UPSC Combined Geo-Scientist, Mains · 2017 · Mains Paper I

Q1(c). Nickel magnetic moment

Question

The effective magnetic moment of [Ni(NH₃)₆]²⁺ is ~3.20 B.M. Is it higher or lower or equal to the μ_(spin-only) value? Provide an explanation.

The observed 3.20μ_B is higher than the spin-only value √8μ_B=2.83μ_B.

Ni(II) is d⁸. In an octahedral field its configuration is t_(2g)⁶e_g², with two unpaired electrons.
Hence μₛₒ=√(n(n+2))μ_B=√(2(4))μ_B=2.83μ_B.
The orbital-singlet ground state has largely quenched first-order orbital angular momentum, but spin–orbit coupling mixes excited states into it.

The full model answer

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