Oxidation state and coordination number: Class 12 practice
In a complex, the metal's oxidation state x satisfies: x + Σ(ligand charges) = charge of the complex ion. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Ion charges
The idea behind oxidation state and coordination number
In a complex, the metal's oxidation state x satisfies: x + Σ(ligand charges) = charge of the complex ion. Coordination number = number of donor atoms (a bidentate ligand like en counts 2).
Step-by-step method
- Find the complex ion's charge from the counter-ions.
- Add ligand charges (NH₃, H₂O, CO = 0; Cl⁻, CN⁻ = −1; en = 0, bidentate).
- Solve for the metal's oxidation state.
Worked example 1 (Basic)
Find the oxidation state and coordination number of the metal in [Cr(H₂O)₆]Cl₃.
- Balance charges with counter-ions and ligands
- Oxidation state = +3
- Donor atoms = 6
- Answer: +3, CN 6
Worked example 2 (Standard)
Find the oxidation state and coordination number of the metal in [Pt(NH₃)₂Cl₂].
- Balance charges with counter-ions and ligands
- Oxidation state = +2
- Donor atoms = 4
- Answer: +2, CN 4
Worked example 3 (Challenge)
Find the oxidation state and coordination number of the metal in [Fe(C₂O₄)₃]³⁻.
- Balance charges with counter-ions and ligands
- Oxidation state = +3
- Donor atoms = 6
- Answer: +3, CN 6
Common mistakes and misconceptions
- Ignoring counter-ions
- Counting en as one donor
Practice questions
- (Basic) Find the oxidation state and coordination number of the metal in K₄[Fe(CN)₆].
- (Basic) Find the oxidation state and coordination number of the metal in [Cu(NH₃)₄]SO₄.
- (Standard) Find the oxidation state and coordination number of the metal in [Co(en)₃]Cl₃.
- (Standard) Find the oxidation state and coordination number of the metal in K₂[PtCl₆].
- (Challenge) Find the oxidation state and coordination number of the metal in Na₂[Zn(OH)₄].
- (Challenge) Find the oxidation state and coordination number of the metal in [Co(NH₃)₅Cl]Cl₂.
Answer key with full working
- Q1: Balance charges with counter-ions and ligands → Oxidation state = +2 → Donor atoms = 6 → Answer: +2, CN 6
- Q2: Balance charges with counter-ions and ligands → Oxidation state = +2 → Donor atoms = 4 → Answer: +2, CN 4
- Q3: Balance charges with counter-ions and ligands → Oxidation state = +3 → Donor atoms = 6 → Answer: +3, CN 6
- Q4: Balance charges with counter-ions and ligands → Oxidation state = +4 → Donor atoms = 6 → Answer: +4, CN 6
- Q5: Balance charges with counter-ions and ligands → Oxidation state = +2 → Donor atoms = 4 → Answer: +2, CN 4
- Q6: Balance charges with counter-ions and ligands → Oxidation state = +3 → Donor atoms = 6 → Answer: +3, CN 6
Syllabus note
This skill is listed under "Coordination Compounds" for Class 12 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is oxidation state and coordination number in?
"Coordination Compounds", Class 12 Chemistry (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.