Coordination Compounds

Chemistry · Class 12

Simulation · Chemistry · Class 12

Count the ligands or the donor atoms?

"Count the ligands or the donor atoms?" is a free, interactive Class 12 Chemistry simulation for NEET and JEE, built on the NCERT section "Coordination number and oxidation number" in Coordination Compounds. A metal ion with ligands snapping on through their donor atoms: unidentate, didentate (en, ox) and hexadentate EDTA; CN counts donor atoms, oxidation state from charge balance. NEET has asked about this idea 3 times in 10 years of papers. JEE Main asked 80 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).

From the lesson Coordination number and oxidation number in Coordination Compounds. Change the values and watch what happens.

Count the ligands or the donor atoms?Chemistry · Class 12

What it shows

NCERT §5.2; §5.3

A metal ion with ligands snapping on through their donor atoms: unidentate, didentate (en, ox) and hexadentate EDTA; CN counts donor atoms, oxidation state from charge balance.

Controls

3 to try

  • Dragligands Cl⁻, NH₃, H₂O, CN⁻, en, C₂O₄²⁻, EDTA⁴⁻ onto the metal
  • Slideroverall charge of the entity −4 to +3
  • Readoutscoordination number, metal oxidation state, homoleptic/heteroleptic

Try this

Reach CN = 6 using only three ligands, and find the oxidation state of Fe in K₃[Fe(C₂O₄)₃].

Common mistake

Counting ligands instead of donor atoms.

In NEET

10 years of papers

Asked 3 times in 10 years of NEET.

See the past questions: Coordination Compounds, Coordination Compounds (chapter)

In JEE Main

42 shifts analysed

80 questions from Coordination Compounds across the 42 JEE Main shifts Lumi has analysed (2017–2026).

See the past questions

The idea behind it

NCERT §5.2

  • The coordination number (CN) is the number of ligand donor atoms bonded directly to the metal: 6 for Pt in [PtCl₆]²⁻ and 4 for Ni in [Ni(NH₃)₄]²⁺.
  • A didentate ligand counts twice: Fe in [Fe(C₂O₄)₃]³⁻ and Co in [Co(en)₃]³⁺ both have CN = 6.
  • Only σ bonds from ligand to metal fix the coordination number; any π bonds between them are not counted.
  • The coordination sphere is the central atom with its ligands, written in square brackets. Ionisable groups outside the bracket are counter ions: in K₄[Fe(CN)₆] the sphere is [Fe(CN)₆]⁴⁻ and the counter ion is K⁺.
  • The coordination polyhedron is the spatial arrangement of the donor atoms around the central atom. The usual ones are octahedral ([Co(NH₃)₆]³⁺), square planar ([PtCl₄]²⁻) and tetrahedral ([Ni(CO)₄]).
  • The oxidation number of the central atom is the charge left on it if every ligand is taken away together with the electron pair it shares. It is written as a Roman numeral in brackets: Cu in [Cu(CN)₄]³⁻ is +1, Cu(I).
  • Homoleptic complexes carry one kind of donor group, e.g. [Co(NH₃)₆]³⁺; heteroleptic complexes carry more than one, e.g. [Co(NH₃)₄Cl₂]⁺.

Questions

5 answers

What does the "Count the ligands or the donor atoms?" simulation show?
A metal ion with ligands snapping on through their donor atoms: unidentate, didentate (en, ox) and hexadentate EDTA; CN counts donor atoms, oxidation state from charge balance.
What mistake does the "Count the ligands or the donor atoms?" simulation fix?
A common mistake it clears up: counting ligands instead of donor atoms.
Has NEET asked about this idea?
Yes. NEET has asked about this idea 3 times in 10 years of papers.
Does JEE Main ask questions from Coordination Compounds?
Yes. JEE Main asked 80 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
Is this simulation free?
Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Coordination number and oxidation number" in Coordination Compounds (Class 12 Chemistry).

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