Explain:
(i)Transition elements mostly form the complex compounds. Why?
(ii)Transition elements are a good catalyst. Why?
(iii)Transition elements show variable valency. Why?

Hint: We know that the middle large section of the periodic table is occupied by transition elements. They are named as transition elements due to their location between s-block and p-block elements.

Complete step by step answer:
(a) We know that complex compounds are those in which binding of metal ions to a number of anion or neutral molecules takes place. These complex compounds have characteristic properties. Some examples of complex compounds are ${\left[ {{\rm{Fe}}{{\left( {{\rm{CN}}} \right)}_{\rm{6}}}} \right]^{3 - }}$, \({\left[ {{\rm{Cu}}{{\left( {{\rm{N}}{{\rm{H}}_{\rm{3}}}} \right)}_4}} \right]^{2 + }}\)etc.\({\left[ {{\rm{Cu}}{{\left( {{\rm{N}}{{\rm{H}}_{\rm{3}}}} \right)}_4}} \right]^{2 + }}\).
The reason for formation of a huge number of complex compounds by transition metals is smaller size of metal ions, high charge of ions and the availability of d orbitals for bond formation.

(b) Catalysts are the substances which speed up a chemical reaction without getting consumed in the reaction. We know that transition metals and the compounds of transition metals are well known for catalytic properties. The catalytic property of them is due to their ability to acquire multiple oxidation states and to form complexes.
In the Contact process, Vanadium (V) oxide is used as catalyst, in Haber’s process finely divided iron is used as catalyst and in catalytic halogenations, nickel is used.

(c) We know that a great variety of oxidation states are shown by transition elements in different compounds. Most of the transition elements show variable valency. Manganese exhibits oxidation states from +2 to +7. The energy subshells $ns$ and $\left( {n - 1} \right)d$ subshells are very in transition elements. Therefore, the possibility of losing electrons from both $\left( {n - 1} \right)d$ and $ns$ is very high if an unpaired electron is present. Hence, transition elements show variable valency.

Note:
The transition elements possess electronic configuration (general) of $\left( {n - 1} \right){d^{1 - 10}}n{s^{1 - 2}}$.The transition elements possess variable valency because of presence of valence electrons in two set of orbital, that is, $\left( {n - 1} \right)d$ and $ns$. Because of incomplete d-orbital they are colorful in nature.