Acids and Bases: Lewis Theory

Cationic Lewis Acids

Metal cations have two characteristics that allow them to act as Lewis acids:

1. Their positive charge attracts electrons.
2. They have at least one empty orbital that can accomodate an electron pair.

When a metal cation encounters a substance with a lone electron pair, a coordination compound can form. An example of this is when metal ions are in an aqueous solution they are hydrated (or they form a coordination compound with water). The metal ion is a Lewis acid; water is a Lewis base:

There are many substances that have lone electron pairs, making them potential Lewis bases when combined with metal cations. A few examples are shown below:

One Lewis base that deserves special mention is hydroxide (OH^-). The hydroxide ion binds easily to many metal ions forming metal hydroxides. Some metal hydroxides are amphiprotic (or are able to act as an acid or a base). Their amphiprotic nature allows solid metal hydroxides to dissolve when either an acid or base is added:

Reaction as a base:
	Zn(OH)2(s) + 2 H3O+(aq) Zn2+(aq) + 4 H2O(l)
Reaction as an acid:
	Zn(OH)2(s) + 2 OH-(aq) [Zn(OH)4]2-(aq)

Which of the following molecules or ions can act as Lewis bases and bind with metal cations? (Hint: draw Lewis structures for each molecule!)

F-	Al3+	H2S	CH4