Chapter 3: d-Block Elements and Their Compounds

1. Introduction

The elements in which the differentiating electron enters the d-subshell of the penultimate shell are called d-block elements. They are located between the s-block and p-block elements in the periodic table. The d-block elements are mainly represented by the groups 3 to 12 of the modern periodic table.

Most of the d-block elements are known as transition elements. They occupy an important position in inorganic chemistry because they exhibit variable oxidation states, form coloured ions and compounds, possess magnetic properties, form coordination compounds and show catalytic activity.

The characteristic properties of transition elements arise mainly from the involvement of both (n−1)d and ns electrons in bonding and chemical reactions.

2. Position of d-Block Elements in the Periodic Table

The d-block elements are situated between the s-block and p-block elements. They occur in the central portion of the periodic table and include groups 3 to 12.

The filling of the d-subshell occurs progressively across these elements. Since the d-subshell is being filled in the penultimate shell, these elements are also called inner transition-related elements in a broad descriptive sense, although the term inner transition elements is more specifically used for f-block elements.

3. General Electronic Configuration

The general electronic configuration of d-block elements is:

Here, n represents the principal quantum number of the outermost shell. The d-electrons are present in the penultimate shell, while the s-electrons are present in the outermost shell.

3.1 First Transition Series

The first transition series extends from scandium (Sc) to zinc (Zn). The general electronic configuration is:

4. Transition Series

4.1 First Transition Series

The first transition series contains Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn. It involves progressive filling of the 3d orbitals.

4.2 Second Transition Series

The second transition series extends approximately from yttrium to cadmium and involves filling of the 4d orbitals.

4.3 Third Transition Series

The third transition series involves the filling of the 5d orbitals. It includes elements such as Hf, Ta, W, Re, Os, Ir, Pt, Au and Hg.

4.4 Fourth Transition Series

The fourth series involves the filling of the 6d orbitals and contains the heavier synthetic elements.

5. General Characteristics of d-Block Elements

The d-block elements show several characteristic properties that distinguish them from many s-block and p-block elements. These properties are largely related to their partially filled d-orbitals.

5.1 Metallic Character

Almost all d-block elements are metals. They possess high electrical and thermal conductivity, lustre, high density and relatively high melting and boiling points.

The presence of metallic bonding involving d-electrons contributes to their strong metallic character.

5.2 High Melting and Boiling Points

Most transition metals have high melting and boiling points. Strong metallic bonding is responsible for this property. The strength of metallic bonding depends partly on the number of electrons available for delocalization.

5.3 High Density

Transition metals generally have high densities because their atomic masses are relatively high and their atomic volumes are comparatively small.

5.4 Variable Oxidation States

One of the most important properties of transition elements is their ability to exhibit several oxidation states. This occurs because the energies of the ns and (n−1)d electrons are relatively close.

5.5 Formation of Coloured Ions

Many transition-metal ions and compounds are coloured. The colour is generally associated with electronic transitions involving d-orbitals.

5.6 Magnetic Behaviour

Transition-metal ions frequently contain unpaired d-electrons. Therefore, many of them are paramagnetic.

5.7 Complex Formation

Transition metals readily form coordination compounds with ligands such as NH₃, H₂O, CN⁻, Cl⁻ and other donor species.

5.8 Catalytic Activity

Many transition metals and their compounds act as catalysts. Their variable oxidation states and ability to adsorb reactants make them particularly useful in catalytic processes.

5.9 Formation of Alloys

Transition metals readily form alloys with one another and with other metals. Their atomic sizes are often sufficiently similar to permit substitution in metallic lattices.

5.10 Formation of Interstitial Compounds

Small atoms such as hydrogen, carbon, nitrogen and boron can occupy the interstitial spaces in the crystal lattice of transition metals. The resulting substances are called interstitial compounds.

6. Atomic and Ionic Radii

Across a transition series, the atomic radii generally decrease at first and then become nearly constant toward the end of the series.

As the atomic number increases, the nuclear charge increases. The added d-electrons provide some shielding, which reduces the effect of increasing nuclear charge. As a result, the decrease in atomic size is relatively small.

The ionic radii of transition-metal ions generally decrease with increasing oxidation state because greater positive charge attracts the remaining electrons more strongly.

7. Ionization Energy

The ionization energies of transition elements generally increase gradually across a series. The increase is not regular because of changes in electronic configuration and the relative stability of different d-electron arrangements.

The relatively small difference between the energies of the ns and (n−1)d electrons allows both types of electrons to participate in chemical bonding and oxidation-state formation.

