The catalytic activity of transition metals and their compounds is ascribed mainly to:

According to NCERT, the catalytic activity of transition metals and their compounds is ascribed to their ability to adopt multiple oxidation states and to form complexes.

Two main mechanisms explain this activity:

1. Variable Oxidation States: Transition metal ions can change their oxidation states, allowing them to be more effective as catalysts. For example, iron(III) catalyses the reaction between iodide and persulphate ions by alternating between $Fe^{3+}$ and $Fe^{2+}$ oxidation states .
2. Surface Adsorption: Catalysts at a solid surface involve the formation of bonds between reactant molecules and atoms of the surface (utilising 3d and 4s electrons), which increases the concentration of reactants and weakens the bonds in the reacting molecules (lowering activation energy) .

While unfilled d-orbitals are part of the electronic structure that allows these properties, the specific chemical feature cited as the cause of catalytic activity is the ability to adopt variable oxidation states.