Four coordinate nickel complexes can have another geometry other than tetrahedral. Name that geometry, and explain why the absorption spectrum you took strongly suggests a tetrahedral geometry, as opposed to the other possible option. Hint: Think about the allowedness of transitions in the two geometries.

The other geometry is square planar and this is because the four coordinate complexes forms d⁸ configuration.

Explanation:

(1) The other geometry is square planar

(2) This absorption spectrum (square planar) strongly suggests a tetrahedral geometry because  in the molecular geometry, a central atom is surrounded by constituent atoms, which form the corners of a square on the same plane.

This square planar is as a results of d⁸ electronic configuration and this is common for transition metal complexes with d⁸ configuration.