Why do transition elements show variable oxidation state? Explain their catalytic properties in detail.

Hint:The Transition elements which are also known as transition metals, are the elements that have partially filled ‘d’ orbitals. IUPAC defines transition elements as an element which has a d subshell that is partially filled with the electrons, or an element which has the ability to form stable cations with an incompletely filled ‘d’ orbital. The electronic configuration of transition element is \[\left( {n - 1} \right)\]\[{d^5}\] \[n{s^1}\] or it can be \[\left( {n - 1} \right)\] \[{d^{10}}\] \[n{s^1}\].

Complete answer:
The transition elements show variable oxidation state. It is because the valence electrons of the transition elements are in two different sets of orbitals, that is \[\left( {n - 1} \right)\]d and ns and the energy difference between these orbitals is very less i.e. there is a little distinction in energies both the energy levels can be utilised as a part of bond development, so both the energy levels can be used for bond formation. Thus, transition elements have variable oxidation states.
Example: Manganese (Mn) having atomic number 25 has the highest number of unpaired electrons in the d-subshell and it shows high oxidation state \[\left( { + 7} \right).\] Scandium (Sc) only exhibits a \[ + 3\]oxidation state in these series.
The transition elements demonstrate +2 oxidation state, it is because of 2 electrons in ‘ns’ orbitals when the electron of \[\left( {n - 1} \right)\]d orbital stays unaffected.
The transition elements exhibit catalytic properties due to the ability of transition metals to accept electrons from ligands due to unfilled d-orbitals made them good for catalysis. Iron and vanadium are the most important catalysts. Iron (Fe) is used as a catalyst in the manufacture of ammonia in Haber’s process. Vanadium is used in the form of vanadium pentoxide (\[{V_2}{O_5}\]) in the manufacture of sulphuric acid.

Note:
Transition metals show catalytic behaviour mainly due to the presence of vacant d orbitals, they have the ability to exhibit variable valencies and they have a tendency to form complex compounds.