8. Variable Oxidation States

Variable oxidation state is one of the most characteristic properties of transition elements. Unlike many main-group elements, a transition metal can form several stable ions with different oxidation numbers.

This behaviour occurs because the energy difference between the outer ns and inner (n−1)d orbitals is relatively small. Therefore, different numbers of these electrons can participate in bonding.

8.1 Stability of Oxidation States

The stability of an oxidation state depends on several factors, including electronic configuration, lattice energy, hydration energy, ionization energy and the nature of ligands.

Higher oxidation states are often stabilized by highly electronegative elements such as oxygen and fluorine. For example, chromium(VI) is commonly found in chromate, dichromate and chromium(VI) oxide compounds.

Similarly, manganese exhibits oxidation states ranging from +2 to +7, with high oxidation states particularly stabilized in oxygen-containing compounds.

9. Colour of Transition-Metal Ions and Compounds

Many transition-metal ions are coloured. This property is mainly associated with the presence of partially filled d-orbitals.

When a transition-metal ion is placed in a ligand field, the d-orbitals split into groups of different energies. Electrons can absorb visible light and move from a lower energy d-orbital to a higher energy d-orbital.

This process is called a d–d transition. The wavelength of light absorbed determines the colour observed by the eye.

9.1 Why are d⁰ and d¹⁰ ions often colourless?

Ions having d⁰ or d¹⁰ configurations do not normally undergo d–d transitions. Therefore, many simple ions with these configurations are colourless.

10. Magnetic Properties

The magnetic behaviour of transition-metal ions is mainly determined by the number of unpaired electrons present in their d-orbitals.

10.1 Paramagnetism

A substance containing one or more unpaired electrons is generally paramagnetic. It is attracted by an external magnetic field.

Most transition-metal ions are paramagnetic because their d-orbitals are incompletely filled.

10.2 Diamagnetism

A substance in which all electrons are paired is diamagnetic. It is weakly repelled by an external magnetic field.

For example, Zn²⁺ has a d¹⁰ configuration and is diamagnetic.

10.3 Spin-only Magnetic Moment

The magnetic moment can be approximately calculated using the spin-only formula:

Here, n is the number of unpaired electrons and BM represents Bohr Magneton.

11. Complex Formation

Transition metals have a strong tendency to form coordination or complex compounds. In these compounds, a central metal ion is surrounded by ions or molecules called ligands.

11.1 Reasons for Complex Formation

• Small size of transition-metal ions.
• Relatively high charge density.
• Availability of suitable orbitals for bonding.
• Variable oxidation states.
• Ability to accept electron pairs from ligands.

11.2 Examples

12. Catalytic Properties

Many transition metals and their compounds act as catalysts. Their catalytic activity is one of the most important industrial applications of transition elements.

12.1 Reasons for Catalytic Activity

• Variable oxidation states.
• Ability to form intermediate compounds.
• Ability to adsorb reactants on their surfaces.
• Availability of vacant or partially occupied orbitals.
• Ability to provide an alternative reaction pathway.

12.2 Examples

13. Alloy Formation

Transition metals readily form alloys because many of them have similar atomic sizes and crystal structures. Atoms of one metal can therefore replace atoms of another metal in the metallic lattice.

Alloys often possess improved hardness, strength, corrosion resistance and other useful properties.

14. Interstitial Compounds

Transition metals can form compounds by allowing small atoms such as hydrogen, carbon, nitrogen or boron to occupy interstitial spaces in the metal lattice.

These compounds are known as interstitial compounds. They are generally hard and may retain metallic conductivity.

Important Characteristics

• They are often hard and rigid.
• They may have high melting points.
• They often retain metallic conductivity.
• They are frequently non-stoichiometric.

15. Detailed Study of Important First-Row Transition Elements

15.1 Scandium (Sc)

Scandium is the first member of the first transition series. Its atomic number is 21 and its electronic configuration is:

The most stable oxidation state of scandium is +3. The Sc³⁺ ion has a d⁰ configuration, so its compounds are generally colourless and diamagnetic.

Important Properties

• Predominantly exhibits +3 oxidation state.
• Sc³⁺ has d⁰ configuration.
• Its compounds are generally colourless.
• It forms coordination compounds.

Uses

Scandium compounds and scandium-containing alloys have specialized applications, including aerospace materials and high-intensity lighting systems.

15.2 Titanium (Ti)

Titanium has atomic number 22 and electronic configuration:

Titanium commonly exhibits +2, +3 and +4 oxidation states. The +4 state is the most stable under many conditions.

