Colour Magnetic Coordination

The trap: Students default to one spin state when asked about colour or magnetic behaviour of a complex — they forget that the ligand's field strength determines whether a d-orbital configuration is high-spin or low-spin, which directly decides the number of unpaired electrons (magnetic moment) and the magnitude of Δ₀ (colour).

Colour in coordination compounds arises because the d–d transition absorbs a specific wavelength from visible light; the complementary colour is what we observe. The energy gap Δ₀ between t₂g and e_g sets determines which wavelength is absorbed. A larger Δ₀ (strong-field ligand) shifts absorption toward violet/blue → complex appears yellow/orange. A smaller Δ₀ (weak-field ligand) shifts absorption toward red → complex appears green/blue (NCERT Class 12 Chemistry Chapter 5, page 23).

Magnetic properties depend on unpaired electrons. The spin-only magnetic moment is μ = √(n(n+2)) BM. The critical question: what is n? That depends on whether the complex is high-spin or low-spin — controlled entirely by the ligand field strength relative to pairing energy P.

The spectrochemical series decides everything: I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < CN⁻ < CO

• Strong-field (CN⁻, CO, NH₃/en for certain metals): Δ₀ > P → electrons pair in t₂g → low-spin → fewer unpaired electrons → lower μ.
• Weak-field (F⁻, Cl⁻, H₂O): Δ₀ < P → electrons spread across both sets → high-spin → more unpaired electrons → higher μ.

Watch-out: A d⁶ metal with CN⁻ has n = 0 (diamagnetic, low-spin), but the same d⁶ metal with H₂O has n = 4 (paramagnetic, high-spin). One ligand swap changes μ from 0 to 4.90 BM. NEET exploits this gap regularly.

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