In a nickel-containing enzyme, various groups of atoms in the enzyme form a complex with the metal, which was found to have oxidation number +2 and to have no unpaired electrons. What is the most probable geometry of the Ni²⁺ complex:

(a) Octahedral;

(b) Tetrahedral;

(c) Square planar (see Exercise 9.13)? Justify your answer by drawing the orbital energy-level diagram of the ion.

Exercise 9.13

Suggest the form that the orbital energy-level diagram would take for a square planar complex with the ligands in the xy plane, and discuss how the building-up principle applies. The d_z²-orbital has a greater electron density in the xy-plane than the d_zx- or d_yz-orbitals but less than the d_xy-orbital.