16 Reactions of inorganic compounds in aqueous solution 16.1 Lewis acids and bases 16.2 Ligand substitution reactions 16.3 Summary: Acid-Base and substitution reactions of some metal ions

16.1 Lewis Acids and Bases Learning Objectives: 1.Understand the Lewis definition for acids and bases. 2.Compare Lewis acid/bases to Bronsted-Lowry acid/bases. 3.Describe what a metal-aqua ion is. 4.Explain what determines the acidity of metal-aqua ions in aqueous solutions.

Review: Bronsted-Lowry Theory of Acidity What is the Bronsted-Lowry definition for acids and bases? Bronsted-Lowry Acid = Proton donor (H+) Bronsted-Lowry Base = Proton acceptor (OH-)

Lewis Theory of Acidity Lewis acid = Electron pair acceptor Lewis base = Electron pair donor In the formation of a co-ordinate bond. Identify the Lewis acid and base for the following reaction: NH 3 + BF 3  NH 3 BF 3

All Bronsted-Lowry Acids are Lewis Acids H+ ions have no electrons, so always accept electrons when forming bonds. So, Bronsted-Lowry acids are always Lewis acids. Lewis Acids Bronsted- Lowry Acids

Water can act as both Bronsted-Lowry base and a Lewis base. Water has two lone pairs of electrons. It can use one to accept a proton (Bronsted-Lowry): H 2 O + H +  H 3 O + Or it can use one to form a co- ordinate bond (Lewis) such as when acting as a ligand in a complex ion.

Practice Questions 1. a) both b) Lewis acid c) Lewis acid 2. a)Both b)Both c)Lewis base 3. a) The H+ ion is the acid and the NH 3 molecule is the base. b) the H+ ion (the acid) is acting as a Lewis acid because it is accepting a pair of electrons from the NH 3 molecule. It is also acting a Bronsted-Lowry acid because it is donating a proton to the NH 3 molecule.

Metal-Aqua Ions Water molecules donate an electron pair to the metal ion (Lewis base). The metal ion accepts the electron pair so is a Lewis acid. Metal aqua ions are usually octahedral with six water molecules co-ordinately bonded to the metal ion.

How do metal-aqua ions form? Metal-aqua ions form when transition metal salts dissolve in water. Example: iron(II)nitrate is soluble so forms the metal-aqua ion [Fe(H 2 O) 6 ] 2+ when dissolved in water.

How acidic are solutions containing metal-aqua ions? [Fe(H 2 O) 6 ] 2+ will weakly dissociate in water to form an acidic solution However, metal-aqua ions of Fe 3+, [Fe(H 2 O) 6 ] 3+ dissociate more to form a more acidic solution containing [Fe(H 2 O) 5 (OH)]

Why are solutions with metal-aqua 3+ ions more acidic? The small, highly charged metal ion has a high charge density. This pulls electron density away from the oxygens, weakening the O-H bond. This makes it easier for the H+ to be removed.

This reactions is called a hydrolysis reaction because it usually involves water molecules [Fe(H 2 O) 6 ] 2+ + H 2 O  [Fe(H 2 O) 5 (OH)] + + H 3 O + Here the metal-aqua ion is donating a H+ (proton) to the water molecule and so is behaving as a Bronsted-Lowry acid. The water molecule is accepting a proton (acting as a Bronsted-Lowry base) and is also donating an electron pair to the H+ ion (acting as Lewis base).

Further hydrolysis of metal-aqua ions The following acid dissociation equilibrium exists. When OH- ions are added, the equilibrium shifts to the RIGHT

As more [M(H 2 O) 5 (OH)] + is made, a new equilibrium is set up. Again, adding OH- ions shifts the equilibrium to the right. This forms a neutral complex [M(H 2 O) 4 (OH) 2 ]. This neutral hydroxide is insoluble and forms a precipitate. It is written as M(OH) 2 as it is now an insoluble precipitate and no longer a metal-aqua ion.

Practice Question Write out the reactions involved when a metal-aqua 3+ ion reacts with hydroxides. (HINT: there are a different number of steps).

Similar reactions happen when metal- aqua ions react with ammonia and carbonates Adding ammonia gives the same results as adding OH- [M(H 2 O) 6 ] NH 3  [M(H 2 O) 3 (OH) 3 ] (s) + 3NH 4 + Metal 2+ ions will form metal carbonate precipitates. [M(H 2 O) 6 ] 2+ + CO 3 2-  MCO 3 (s) + 6H 2 O However, metal 3+ ions are strong enough acids that they form metal hydroxide precipitates instead. 2[M(H 2 O) 6 ] CO 3 2-  2[M(H 2 O) 3 (OH) 3 ] (s) + 3CO 2 + 3H 2 O

Amphoteric Metal Hydroxides Some metal hydroxides can then act as both acids and bases (amphoteric). Acting as an acid [Al(H 2 O) 3 (OH) 3 ] (s) + OH -  [Al(H 2 O) 2 (OH) 4 ] – (aq) + H 2 O Acting as a base [Al(H 2 O) 3 (OH) 3 ] (s) + 3H +  [M(H 2 O) 6 ] 3+ (aq)

16.2 Ligand Substitution Reactions Learning Objectives: 1.Identify if the coordination number or charges changes during a ligand substitution reaction. 2.Understand the ligand substitution reactions may be incomplete. 3.Compare the stabilities between complex ions formed from unidentate and multidentate ligands. 4.Explain the chelate affect in terms of entropy change.

Ligand Substitution Reactions (Ligand Exchange) Ligand exchange results in a colour change. In can sometimes result in a change in coordination number.

Ligand Substitution – Ligands of Similar Size If ligands are substituted for ligands of similar size (ie H 2 O and NH 3 ) than there is no change in coordination number and no change in shape. But there will be a change in colour.

Ligand Substitution – Different Sized Ligands If the ligands are different sizes (ie H 2 O and Cl - ) there is a change in coordination number and shape. The larger the ligand, the fewer will be able to fit around the central metal ion and the lower the coordination number.

Review: Shapes of Complex Ions Coordination number of 2 …usually linear. Coordination number of 4 …usually tetrahedral. Coordination number of 6 …usually octahedral.

Partial Substitution Sometimes the ligand substitution is partial, for example only one ligand is substituted out of an octahedral complex ion.

Complex Ion Stability Ligand exchange reactions can be easily reversed, unless the new complex ion is more stable. More stable complex ions… Have ligands with stronger coordinate bonds to the metal ion -OR- Have multidentate ligands

Multidentate Ligands – The Chelate Effect When unidentate ligands are exchanged for multidentate ligands, entropy increases because there are overall more molecules after the exchange. This is called the chelate effect and is why multidentate ligands form more stable complex ions.

Free-Energy Change The enthalpy change for a ligand substitution reaction is usually very small as the strength of bond broken and bonds formed will be similar. However, because entropy increases due to the increase in number of molecules the Gibbs free energy is more likely to be negative and thus the reaction will be feasible.