The transition elements exhibit variable oxidation states because (n−1)d electrons can also participate in bonding, as the energy difference between (n−1)d and ns orbitals is very small. The maximum oxidation state that can be exhibited by a transition element is equal to the total number of electrons present in both ns and (n−1)d orbitals. The elements in the middle exhibit more oxidation states, e.g., manganese exhibits all oxidation states from +2 to +7. In the second series, a stable maximum oxidation state is exhibited by technetium, and an unstable maximum oxidation state of +8 is exhibited by ruthenium. In the third series, a stable maximum oxidation state of +8 is exhibited by osmium. Transition metal ions having a completely filled or an exactly half-filled d subshell are stable. The Fe3+ ion is more stable than the Fe2+ ion because of the stable half-filled 3d5 electronic configuration of Fe3+.
Transition elements exhibit zero or low oxidation states in
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → coordination compounds having ligands with \(\pi\)-acceptor character
Some ligands, such as CO, do more than donate an electron pair to the metal (σ-bonding): they also accept electron density back from the metal into their own empty orbitals (π-acceptor character). This lets the metal stay in a very low oxidation state.
In \( \ce{[Ni(CO)4]} \) and \( \ce{[Fe(CO)5]} \), CO is neutral, so nickel and iron are in the zero oxidation state.
Why the other options are incorrect
Option (1) oxocations — Incorrect. Oxocations such as \( \ce{VO2^+} \) carry the metal in a high oxidation state (+5 for vanadium here).
Option (2) oxoanions — Incorrect. Oxoanions such as \( \ce{MnO4^-} \) (Mn +7) and \( \ce{CrO4^2-} \) (Cr +6) show the metal in its highest oxidation states.
Option (4) all coordination compounds — Incorrect. Most complexes have the metal in a positive oxidation state; in \( \ce{[Co(NH3)6]^3+} \), for example, cobalt is +3.
NCERT (2026-27 edition):
4.3.4 Oxidation States – “Low oxidation states are found when a complex compound has ligands capable of π-acceptor character in addition to the σ-bonding. For example, in Ni(CO)4 and Fe(CO)5, the oxidation state of nickel and iron is zero.”