Titanium Dioxide

Titanium dioxide, TiO₂, is an important compound of titanium. It is a white solid and is widely used as a pigment because of its high refractive index and stability.

Important Properties of TiO₂

• White solid.
• High melting point.
• Chemically stable.
• Strong covering and pigmenting power.

Uses of Titanium

• Aerospace materials.
• Medical implants.
• Corrosion-resistant equipment.
• Pigment production through TiO₂.

15.3 Vanadium (V)

Vanadium has atomic number 23 and electronic configuration:

Vanadium exhibits several oxidation states, commonly +2, +3, +4 and +5.

Vanadium Pentoxide

Vanadium pentoxide, V₂O₅, is an important vanadium compound and is used as a catalyst in the Contact process.

V₂O₅ facilitates the oxidation of sulfur dioxide to sulfur trioxide.

15.4 Chromium (Cr)

Chromium has atomic number 24 and possesses the exceptional electronic configuration:

Chromium exhibits several oxidation states, particularly +2, +3 and +6.

Chromium(II)

Chromium(II) compounds are generally reducing agents. The Cr²⁺ ion has a d⁴ configuration.

Chromium(III)

Chromium(III) is relatively stable. The Cr³⁺ ion has a d³ configuration and commonly forms coordination compounds.

Chromium(VI)

Chromium(VI) occurs mainly in oxygen-containing compounds such as chromate and dichromate.

Chromate Ion

Chromate ion is generally yellow in aqueous solution.

Dichromate Ion

Dichromate ion is orange in aqueous solution.

Chromate–Dichromate Equilibrium

Addition of acid shifts the equilibrium toward dichromate, while addition of base favours chromate.

Potassium Dichromate

Potassium dichromate, K₂Cr₂O₇, is an important orange crystalline compound. It is a strong oxidizing agent, especially in acidic medium.

Chromium Trioxide

Chromium trioxide, CrO₃, is another important Cr(VI) compound and is a powerful oxidizing substance.

15.5 Manganese (Mn)

Manganese has atomic number 25 and electronic configuration:

Manganese is particularly important because it exhibits a wide range of oxidation states from +2 to +7.

Manganese Dioxide

Manganese dioxide, MnO₂, is a dark solid and an important manganese compound.

It acts as an oxidizing agent and is also used as a catalyst in several reactions.

Preparation of Oxygen from Hydrogen Peroxide

MnO₂ acts as a catalyst for this decomposition.

Potassium Permanganate

Potassium permanganate, KMnO₄, is one of the most important compounds of manganese. It is a dark purple crystalline solid and a strong oxidizing agent.

Reduction of Permanganate in Acidic Medium

Thus, permanganate ion is reduced from Mn(VII) to Mn(II) in acidic medium.

Reduction in Neutral or Weakly Alkaline Medium

Reduction in Strongly Alkaline Medium

Therefore, the product formed during reduction of permanganate depends strongly on the reaction medium.

15.6 Iron (Fe)

Iron has atomic number 26 and electronic configuration:

The most important oxidation states of iron are +2 and +3.

Iron(II)

Iron(II), Fe²⁺, has a d⁶ configuration. Ferrous compounds are often pale green or greenish in aqueous solution.

Iron(III)

Iron(III), Fe³⁺, has a d⁵ configuration. Ferric compounds are commonly yellow, brown or reddish depending on the compound and medium.

Iron(II) Chloride

Ferrous chloride can be prepared by the reaction of iron with dilute hydrochloric acid.

Iron(III) Chloride

Oxidation of Fe(II) to Fe(III)

Fe(II) can be oxidized to Fe(III) by suitable oxidizing agents.

Importance of Iron

• Major component of steel and cast iron.
• Used in construction and engineering.
• Essential element in biological systems.
• Present in haemoglobin and several enzymes.

15.7 Cobalt (Co)

Cobalt has atomic number 27 and electronic configuration:

The common oxidation states of cobalt are +2 and +3. Cobalt is particularly important in coordination chemistry.

Cobalt(II)

Cobalt(II) compounds are generally more stable than simple cobalt(III) compounds in aqueous conditions.

Cobalt(III) Complexes

Cobalt(III) forms numerous stable coordination complexes with ligands such as ammonia and chloride.

Cobalt compounds also have applications in pigments, alloys and catalysis.

15.8 Nickel (Ni)

Nickel has atomic number 28 and electronic configuration:

Nickel commonly exhibits the +2 oxidation state. It forms many coordination compounds and alloys.

Nickel as a Catalyst

Finely divided nickel is widely used as a catalyst for hydrogenation reactions.

