Lewis acids and bases Definitions: Lewis acid: electron pair acceptor Lewis base: electron pair donator In the formation of complex ions the ligand is the Lewis base because it is donating a pair of electrons in the dative covalent bond and the metal ion is the Lewis acid. Metal-aqua ions Metal aqua ions are formed in aqueous solution. [M(H2O)6 ] 2+, limited to M = Fe (green) and Cu (blue); [M(H2O)6 ] 3+, limited to M = Al (colourless), and Fe (violet) In solution and Fe(III) appears yellow/brown due to hydrolysis reactions. The violet colour is only really seen in solid hydrated salts that contain these complexes. Acidity or hydrolysis reactions [M(H2O)6 ] 3+ + H2O [M(H2O)5 (OH)]2+ + H3O+ The following equilibria happen in aqueous solutions of metal ions. The equilibria lead to generation of acidic solutions with M3+ ions, and very weakly acidic solutions with M2+ ions. The 3+ ions are noticeably more acidic. The acidity of [M(H2O)6 ] 3+ is greater than that of [M(H2O)6 ] 2+ in terms of the greater polarising power (charge/size ratio) of the 3+ metal ion. The greater the polarising power, the more strongly it attracts the water molecule. This weakens the O-H bond so it breaks more easily.
3.2.6 Reactions of ions in aqueous solution (A-level only)
In aqueous solution, the following metal-aqua ions are formed: [M(H2O)6] 2+, limited to M = Fe and Cu [M(H2 O)6 ] 3+, limited to M = Al and Fe
The acidity of [M(H2O)6] 3+ is greater than that of [M(H2O)6] 2+
Students should be able to:
• explain, in terms of the charge/size ratio of the metal ion, why the acidity of [M(H2O)6 ] 3+ is greater than that of [M(H2O)6 ] 2+