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 oxidation states of transition elements change in units of one whereas in p-block elements oxidation states normally differ by two units. The minimum oxidation state exhibited by a transition element is equal to the number of electrons in the ns orbital. 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 from +2 to +7. The elements at the extreme ends exhibit a lesser number of oxidation states. This is because of the availability of lesser electrons to lose or to share in the earlier elements or too many d electrons due to which lesser number of unpaired electrons to share at the end. Due to anomalous electronic configuration, chromium and copper can exhibit a minimum oxidation state of +1. In the first series, the maximum oxidation state increases up to Mn and then decreases from Fe onwards. In the first series, Mn exhibits the maximum oxidation state +7. In the second series, a stable maximum oxidation state is exhibited by technetium, and an unstable maximum oxidation state +8 is exhibited by ruthenium. In the third series, stable maximum oxidation +8 is exhibited by osmium. The transition metal ions having completely filled and exactly half-filled d-sub level and those having octet in their outermost shell are stable. The stabilities of Cr³⁺ and Mn⁴⁺ ions is due to high lattice energy in solid state and high hydration energy in their aqueous solutions. Fe³⁺ ion is more stable than Fe²⁺ ion because of the stable half-filled 3d⁵ electronic configuration in Fe³⁺. In the last five elements of the 3d series, the 3d electrons are stabilized and require more energy for their removal because the 3d orbital contracts more and come nearer to the nucleus with an increase in nuclear charge. Thus in the last five elements, the +2 oxidation state becomes more stable (except Fe³⁺ ).

Transition elements exhibit zero or negative oxidation states in

coordination compounds having ligands with \(\pi\)-acceptor character

The answer is (3), coordination compounds having ligands with π-acceptor character.

Transition elements can exhibit zero or negative oxidation states in coordination compounds that have ligands with π-acceptor character. These ligands can donate their π electrons to the transition metal, which can result in a negative oxidation state for the metal.

For example, the complex [Fe(CO)₅] has a zero oxidation state for iron. The CO ligand is a π-acceptor ligand, and it donates its π electrons to the iron atom. This gives the iron atom a negative charge, which cancels out the positive charge from the five CO ligands.

Oxocations and oxoanions are also coordination compounds, but they do not have π-acceptor ligands. Therefore, transition elements do not exhibit zero or negative oxidation states in oxocations or oxoanions.

All coordination compounds do not have π-acceptor ligands, so transition elements do not always exhibit zero or negative oxidation states in coordination compounds.