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Werner's Theory, Coordination Sphere and Basic Terms

Werner's theory states that a metal in a coordination compound shows primary valence, usually ionisable and related to oxidation state, and secondary valence, non-ionisable and equal to the coordination number. The metal and directly attached ligands form the coordination sphere written inside square brackets.

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Student-friendly explanation

A coordination compound has a central metal atom or ion bonded to ligands. Ligands donate an electron pair to the metal. The species inside square brackets is the coordination entity, while ions outside the brackets act as counter ions. Primary valence is satisfied by negative ions and is ionisable in solution. Secondary valence is satisfied by ligands and fixes the number and spatial arrangement of ligand attachments around the metal.

How to write this in exams

  1. 1

    Start with the exact idea

    Werner's theory states that a metal in a coordination compound shows primary valence, usually ionisable and related to oxidation state, and secondary valence, non-ionisable and equal to the coordination number. The metal and directly attached ligands form the coordination sphere written inside square brackets.

  2. 2

    Then show how to use it

    Write the coordination sphere separately. Identify ligand charges. Find the charge on the complex ion from counter ions. Apply charge balance for oxidation state. Count only donor atoms inside the brackets for coordination number.

  3. 3

    Add one concrete example

    In [Co(NH3)6]Cl3, the coordination sphere is [Co(NH3)6]3+. Ammonia is a neutral ligand, the coordination number of cobalt is 6, and three chloride ions are outside the coordination sphere as counter ions.

  4. 4

    Avoid this incomplete answer

    A common wrong answer is coordination number 7 for [Co(NH3)6]Cl3 because the student adds one chloride or all counter ions to the six ammonia ligands.

Definition

Werner's theory states that a metal in a coordination compound shows primary valence, usually ionisable and related to oxidation state, and secondary valence, non-ionisable and equal to the coordination number. The metal and directly attached ligands form the coordination sphere written inside square brackets.

Example

In [Co(NH3)6]Cl3, the coordination sphere is [Co(NH3)6]3+. Ammonia is a neutral ligand, the coordination number of cobalt is 6, and three chloride ions are outside the coordination sphere as counter ions.

Rule to remember

Coordination number = number of donor atoms directly attached to the central metal. Oxidation state rule: charge on metal + total ligand charge = charge on coordination entity.

Memory hook

Square brackets show the bonded team; anything outside balances charge but does not increase coordination number.

Examples and method

Worked example

Find the oxidation state of Co in [Co(NH3)5Cl]Cl2. Let oxidation state of Co be x. Inside the sphere, NH3 is neutral and coordinated Cl is -1. The two outside Cl- ions show that the complex ion is +2. Therefore x + 0 - 1 = +2, so x = +3.

Method to apply

Write the coordination sphere separately. Identify ligand charges. Find the charge on the complex ion from counter ions. Apply charge balance for oxidation state. Count only donor atoms inside the brackets for coordination number.

Diagram support

A bracket-labelled structural sketch can help show the central metal, ligands inside the coordination sphere, and counter ions outside it, but a diagram is not essential for this terminology concept.

How CBSE asks it

Questions usually ask for oxidation state, coordination number, ligand identification, primary and secondary valence, or the number of ions formed in solution.

Avoid common mistakes

Common confusion

Students often count counter ions outside the square brackets while finding coordination number. In [Co(NH3)6]Cl3, Cl- ions are not directly bonded to cobalt, so they are not counted in the coordination number.

Common wrong answer

A common wrong answer is coordination number 7 for [Co(NH3)6]Cl3 because the student adds one chloride or all counter ions to the six ammonia ligands.

Exam tip

First separate the part inside square brackets from the ions outside. Use only the ligands directly attached to the metal to find coordination number, and use charge balance to find oxidation state.

Quick check

In [Pt(NH3)4]Cl2, what is the coordination number of Pt and which ions are outside the coordination sphere?

The coordination number of Pt is 4 because four NH3 ligands are directly attached. Two Cl- ions are outside the coordination sphere as counter ions.

Answer writing and exam use

1-mark answer

Werner's theory states that a metal in a coordination compound shows primary valence, usually ionisable and related to oxidation state, and secondary valence, non-ionisable and equal to the coordination number. The metal and directly attached ligands form the coordination sphere written inside square brackets.

2-mark answer

Werner's theory states that a metal in a coordination compound shows primary valence, usually ionisable and related to oxidation state, and secondary valence, non-ionisable and equal to the coordination number. The metal and directly attached ligands form the coordination sphere written inside square brackets. Coordination number = number of donor atoms directly attached to the central metal. Oxidation state rule: charge on metal + total ligand charge = charge on coordination entity. In [Co(NH3)6]Cl3, the coordination sphere is [Co(NH3)6]3+. Ammonia is a neutral ligand, the coordination number of cobalt is 6, and three chloride ions are outside the coordination sphere as counter ions.

3-mark answer

A coordination compound has a central metal atom or ion bonded to ligands. Ligands donate an electron pair to the metal. The species inside square brackets is the coordination entity, while ions outside the brackets act as counter ions. Primary valence is satisfied by negative ions and is ionisable in solution. Secondary valence is satisfied by ligands and fixes the number and spatial arrangement of ligand attachments around the metal. Coordination number = number of donor atoms directly attached to the central metal. Oxidation state rule: charge on metal + total ligand charge = charge on coordination entity. Find the oxidation state of Co in [Co(NH3)5Cl]Cl2. Let oxidation state of Co be x. Inside the sphere, NH3 is neutral and coordinated Cl is -1. The two outside Cl- ions show that the complex ion is +2. Therefore x + 0 - 1 = +2, so x = +3. Questions usually ask for oxidation state, coordination number, ligand identification, primary and secondary valence, or the number of ions formed in solution. A common wrong answer is coordination number 7 for [Co(NH3)6]Cl3 because the student adds one chloride or all counter ions to the six ammonia ligands.
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