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Compounds Werner Theory
The early study of inorganic compounds merely dealt with the attempts to explain the existence and structure of
hydrated double salts like Mohrs salt, carnalite etc. though, the early theories developed during the era could
provide a satisfactory explanation of the formation and properties of mixed salts yet these molecules with ions t
form complex compounds. The chemists wondered about how the atoms in these complexes were connected to
each other and the possible effect of the arrangement on the properties of the complexes.
The first successful theory explained the properties of co-ordination compounds in terms of their structure was
proposed by Alfred Werner around 1893, long before sophisticated techniques were developed and the concept
of electronic structure were applied to chemical system. in recognition of the work, Werner was awarded nobel
prize in 1913. The brief outlines of his theory are:
(i) Metals have two types of linkages (valencies) i.e. primary and secondary linkages. The primary linkages are
satisfied by the negative ions whereas the secondary linkages may be satisfied either by the negative ions or by
neutral molecules. The primary valencies are insoluble valencies whereas the secondary valencies are non-
ionisable valencies.
According to the modern concept the primary valencies usually indicate the oxidation number of the metal ion
whereas the secondary valencies refer to the co-ordination number.
For example, in the compound CoCl2 the primary valency of Co is 3. When addition of ammonia is done toCoCl3,
yields a complex compound CoCl3.6NH3. In this case the primary valencies are satisfied by three chloride ions
whereas the secondary valencies are satisfied are satisfied by ammonia molecules. Thus, every metal atom has a
tendency to satisfy as well as secondary valencies.
(ii) Each metal atom can have a characteristic number of secondary valencies about the central atom. Thesevalencies are projected in space about the central atom in such a manner that these assign a definite geometry t
the co-ordination compound.
For example, in CoCl3.6NH3, the six secondary valencies are arranged in an octahedral fashion around the centra
metal ion. On the other hand, if there are four secondary valencies as in the case of CoCl2.4NH3then these shall
have either a tetrahedral or a square planar arrangement.
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(iii) Not only the neutral molecules are co-ordinated to the central atoms but ions may also be co-ordinated in
same fashion. When an ion replaces a neutral molecule in the coordination sphere, it loses its electrovalent
character and gets attached to the central ion by non-ionisable linkage.
For example, the co-ordination compounds [Co(NH3)5Br]2+SO42- and [Co(NH3)5SO4]+Br- have the same formula. In
the former, as the SO42- ion is present outside the co-ordination sphere it gives a white precipitate with BaCl2.
When this ion replaces the Br ion in the co-ordination sphere it loses its ionic character i.e. no precipitates are
obtained with BaCl2.
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