Which of the following has a square planar geometry? (a) [PtCl4]2– (b) [CoCl4]2– (c) [FeCl4]2– (d) [NiCl4]2– (At. nos.: Fe = 26, Co = 27, Ni = 28, Pt = 78) ?

To determine which of the given complexes has a square planar geometry, we need to consider the coordination number and the electronic configuration of the central metal ions in each complex. Let's break down each option one by one.

Understanding Coordination Complexes

Coordination complexes consist of a central metal atom or ion surrounded by molecules or ions called ligands. The geometry of these complexes is influenced by factors such as the oxidation state of the metal, the type of ligands, and the number of ligands attached to the metal.

Analyzing Each Complex

• [PtCl4]²–: Platinum (Pt) in this complex is in the +2 oxidation state. Platinum typically exhibits a square planar geometry, especially in its +2 oxidation state due to its d8 electron configuration. The four chloride ligands arrange themselves in a square planar fashion around the platinum ion.
• [CoCl4]²–: Cobalt (Co) here is in the +2 oxidation state. Cobalt in this state usually adopts a tetrahedral geometry when coordinated with four ligands. Therefore, this complex does not have a square planar geometry.
• [FeCl4]²–: Iron (Fe) is also in the +2 oxidation state in this complex. Similar to cobalt, iron tends to form tetrahedral complexes with four ligands. Hence, this complex does not exhibit square planar geometry either.
• [NiCl4]²–: Nickel (Ni) in this case is in the +2 oxidation state as well. Nickel typically forms tetrahedral complexes when coordinated with four ligands, so this complex is not square planar.

Conclusion on Geometry

From the analysis above, the only complex that exhibits a square planar geometry is [PtCl4]²–. This is primarily due to the d8 electron configuration of platinum in the +2 oxidation state, which favors a square planar arrangement to minimize electron pair repulsion and achieve stability.

Summary

In summary, when considering the geometry of coordination complexes, it is essential to look at the oxidation state and electron configuration of the central metal ion. In this case, [PtCl4]²– is the only complex with a square planar geometry among the options provided.