Class 12 Chemistry notes d-Block Elements: Oxidation States

d-Block Elements: Oxidation States (Comprehensive Notes & High-Yield Questions)

1. General Features and Electronic Basis of d-Block Oxidation States

Transition elements are uniquely characterised by their ability to exhibit a wide variety of oxidation states. This variability is fundamentally rooted in the fact that the energy levels of the (n-1)d and ns orbitals are remarkably close, allowing electrons from both subshells to participate in chemical bonding.

3d Series Oxidation State Matrix

The following table presents the oxidation states for the elements from Scandium (Z=21) to Zinc (Z=30). Bold values indicate the most common and stable states found in nature and laboratory reagents.

Element	Outer Configuration	Oxidation States
Scandium (Sc)	3d1 4s2	+3
Titanium (Ti)	3d2 4s2	+2, +3, +4
Vanadium (V)	3d3 4s2	+2, +3, +4, +5
Chromium (Cr)	3d5 4s1	+2, +3, +4, +5, +6
Manganese (Mn)	3d5 4s2	+2, +3, +4, +5, +6, +7
Iron (Fe)	3d6 4s2	+2, +3, (+4, +6)*
Cobalt (Co)	3d7 4s2	+2, +3, +4
Nickel (Ni)	3d8 4s2	+2, +3, +4
Copper (Cu)	3d10 4s1	+1, +2
Zinc (Zn)	3d10 4s2	+2

*Note: High oxidation states for Iron (+4, +6) are extremely rare and generally only exist in alkaline media as oxoanions (e.g., Ferrate ions).

Strategist's Pro-Tip (IUPAC Definition): According to IUPAC, transition metals are defined as metals having an incomplete d subshell in the neutral atom or in their common ions. Consequently, Zinc, Cadmium, and Mercury (Group 12) are not strictly transition metals because they possess a full d¹⁰ configuration in both their ground state and their common ionised states.

Matrix Footer: The stability of the +2 state generally increases across the series after Chromium. As the nuclear charge increases, the d-electrons are held more tightly, making them increasingly difficult to remove for higher oxidation states.

• The "Middle-of-the-Series" Peak: The maximum number of oxidation states is observed at Manganese (+2 to +7) in the middle of the series due to the availability of the maximum number of unpaired electrons (3d⁵ 4s²) for bonding. Moving from Sc to Mn, the number of states increases as more d-electrons are available for sharing.
• Boundary Limitations:
  • Sc & Ti: At the beginning of the series, there are simply too few electrons available to allow for extensive variability.
  • Cu & Zn: Towards the end of the series, the d-orbitals become full or nearly full. The significant increase in effective nuclear charge holds the remaining d-electrons more firmly, limiting the number of orbitals available for sharing.
• Anomalous & Specific Stability Cases:
  • Scandium (+3): Scandium exhibits only the +3 state because the loss of all three valence electrons results in the stable noble gas configuration of Argon ([Ar]).
  • Zinc (+2): Zinc possesses a completely filled 3d¹⁰ subshell. It only loses the 4s² electrons to form the Zn²⁺ ion, maintaining the stability of the full d-shell.
  • Titanium (IV): Ti(IV) is significantly more stable than Ti(III) or Ti(II) because it achieves a d⁰ configuration, matching the stable electronic configuration of the preceding noble gas.
• Post-Manganese Stability Decline: Following Manganese, there is an abrupt decrease in the stability of higher oxidation states. This is caused by the pairing of electrons in the d-orbitals, which reduces the number of unpaired electrons available for bonding. Common stable ions in this region shift towards lower states: Fe(II, III), Co(II, III), Ni(II), Cu(I, II), and Zn(II).

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2. Key Concepts & Distinctive Trends

• Variable Oxidation States (d-block vs p-block):
  • d-block: Oxidation states typically vary by unity (e.g., +2, +3, +4). This occurs because electrons are being progressively removed from (n-1)d orbitals which are very close in energy.
  • p-block: Oxidation states often vary by two units (e.g., Sn^II/Sn^IV, Pb^II/Pb^IV). This is a result of the Inert Pair Effect, where the ns² electrons remain unshared while the np electrons are involved in bonding.
• Heavier Element Group Trends: Unlike the p-block, where lower oxidation states are often more stable for heavier elements, heavier d-block members (4d and 5d series) favour higher oxidation states.
  • Group 6 Comparison: In Group 6, Molybdenum (Mo) and Tungsten (W) are far more stable in the +6 state than Chromium (Cr).
  • Chemical Consequence: Cr(VI) in the form of acidic dichromate (Cr₂O₇²⁻) is a powerful oxidising agent, as it seeks to reduce to the more stable Cr(III). Conversely, MoO₃ and WO₃ are highly stable and show no significant oxidising power.
• Low Oxidation State Complexes: Transition metals can be stabilised in zero or negative oxidation states within specific complexes.
  • Metal Carbonyls: Examples include Ni(CO)₄ and Fe(CO)₅.
  • Stabilisation Mechanism: These states are stabilised by ligands like Carbon Monoxide (CO) which possess σ-bonding and π-acceptor character. This allows for "back-bonding" (synergic bonding) where the metal donates electron density back into the ligand's empty π* orbitals.

