Question

Explain the square planar shape of the tetracyanidonickelate (II) ion. 

Answer 1

The complex [Ni(CN)₄]²⁻ exhibits a square planar geometry and is diamagnetic in nature. In this complex, nickel is in the +2 oxidation state, with an electronic configuration of 3d⁸. The cyanide ion (CN⁻) is a strong field ligand, which induces pairing of electrons in the 3d orbitals of the nickel ion. As a result, all eight electrons in the 3d orbitals become paired. This electron pairing frees up one 3d orbital, which combines with one 4s and two 4p orbitals to undergo dsp² hybridisation. This hybridisation leads to the formation of four equivalent hybrid orbitals that arrange themselves in a square planar geometry, accommodating lone pairs from the four cyanide ligands. This geometry is characteristic of low-spin d⁸ complexes with strong field ligands. Since all the electrons are paired after hybridisation, there are no unpaired electrons, and therefore, the complex is diamagnetic, meaning it is not attracted to a magnetic field.