [CrCl_4]^- + 6H_2O \)

The shape changes from octahedral to tetrahedral and the colour changes from violet to green.

Ligand exchange reactions let you predict colour changes, coordination number changes and relative stabilities of transition metal complexes just from the identity and denticity of the ligands involved.

Related lessons

• Uk A Level Chemistry