Nickel is also used in the hydrogenation of vegetable oils.

Uses of Nickel

• Manufacture of stainless steel and other alloys.
• Electroplating.
• Hydrogenation catalyst.
• Coinage and engineering applications.

15.9 Copper (Cu)

Copper has atomic number 29 and the exceptional electronic configuration:

Copper mainly exhibits +1 and +2 oxidation states. Copper(II) is generally more stable in aqueous conditions.

Copper(I)

Copper(I) compounds contain Cu⁺, which has a d¹⁰ configuration.

Copper(II)

Copper(II) contains Cu²⁺, which has a d⁹ configuration. Many Cu(II) compounds are blue or green.

Copper(II) Oxide

Copper(II) oxide is a black solid and reacts with acids to form copper(II) salts.

Copper(II) Hydroxide

Copper(II) hydroxide is a blue precipitate.

Uses of Copper

• Electrical wiring.
• Heat exchangers.
• Manufacture of brass and bronze.
• Plumbing and engineering applications.
• Electroplating and electrorefining.

15.10 Zinc (Zn)

Zinc has atomic number 30 and electronic configuration:

Zinc commonly exhibits only the +2 oxidation state. Zn²⁺ has a completely filled d¹⁰ configuration.

Important Uses

• Galvanization of iron.
• Manufacture of brass.
• Dry cells.
• Die-casting alloys.

16. Comparison of Important First-Row Transition Elements

17. Important Trends Across the First Transition Series

17.1 Atomic Size

Atomic size generally decreases from Sc to approximately the middle of the series and then remains nearly constant. This occurs because increasing nuclear charge is partly balanced by increased shielding from the added d-electrons.

17.2 Ionization Energy

Ionization energy generally shows a gradual increase across the series, although irregularities occur because of changes in electronic configuration.

17.3 Oxidation States

The number of accessible oxidation states increases toward the middle of the first transition series and then becomes more restricted toward the end.

17.4 Stability of +2 Oxidation State

The +2 oxidation state is common among many first-row transition elements because the two 4s electrons are generally removed first.

17.5 Higher Oxidation States

Higher oxidation states are particularly stable when the metal is bonded to electronegative elements such as oxygen and fluorine.

18. Why Do Transition-Metal Compounds Show Different Colours?

The colour of transition-metal compounds depends on the electronic configuration of the metal ion, the oxidation state, the surrounding ligands and the geometry of the coordination sphere.

When white light falls on a transition-metal compound, certain wavelengths may be absorbed because of electronic transitions. The remaining wavelengths are transmitted or reflected and produce the observed colour.

Consequently, changing the oxidation state or ligand can change the colour of a transition-metal compound.

19. Coordination Number and Geometry

Transition-metal complexes may possess different coordination numbers and geometries depending on the metal ion, oxidation state and ligand.

20. Redox Behaviour of Transition Elements

Transition metals participate readily in oxidation-reduction reactions because they can exist in multiple oxidation states.

A metal ion can be oxidized to a higher oxidation state or reduced to a lower oxidation state depending on the reaction conditions.

Example: Fe(II)/Fe(III)

Example: Mn(VII)/Mn(II)

21. Biological Importance of Transition Elements

Several transition elements are essential for living organisms. They participate in oxygen transport, electron transfer, enzyme activity and other biochemical processes.

Iron

Iron is an essential component of haemoglobin and myoglobin. It is also present in cytochromes and several iron-containing enzymes.

Cobalt

Cobalt is a constituent of vitamin B₁₂, which is essential for normal cellular metabolism and blood formation.

Copper

Copper acts as a component of several enzymes involved in oxidation-reduction reactions.

Manganese

Manganese is required in trace amounts for the activity of several enzymes and metabolic processes.

22. Industrial Importance of d-Block Elements

Transition metals are extremely important in modern industries because of their strength, catalytic activity, corrosion resistance, electrical properties and ability to form useful alloys.

23. Important Chemical Equations for Revision

Formation of titanium dioxide:

Oxidation of sulfur dioxide:

Chromate-dichromate equilibrium:

Reduction of dichromate in acidic medium:

Reduction of permanganate in acidic medium:

Reduction of permanganate in neutral medium:

Preparation of ferrous chloride:

Formation of ferric chloride:

Formation of copper(II) chloride:

Formation of copper hydroxide:

Decomposition of hydrogen peroxide:

24. d-Block Elements vs Typical Main-Group Elements

25. Important Concepts to Remember

26. Quick Revision of d-Block Elements