The d-and f-Block Elements — Chemistry STD 12 Science — Question

→ One of the notable features of a transition element is the great variety of oxidation states it may show in its compounds.

Sc	Ti	V	Cr	Mn	Fe	Co	Ni	Cu	Zn
	+2	+2	+2	+2	+2	+2	+2	+1	+2
+3	+3	+3	+3	+3	+3	+3	+3	+2
	+4	+4	+4	+4	+4	+4	+4
		+5	+5	+5
			+6	+6	+6
				+7

Oxidation States of the first row Transition Metals
(the most common ones are in bold types)
→ The elements which give the greatest number of oxidation states occur in or near the middle of the series. Manganese, for example, exhibits all the oxidation states from +2 to +7.
→ In the starting of series, very less number of d-electrons are available for chemical bonding. Hence, less number of oxidation states are shown by elements present at the starting of series.
e.g.: Sc⁺³, Ti⁺², Ti⁺³, Ti⁺⁴
→ At the end of the series there are too many d-electrons and d-orbitals are completely occupied. Hence, these elements show very less number of oxidation states.
→ Down the group the stability of elements in higher oxidation states increases because removal of electrons from d-orbitals become easy.
→ For example, in group 6, Mo(VI) and W(VI) are found to be more stable than Cr(VI). Thus, Cr(VI) in the form of dichromate in acidic medium is a strong oxidising agent, whereas MoO₃ and WO₃ are not.
→ Low oxidation states are found when a complex compound has ligands capable of π-acceptor character in addition to the o-bonding. For example, in Ni(CO)₄ and Fe(CO)₅, the oxidation state of nickel and iron is zero.