Why do some elements have more than one charge in specific reactions?

[Urban Chaos]

Why do certain elements (particularly transition metals) possesses varying charges in different reactions? For example: Iron (Fe), is known to exist as Iron2+ and 3+.

 

[Mike H]

Transition metals, given the presence of the d-orbitals, are known to form complexes with varying oxidation states. The d-orbitals basically give the atoms of the elements "more room" to work with, chemically speaking. Iron, for example, is known to be oxidized to iron(IV) in certain catalytic reactions, and more reduced species are known to be useful chemical reagents. There is, of course, the issue that it will also depend on what ligands are bound to said transition metal. Iron, for example, mostly is found in ferric and ferrous complexes, although the latter do exhibit a tendency to be oxidized.