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 correct answer is option 3. coordination compounds having ligands with \(\pi\)-acceptor character.

To understand why transition elements exhibit zero or negative oxidation states specifically in coordination compounds with π-acceptor ligands, let’s delve into some details:

Coordination Compounds with π-Acceptor Ligands

\(\pi \)-Acceptor Ligands:

\(\pi \)-acceptor ligands are those that can accept electron density from the metal center through π-backbonding. Examples include carbon monoxide \((CO)\), phosphines \((PR_3)\), and olefins (alkenes). These ligands have empty \(\pi ^*\) orbitals that can accept electron density from the metal’s d orbitals.

Effect on Oxidation States:

The ability of π-acceptor ligands to accept electron density from the metal stabilizes the metal in lower oxidation states, including zero. This is because the \(\pi \)-backbonding reduces the electron deficiency of the metal center. For instance, carbon monoxide \((CO)\) is a well-known π-acceptor ligand. In complexes like \([Ni(CO)_4]\), nickel is in the zero oxidation state. The \(CO\) ligands stabilize this zero oxidation state through strong \(\pi \)-backbonding

Examples and Mechanism

Zero Oxidation State Complexes:

\([Ni(CO)_4]\): In this complex, nickel is in the zero oxidation state. Carbon monoxide acts as a \(\pi \)-acceptor ligand, donating electron density back to the metal. This interaction stabilizes the nickel in its zero oxidation state

\([Fe(CO)_5]\): Iron is in the zero oxidation state in this complex, with \(CO\) ligands stabilizing this state through \(\pi \)-backbonding.

Negative Oxidation States:

Transition metals typically do not exhibit stable negative oxidation states in simple coordination compounds. However, certain transition metals can form complexes where they appear to have negative oxidation states due to the nature of the ligands and their interactions.

Why Not in All Coordination Compounds

Not all coordination compounds involve \(\pi \)-acceptor ligands. In many coordination complexes, the transition metal can be in various positive oxidation states, depending on the ligands and the overall charge of the complex. For instance, in complexes with simple ligands like chloride (Cl⁻) or water (H₂O), transition metals usually exhibit positive oxidation states.

Summary

Transition metals exhibit zero oxidation states primarily in coordination compounds with \(\pi \)-acceptor ligands, such as \(CO\), where the π-backbonding stabilizes these low oxidation states. In other coordination environments, the oxidation states can vary widely, and transition metals do not generally exhibit zero or negative oxidation states.

Thus, the stabilization of zero oxidation states in transition metals is strongly associated with the presence of \(\pi \)-acceptor ligands, making this the primary scenario where such oxidation states are observed.