One topic involving the solution chemistry of complex ions is the underlying reason simple salts of metal ions can be acidic. These concepts would represent a hard pitch in the test (probably a quarter of students going into the exam have a clue about this) Why is an aluminum sulfate (Al2(SO4)3) solution acidic? A metal ion can't donate a proton directly to water to produce H3O+. A metal ion can act as a Lewis acid. That's true. A Lewis acid is an electron pair receiver. What does that have to do with lowering the pH?
In fact, it's the Lewsis acid property which is the key. Al3+ interacts with water, a Lewis base, by coordinating to a lone pair of electrons on the oxygen atom to form the hydrated metal ion [Al(H2O)6]3+. Being in this complex makes the water molecules more acidic. The metal ion simultaneously polarizes the O-H bonds in the water by pulling electron density from the oxygen atoms while it repels the hydrogen atoms making it easier for the coordinated water to lose a proton. The [Al(H2O)6]3+ then becomes [Al(H2O)5OH]2+. This is why metal salts are often acidic. The greater the charge of the particular metal ion and the smaller its radius, the larger the effect will be. Small, highly charged metal ions, such as Al3+ and Fe3+ are the most acidic. [Al(H2O)6]3+ is a very respectable weak acid, on the order of a carboxylic acid.