Square planar complex

Worked square planar example: [Ni(CN)4]2-. Nickel's oxidation state here is again +2, so the free-ion valence configuration is again 3d8. Four CN- ligands (a STRONG-field ligand, high in the spectrochemical series) again require four vacant metal orbitals, but because the resulting complex is square planar rather than tetrahedral, nickel uses dsp2 hybrid orbitals instead of sp3 -- specifically one 3d orbital, one 4s orbital, and two of the three 4p orbitals. To free up that one needed 3d orbital, the eight 3d electrons must first PAIR UP completely before hybridisation, filling four of the five 3d orbitals fully (paired) and leaving exactly one 3d orbital genuinely vacant for the dsp2 mix. The four resulting dsp2 hybrid orbitals point toward the four corners of a square (not a tetrahedron), giving [Ni(CN)4]2- its square planar geometry, and each overlaps with one CN- ligand's donor orbital to form the four Ni-CN coordinate bonds. Because every one of nickel's eight d electrons ends up paired in this arrangement, [Ni(CN)4]2- has no unpaired electrons at all and is diamagnetic -- a direct contrast with the paramagnetic, tetrahedral [NiCl4]2- of the previous section, even though both start from the identical Ni2+ (3d8) free-ion …