Short Answer

Transition metals form complex ions because they have partially filled d-orbitals that can accept electron pairs from ligands.

• The empty or partially filled d-orbitals allow coordination with molecules or ions, forming coordination complexes.
• Complex formation stabilizes the metal ion, alters its physical and chemical properties, and is common in transition metal chemistry.

Detailed Explanation :

Definition of Complex Ions

A complex ion consists of a central metal ion bonded to surrounding molecules or ions (ligands) via coordinate covalent bonds.

• Ligands donate lone pairs of electrons to the metal’s vacant orbitals.
• Transition metals are especially suited for this due to their electronic configuration (n-1)d¹⁻¹⁰ ns¹⁻².

Reasons for Complex Formation

1. Availability of d-Orbitals:
   • Partially filled d-orbitals can accept electron pairs, providing multiple sites for coordination.
   • Example: Cu²⁺ ([Ar] 3d⁹) can coordinate with 4 H₂O molecules to form [Cu(H₂O)₄]²⁺.
2. Variable Oxidation States:
   • Transition metals can exhibit multiple oxidation states (+2, +3, etc.), influencing complex stability.
   • Example: Fe²⁺ and Fe³⁺ form [Fe(H₂O)₆]²⁺ and [Fe(H₂O)₆]³⁺ respectively.
3. Formation of Stable Coordination Numbers:
   • Common coordination numbers: 4 (tetrahedral/square planar) and 6 (octahedral).
   • Ligand attachment minimizes electron repulsion and stabilizes the complex.
4. Energetic Favorability:
   • Formation of complexes releases energy, often via enthalpy of formation, making the process thermodynamically favorable.
   • Ligands stabilize the metal ion and reduce its reactivity.
5. Magnetic and Color Effects:
   • Complex formation causes d-orbital splitting, giving rise to colored compounds.
   • Example: [Ti(H₂O)₆]³⁺ appears violet due to d-d transitions.

Role of Ligands

1. Nature of Ligands:
   • Ligands can be neutral molecules (H₂O, NH₃) or anions (Cl⁻, CN⁻).
   • Ligand strength affects stability and geometry of the complex.
2. Chelation:
   • Ligands with multiple donor sites form chelates, which are more stable.
   • Example: EDTA forms strong complexes with Fe³⁺, Ca²⁺, etc.
3. Crystal Field Stabilization:
   • Ligands split metal d-orbitals → stabilization of low-spin or high-spin complexes, depending on ligand type.

Examples of Complex Ions

1. Copper Complexes:
   • [Cu(NH₃)₄]²⁺ → deep blue solution.
   • [Cu(H₂O)₄]²⁺ → pale blue solution.
2. Iron Complexes:
   • [Fe(CN)₆]³⁻ → yellow solution.
   • [Fe(H₂O)₆]³⁺ → pale violet solution.
3. Cobalt Complexes:
   • [Co(NH₃)₆]³⁺ → violet solution.

Significance

• Complex formation explains color, magnetism, and catalytic properties of transition metals.
• Central to bioinorganic chemistry: hemoglobin ([Fe(Hemoglobin)]) and chlorophyll ([Mg(Chlorophyll)]).
• Industrially important in catalysis, electroplating, and metal recovery.

Conclusion

Transition metals form complex ions because partially filled d-orbitals accept electron pairs from ligands, stabilizing the metal ion. The formation of complexes is influenced by oxidation state, ligand nature, coordination number, and crystal field effects. These complexes are crucial for understanding color, magnetism, catalysis, and biological functions of transition metals in chemistry and industry.