Introduction to Transition Metal Chemistry:

As discussed earlier, the elements zinc, cadmium, and mercury are not considered transition elements since their electronic configurations are different from other transition metals. However, the rest of the d-block elements are somewhat similar in properties and this similarity can be observed along each specific row of the periodic table. These properties of the transition elements are listed below.

1. Variable Oxidation States
• Can lose different numbers of d and s electrons → form multiple oxidation states.
• Example: Fe²⁺ (ferrous) & Fe³⁺ (ferric), Cu⁺ & Cu²⁺.
2. Formation of Colored Compounds
• d-electrons absorb visible light → produce colored solutions.
• Example:
• Fe²⁺ (pale green)
• Cu²⁺ (blue)
• Cr³⁺ (violet/green)
3. Complex Formation
• Readily form coordination complexes with ligands like H₂O, NH₃, Cl⁻.
• Example: [Fe(CN)₆]³⁻, [Cu(NH₃)₄]²⁺.
4. Catalytic Properties
• Can provide surfaces or variable oxidation states for catalysis.
• Example: Fe in Haber process, V₂O₅ in Contact process.
5. High Melting & Boiling Points
• Strong metallic bonding due to delocalized d-electrons.
6. Good Conductors of Heat & Electricity
• Free d-electrons allow conductivity.
7. Paramagnetism
• Unpaired d-electrons → magnetic behavior (Fe, Co, Ni strongly magnetic).

Atomic Ionic Radii

The atomic and ionic radii of the transition elements decrease from group 3 to group 6 due to the poor shielding offered by the small number of d-electrons. Those placed between groups 7 and 10 have somewhat similar atomic radii and those placed in groups 11 and 12 have larger radii. This is because the nuclear charge is balanced out by the electron-electron repulsions.

While traversing down the group, an increase in the atomic and ionic radii of the elements can be observed. This increase in the radius can be explained by the presence of a greater number of subshells.

Ionization Enthalpy

Ionization enthalpy refers to the amount of energy that must be supplied to an element for the removal of a valence electron. The greater the effective nuclear charge acting on the electrons, the greater the ionization potential of the element. This is why the ionization enthalpies of transition elements are generally greater than those of the s-block elements.

In a way, the ionization energy of an element is closely related to its atomic radius. Atoms with smaller radii tend to have greater ionization enthalpies than those with relatively larger radii. The ionization energies of the transition metals increase while moving along the row (due to the increase in atomic number).

These elements also exhibit a wide variety of oxidation states and tend to form compounds that act as catalysts in many chemical processes.