– , Mn VII O – ) followed by a rather abrupt decrease in stability of higher oxidation states, so that the typical species to follow are Fe II,III , Co II,III , Ni II , Cu I,II , Zn II . The variability of oxidation states, a characteristic of transition elements, arises out of incomplete filling of d orbitals in such a way that their oxidation states differ from each other by unity, e.g., V II , V III , IV , V V . This is in contrast with the variability of oxidation states of non transition elements where oxidation states normally differ by a unit of two. An interesting feature in the variability of oxidation states of the d – block elements is noticed among the groups (groups through ).
Although in the p –block the lower oxidation states are favoured by the heavier members (due to inert pair effect), the opposite is true in the groups of d -block. For example, in group , 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 MoO and WO are not. Low oxidation states are found when a complex compound has ligands capable of p -acceptor character in addition to the s -bonding.
For example, in Ni(CO) and Fe(CO) , the oxidation state of nickel and iron is zero. Name a transition element which does not exhibit variable oxidation states. Scandium ( Z = ) does not exhibit variable oxidation states.