Postulates are:
(i) In coordination compounds metals show two types of linkages (valencies) - primary and secondary.

(ii) The primary valencies are normally ionisable and are satisfied by negative ions.

(iii) The secondary valencies are non-ionisable. These are satisfied by neutral molecules or negative ions. The secondary valence is equal to the coordination number and is fixed for a neutral.

(iv)The ions/groups bound by the secondary linkages to the metal have characteristic spatial arrangements corresponding to different numbers.
In the modern formulations, such spatial arrangements are called coordination polyhedra. The species with the square brackets 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.