28.1.5—Why transition elements behave as catalysts

Transition-metal catalysts can provide alternative pathways because they access multiple oxidation states and have energetically accessible vacant or partially occupied d orbitals that form dative bonds to reactants.

A catalyst may bind, activate and release a substrate, or shuttle electrons between oxidation states. It is regenerated overall and lowers activation energy without changing equilibrium.

Fe²⁺/Fe³⁺ can catalyse a redox chain by accepting an electron in one step and donating it in another.

The catalyst is not consumed permanently, and “vacant d orbital” does not mean every d orbital is empty.