medium difficulty9 min

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.

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

Practice This Concept

Understand it

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.

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.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

Find the oxidation state of Co in [Co(NH3)5Cl]Cl2.

1Let oxidation state of Co be x.
2Inside the sphere, NH₃ is neutral and coordinated Cl is -1.
3The two outside Cl^- ions show that the complex ion is +2.
4Therefore x + 0 - 1 = +2, so x = +3.

Method you can reuse

1Write the coordination sphere separately.
2Identify ligand charges.
3Find the charge on the complex ion from counter ions.
4Apply charge balance for oxidation state.
5Count only donor atoms inside the brackets for coordination number.

Visual tip

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.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

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

Think first, then show answer

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.

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.

Don't mix this up

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.

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.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

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.

1
Enough for 1 mark
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
Add for 2 marks
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.
3
Add for 3 marks
A coordination compound has a central metal atom or ion bonded to ligands.

MCQ Quiz

Practice this concept with focused MCQs

Open the concept quiz intro first, review the test details, and then start a focused MCQ set from this concept only. Instant score and answer review are live now.

10 MCQs5 MinutesInstant Results