Question 1

What is crystal field splitting?

Accepted Answer

Crystal field splitting is the breaking of the five degenerate d-orbitals of a free metal ion into different energy levels when ligands approach it. In an octahedral complex the d-orbitals split into a higher e_g set (d_z², d_x²−y²) and a lower t₂g set (d_xy, d_yz, d_zx), separated by an energy Δ₀.

Question 2

Why do e_g orbitals have higher energy than t₂g orbitals in an octahedral complex?

Accepted Answer

Because the six ligands approach along the x, y and z axes. The e_g orbitals (d_z² and d_x²−y²) have their lobes pointing directly at these approaching ligands, so electrons in them experience maximum repulsion and are raised in energy (+0.6 Δ₀). The t₂g orbitals point between the axes, feel less repulsion, and are lowered (−0.4 Δ₀).

Question 3

Why are tetrahedral complexes always high spin?

Accepted Answer

In a tetrahedral field the splitting is inverted and much smaller: Δ_t is only 4/9 of Δ₀. Because Δ_t is almost never larger than the pairing energy P, electrons prefer to occupy the higher set singly rather than pair up in the lower set. Hence tetrahedral complexes are essentially always high spin.

Question 4

What is the spectrochemical series?

Accepted Answer

It is the experimentally determined ordering of ligands by the crystal field splitting they produce, from weak-field to strong-field: I⁻ < Br⁻ < S²⁻ < SCN⁻ < Cl⁻ < NO₃⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < EDTA⁴⁻ < NH₃ < en < CN⁻ < CO. Strong-field ligands favour low-spin complexes; weak-field ligands favour high-spin ones.

Question 5

Why are coordination compounds coloured?

Accepted Answer

In the NCERT model, an electron absorbs a photon and undergoes a d–d transition whose energy equals the crystal field splitting Δ₀. A larger Δ₀ means shorter-wavelength light is absorbed, and the complementary colour is observed. Note that charge-transfer transitions can also give colour in coordination compounds, so d–d transitions are not the only cause.

Crystal Field Splitting: Geometry, Not Decree

Practice

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