Chemistry of transition elements: describe question

In the Contact process, $\ce{V2O5}$ is used as a catalyst for the oxidation of $\ce{SO2}$ to $\ce{SO3}$.

(a) State the two features of a transition element that allow it to behave as a catalyst.

(b) The vanadium in $\ce{V2O5}$ is in oxidation state +5. During the catalytic cycle it is reduced to $\ce{VO2}$ and then re-oxidised back to $\ce{V2O5}$. Explain, in terms of oxidation states, why this allows vanadium to act as a catalyst.

(c) Explain why transition elements form complex ions, referring to the electronic structure of the metal ion.