Primary and Secondary Valencies
Distinguishing between the ionizable (primary) and non-ionizable (secondary) valencies and their roles in determining oxidation state and coordination number.
What is Primary and Secondary Valencies?
According to Werner's theory, it represents the oxidation state of the central metal ion and is ionizable, typically satisfied by counter-anions.
Key formula / rule: Oxidation State Calculation
Key points
- Define primary and secondary valencies according to Werner's theory.
- Distinguish between primary and secondary valencies based on their nature (ionizable/non-ionizable), satisfaction, and role.
- Relate primary valency to the oxidation state of the central metal ion.
- Relate secondary valency to the coordination number of the central metal ion.
Common exam trap
Confusing primary valency with secondary valency.
Definitions
- Term
Primary Valency
- Meaning
According to Werner's theory, it represents the oxidation state of the central metal ion and is ionizable, typically satisfied by counter-anions.
- Term
Secondary Valency
- Meaning
According to Werner's theory, it represents the coordination number of the central metal ion and is non-ionizable, satisfied by ligands directly bonded to the metal, determining the complex's geometry.
- Term
Coordination Number
- Meaning
The total number of ligand donor atoms directly attached to the central metal ion in a coordination complex.
- Term
Oxidation State
- Meaning
The charge an atom would have if all bonds were ionic, representing the ° of oxidation (loss of electrons) of an atom in a chemical compound.
- Term
Ligand
- Meaning
An ion or molecule (functional group) that binds to a central metal atom to form a coordination complex.
Learning objectives
Define primary and secondary valencies according to Werner's theory.
Distinguish between primary and secondary valencies based on their nature (ionizable/non-ionizable), satisfaction, and role.
Relate primary valency to the oxidation state of the central metal ion.
Relate secondary valency to the coordination number of the central metal ion.
Apply Werner's theory to simple coordination compounds to identify primary and secondary valencies.
Formulae
- Name
Oxidation State Calculation
- Note
Ligand charges must be known (e.g., NH₃=0, Cl⁻=-1, H₂O=0, CN⁻=-1, en=0).
- Expression
Sum of oxidation states of all atoms in a neutral complex = 0; Sum of oxidation states of all atoms in a complex ion = charge on the ion.
- Name
Coordination Number Determination
- Note
Monodentate ligands contribute 1, bidentate contribute 2, etc.
- Expression
Number of ligand donor atoms directly bonded to the central metal ion.
Prerequisites
Basic understanding of chemical bonding.
Knowledge of oxidation states.
Familiarity with coordination compounds, central metal atom, and ligands.
Concept of coordination number.
Common mistakes
Confusing primary valency with secondary valency.
Incorrectly determining the oxidation state of the central metal ion.
Incorrectly determining the coordination number of the central metal ion.
Assuming all anions satisfy only primary valency or only secondary valency; some can satisfy both.
Not understanding that secondary valency determines geometry, while primary valency determines charge.
Keywords
Werner's Theory
Primary Valency
Secondary Valency
Oxidation State
Coordination Number
Ligands
Ionizable
Non-ionizable
Coordination Compounds
Geometry
Stereochemistry
Practice preview
The secondary valency of a metal ion in a coordination compound is equal to its:…
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In Werner's theory, the distinction between primary and secondary valencies is crucial for understanding the structure and properties of coordination compounds. Which of the following correctly describes this distinction…
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In the complex [Co(NH₃)₆]Cl₃, what is the primary valency of Cobalt (Co)?…
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