4. Ability to adopt multiple oxidation states and their complexing ability.

Explanation:
The catalytic activity of the transition metals and their compounds is ascribed to their ability to adopt multiple oxidation states and their complexing ability.
Catalysis at a solid surface involve the formation of bonds between reactant molecules and atoms of the surface of the catalyst (first row transition metals utilise 3d and 4s electrons for bonding).
This has the effect of increasing theconcentration of the reactants at the catalyst surface and also weakening of the bonds in the reacting molecules (the activation energy is lowering).
Also because the transition metal ions can change their oxidation states, they become more effective as catalysts. For example, iron(III) catalyses the reaction between iodide and persulphate ions.