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3. Trends in Stability of Higher Oxidation States

• Halide-Based Stabilisation: High oxidation states are often attained in metal halides, specifically fluorides due to fluorine's high electronegativity.
  • Highest Halides: TiX₄, VF₅, and CrF₆.
  • Mechanisms: Fluorine stabilises high states through high lattice energy for ionic compounds (e.g., CoF₃) and high covalent bond enthalpies for higher fluorides like VF₅ and CrF₆.
• Oxygen vs Fluorine Comparison: Oxygen is superior to Fluorine in stabilising high oxidation states. While the highest Manganese fluoride is MnF₄, the highest oxide is Mn₂O₇.
  • Reasoning: Oxygen has the ability to form multiple bonds (double bonds) with metal atoms, allowing for higher coordination without the steric hindrance of seven or eight individual atoms.
• Structural Characteristics:
  • Mn₂O₇: A green covalent oil. Its structure consists of two MnO₄ tetrahedra sharing an oxygen atom, forming an Mn-O-Mn bridge.
  • Tetrahedral Oxoanions: High oxidation states are frequently found in stable oxoanions where the metal is tetrahedrally coordinated: V(V) as VO₄³⁻, Cr(VI) as CrO₄²⁻, and Mn(VII) as MnO₄⁻.

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4. Disproportionation Reactions

Definition: A redox reaction where an element in an intermediate oxidation state is simultaneously oxidised and reduced to two different oxidation states.

• Manganese (VI): In acidic solutions, the manganate ion is unstable and disproportionates:
  3MnO42− + 4H+ → 2MnO4− + MnO2 + 2H2O
• Copper (I): Cu⁺ is unstable in aqueous medium:
  2Cu+(aq) → Cu2+(aq) + Cu(s)

  Thermodynamic Justification: Copper has a positive standard electrode potential (E^⊖ = +0.34V) because its high enthalpy of atomisation (Δ_aH^⊖) and high ionisation enthalpy (IE) are not balanced by its hydration enthalpy. However, for Cu²⁺, the highly negative hydration enthalpy (Δ_hydH^⊖) is so large that it more than compensates for the energy required for the second ionisation enthalpy (IE₂), making Cu²⁺(aq) much more stable than Cu⁺(aq).

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5. Curated Board and Competitive Exam Questions

Section A: Subjective Board-Style Questions

1. Reasoning: Cr²⁺ is a strong reducing agent while Mn³⁺ is a strong oxidising agent, even though both have a d⁴ configuration. Explain.
2. Reasoning: Why does Copper (Cu) have a positive E^⊖ value (+0.34V) compared to other 3d series metals? Mention the thermodynamic variables involved.
3. Reasoning: Explain the exceptional stability of Sc³⁺ in aqueous solution.
4. Reasoning: Why does Zinc exhibit only a +2 oxidation state?

Section B: High-Yield MCQs

1. Which trioxide is the most stable and non-oxidising in nature?
   (a) CrO₃
   (b) MoO₃
   (c) WO₃
   (d) Both (b) and (c)
2. Low oxidation states of transition metals in carbonyls are stabilised by:
   (a) The large size of the CO ligand.
   (b) The π-acceptor character of CO.
   (c) High ionisation enthalpy of the metal.
   (d) Sigma-donor character only.
3. Mn₂O₇ is characterised as:
   (a) A purple solid with octahedral Mn.
   (b) A green covalent oil with tetrahedral Mn.
   (c) A blue liquid with Mn-Mn bonds.
   (d) A yellow gas with linear structure.
4. The stability of Cu²⁺(aq) over Cu⁺(aq) is primarily due to:
   (a) Low enthalpy of atomisation.
   (b) Low first ionisation enthalpy.
   (c) High negative hydration enthalpy of Cu²⁺.
   (d) High covalent bond enthalpy.
5. In which of the following oxoanions is the metal in its highest oxidation state?
   (a) VO₄³⁻
   (b) CrO₄²⁻
   (c) MnO₄⁻
   (d) All of the above
6. Oxygen can stabilise higher oxidation states in metals (like Mn₂O₇) better than Fluorine (like MnF₄) because:
   (a) Oxygen is more electronegative than Fluorine.
   (b) Oxygen can form multiple bonds with the metal.
   (c) Fluorine has a lower electron gain enthalpy.
   (d) The Mn-F bond is too weak to exist.

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6. Comprehensive Answer Key & Explanations

Section A Solutions

1. Cr²⁺ vs Mn³⁺: Cr²⁺ (d⁴) is reducing because it loses an electron to become Cr³⁺ (d³), which provides a stable half-filled t_2g subshell. Mn³⁺ (d⁴) is oxidising because it gains an electron to become Mn²⁺ (d⁵), which is a stable exactly half-filled d-subshell.
2. Copper Potential: Copper is unique because the energy required to transform the metal into ions (sum of enthalpy of atomisation and ionisation enthalpies) is very high and is not compensated by its hydration enthalpy. This results in a positive E^⊖.
3. Sc³⁺ Stability: Scandium has a 3d¹ 4s² configuration. Removing three electrons yields the stable noble gas configuration of Argon ([Ar]), which is energetically the most favourable state for Sc.
4. Zinc Variability: Zinc (3d¹⁰ 4s²) has a completely filled d-subshell which is very stable. Only the two 4s electrons are available for removal, leading to a fixed +2 oxidation state.

Section B Solutions

1. (d) Heavier Group 6 members (Mo, W) favour the +6 state, making their trioxides stable and non-oxidising.
2. (b) The π-acceptor character of CO allows for synergic back-bonding, which stabilises low metal oxidation states.
3. (b) Mn₂O₇ is a green covalent oil containing tetrahedrally coordinated Manganese atoms with an Mn-O-Mn bridge.
4. (c) The high negative hydration enthalpy of Cu²⁺ outweighs its high second ionisation enthalpy.
5. (d) V is +5, Cr is +6, and Mn is +7; each represents the maximum possible oxidation state for that element.
6. (b) Oxygen's ability to form multiple (double) bonds allows it to stabilise the highest oxidation states through a higher total bond order without steric crowding.