What NEET Asks
- Direct questions on identifying primary and secondary valencies.
- Calculations involving oxidation state and coordination number from complex formulas.
- Predicting the number of ions in solution or the conductivity of complexes.
- Conceptual understanding of directional vs. non-directional valencies and their implications on geometry.
Key Points
- Alfred Werner (1893) proposed the theory of coordination compounds.
- Central metal atom/ion exhibits two types of valencies: primary and secondary.
- Primary Valence (Oxidation State): Ionizable, satisfied by anions, non-directional, corresponds to the metal's oxidation state.
- Secondary Valence (Coordination Number): Non-ionizable, satisfied by ligands, directional, determines the geometry of the complex.
- Metal ions tend to satisfy both their primary and secondary valencies.
- Some anions (e.g., Cl-, SO4^2-) can satisfy both primary (as counter-ions) and secondary (as ligands) valencies simultaneously if they are part of the coordination sphere.
Must-Know Formula / Reaction
- Oxidation State Calculation:
Charge of Complex = Sum of (Oxidation State of Metal) + Sum of (Charges of Ligands)- Oxidation State of Metal: Primary Valence.
- Charges of Ligands: Charge contributed by each ligand (e.g., NH3=0, Cl-=-1, H2O=0).
- Coordination Number:
Coordination Number = Sum of (Number of Ligands × Denticity)- Denticity: Number of donor atoms in a single ligand (e.g., monodentate = 1, bidentate = 2).
Common Mistakes
- Students often confuse primary valence (oxidation state) with secondary valence (coordination number).
- Don't forget that a ligand inside the coordination sphere contributes to secondary valence, while a counter-ion outside contributes to primary valence.
- Students often overlook that some ligands (like Cl-) can satisfy both primary (by balancing charge) and secondary (by coordinating) valencies if they are within the coordination sphere.
Rapid Revision
Primary valence = Oxidation state, ionizable, non-directional. Secondary valence = Coordination number, non-ionizable, directional. Ligands satisfy secondary valence. Counter-ions satisfy primary valence. Some ligands can satisfy both.