In modern formulations, such spatial arrangements are called
coordination polyhedra The species within the square bracket are
coordination entities or complexes and the ions outside the square
bracket are called counter ions He further postulated that octahedral, tetrahedral and square planar
geometrical shapes are more common in coordination compounds of
transition metals Thus, [Co(NH3)6]
3+, [CoCl(NH3)5]
2+ and [CoCl2(NH3)4]
+
are octahedral entities, while [Ni(CO)4] and [PtCl4]
2– are tetrahedral and
square planar, respectively