The complex ion, [CoCl4]2-, has a coordination number equal to 4. The structure of [CoCl4]2- might be:
tetrahedral - ligands attached at the corners of a tetrahedron
square planar - ligands attached at the corners of a square
Tetrahedral geometries predominate. However, for transition metal ions with nd8 electron configuration such as Ni2+, Pd2+, Pt2+, Rh2+, Ir+, Au3+, and for the 3d9 electron configuration Cu2+ the square planar configuration dominates. This is due to the requirement of involving only one of the d-orbitals, dx2-y2, which has four lobes pointing to all four ligands. The [CoCl4]2- complex ion is experimentally known to have a tetrahedral structure.
[CoCl4]2- tetrahedral CN = 4
[Ni(CN)4]2- square planar CN = 4
Linear and octahedral are the most common structures for coordination numbers 2 and 6, respectively.
[Ag(NH3)2]+ linear CN = 2
[PtCl6]2- octahedral CN = 6