Coordination Compounds

Chemistry · Class 12

Simulation · Chemistry · Class 12

Pair up or climb the gap?

"Pair up or climb the gap?" is a free, interactive Class 12 Chemistry simulation for NEET and JEE, built on the lesson "Crystal field theory" in Coordination Compounds. Six ligands in an octahedron split the d orbitals into a lower t₂g set of three and an upper eg set of two. Choose the number of d electrons and the kind of ligand. A weak field ligand leaves Δo smaller than the pairing energy P; a strong field ligand makes Δo larger. Δo < P gives high spin (weak field); Δo > P gives low spin (strong field). The field decides n only for d⁴ to d⁷: [FeF₆]³⁻ has 5 unpaired electrons and [Fe(CN)₆]³⁻ has 1, because CN⁻ sits near the strong end of the spectrochemical series. JEE Main asked 80 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).

From the lesson Crystal field theory in Coordination Compounds. Change the values and watch what happens.

Pair up or climb the gap?Chemistry · Class 12

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 §5.5.4

  • CFT treats the metal-ligand bond as purely ionic: anionic ligands are point charges and neutral ligands are point dipoles, and they interact with the metal only electrostatically.
  • In a free metal ion, or in a spherical field of charge, the five d orbitals have equal energy (degenerate). An asymmetric field from real ligands lifts this degeneracy: crystal field splitting.
  • Octahedral field: dx²−y² and dz² point at the ligands, feel more repulsion and rise as the eg pair; dxy, dyz and dxz point between the axes and fall as the t₂g set. The gap is Δo.
  • Relative to the average energy, the two eg orbitals go up by (3/5)Δo and the three t₂g orbitals go down by (2/5)Δo.
  • Δo grows with the field of the ligand and with the charge on the metal ion. The spectrochemical series, found by experiment from absorption spectra, orders ligands by field strength: I⁻ < Br⁻ < SCN⁻ < Cl⁻ < S²⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < edta⁴⁻ < NH₃ < en < CN⁻ < CO.
  • d¹ to d³ ions put their electrons singly into t₂g (Hund's rule). For d⁴ the fourth electron has a choice, set by Δo against the pairing energy P.
  • If Δo < P the fourth electron goes into eg, giving t₂g³ eg¹: weak field ligands, high spin complexes. If Δo > P it pairs in t₂g, giving t₂g⁴ eg⁰: strong field ligands, low spin complexes.
  • Calculations show that complexes of d⁴ to d⁷ ions are more stable in a strong field than in a weak one.
  • Tetrahedral field: the splitting is inverted and smaller, Δt = (4/9)Δo for the same metal, ligands and distances. It is rarely large enough to force pairing, so tetrahedral complexes are almost always high spin.
  • The g subscript (t₂g, eg) is used only for complexes with a centre of symmetry, such as octahedral and square planar ones; tetrahedral levels are written without it.

Questions

3 answers

What does the "Pair up or climb the gap?" simulation show?
Six ligands in an octahedron split the d orbitals into a lower t₂g set of three and an upper eg set of two. Choose the number of d electrons and the kind of ligand. A weak field ligand leaves Δo smaller than the pairing energy P; a strong field ligand makes Δo larger.
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 "Crystal field theory" in Coordination Compounds (Class 12 Chemistry).

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