Q4(d). Diamagnetic [Ni(CN)₄]²⁻ and paramagnetic [NiCl₄]²⁻

Valence bond picture

1. [Ni(CN)₄]²⁻: the strong-field CN⁻ ligands force the two unpaired 3d electrons to pair, leaving one 3d orbital empty. One 3d + 4s + two 4p give dsp² hybrids, pointing to the corners of a square: [Ni(CN)₄]²⁻: square planar, diamagnetic
2. [NiCl₄]²⁻: Cl⁻ is a weak-field ligand; the 3d arrangement of Ni²⁺ (two unpaired) is kept and the outer 4s and 4p are used: sp³, tetrahedral: [NiCl₄]²⁻: tetrahedral, two unpaired electrons

Crystal-field picture (the reason)

1. Square planar: removing the two ligands on z leaves d_(x²-y²) (pointing at the four ligands) far above the others. For d⁸ with a strong-field ligand the gap is larger than the pairing energy: all eight electrons pair in the four lower orbitals, d_(x²-y²) stays empty. The empty orbital also gains extra stabilisation, so d⁸ with strong fields favours square planar geometry (as in [PtCl₄]²⁻).
2. Tetrahedral: Δₜ≈49Δₒ is small, and Cl⁻ is weak, so no pairing: e⁴t₂⁴ with two unpaired electrons in t₂. Four large Cl⁻ ions are also less crowded at 109.5° than at 90°.
3. Magnetic moment: μₛₒ=√(n(n+2))=√(2×4), μ=2.83 BM; the measured value is somewhat higher because the t₂ set has orbital angular momentum.
4. Colour supports the structures: square planar [Ni(CN)₄]²⁻ is yellow; tetrahedral [NiCl₄]²⁻ is blue, with more intense bands (no centre of symmetry).

Application. the square-planar, diamagnetic red complex of Ni(II) with dimethylglyoxime, [Ni(dmgH)₂], is the standard gravimetric precipitate for nickel.