Nickel combines with a uninegative monodentate ligand (X−) to form a paramagnetic complex [NiX4]2−. The hybridisation involved and number of unpaired electrons present in the complex are respectively
sp3, two
For a four-coordinate nickel(II) complex, valence bond theory allows either dsp2 (square planar, electrons paired) or sp3 (tetrahedral, electrons unpaired) hybridisation; the magnetic behaviour decides which one applies.
Step 1 — Oxidation state of nickel in [NiX4]2−: \(x + 4(-1) = -2 \Rightarrow x = +2\), so \(\text{Ni}^{2+}: [\text{Ar}]\,3d^8\).
Step 2 — A d8 ion in a square planar dsp2 complex has all eight electrons paired in four 3d orbitals, making it diamagnetic. Since the complex is paramagnetic, pairing has not occurred.
Step 3 — The 3d orbitals therefore keep two unpaired electrons, and the empty 4s and three 4p orbitals hybridise to sp3, giving a tetrahedral complex with \(\mu = \sqrt{n(n+2)} = \sqrt{8} \approx 2.83\ \text{BM}\).
dsp2 with zero unpaired electrons describes a diamagnetic complex, contrary to the data; dsp2 with one unpaired electron and sp3 with one unpaired electron are impossible for a d8 ion, which has an even number of electrons and gives either zero or two unpaired electrons.
sp3, two option is correct.
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