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Nickel (atomic number 28) forms a paramagnetic complex ion [NiX4]2− with a singly charged, monodentate ligand X−. The number of unpaired electrons on nickel and the shape of this complex are, respectively:

1. one, square planar
2. two, square planar
3. one, tetrahedral
4. two, tetrahedral

Correct answer: two, tetrahedral

Solution

Nickel in the +2 oxidation state has the electron configuration of [Ar] 3d8, which results in two unpaired electrons in its d-orbitals. The tetrahedral geometry of the complex arises from the arrangement of the monodentate ligands around the central nickel ion, minimizing electron pair repulsion.

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