Coordination compounds

Coordination Compounds


Coordination compounds:

These are the complex compounds in which transition metal atoms are bound to a number
of anions or neutral molecules.

Postulates of Werner’s theory of coordination compounds:

 In coordination compounds, there are two types of linkages (valences) – primary and
secondary.

 The primary valences are ionisable and are satisfied by negative ions.

 The secondary valences are non-ionisable and are satisfied by negative ions or neutral
molecules. The secondary valence is equal to the coordination number of a metal and
remains fixed for a metal.

 Different coordination numbers have characteristic spatial arrangement of the ions or
groups bound by the secondary linkages.

Difference between a double salt and a complex:

 In water, a double salt dissociates completely to give simpler ions. Examples of double salt:
carnallite (KCl. MgCl2. 6H2O), Mohr’s salt [FeSO4. (NH4)2 SO4. 6H2O]

 Complex ions do not dissociate further to give simpler ions. For example, [Fe(CN)6]4–,
[Fe(C2O4)3]3–

Ligands:
Ions or molecules bound to the central metal atom or ion in the coordination entity
 Didentate –




 Polydentate –

,  Ambidentate –

(Can bind through two different atoms)




Coordination number: Number of ligand donor atoms bonded directly to the metal
Coordination polyhedral:




Homoleptic and heteroleptic complexes:

 Homoleptic complexes: In these complexes, the metal is bound to only one kind of donor
group, e.g., [Co(NH3)6]3+

 Heteroleptic complexes: In these complexes, the metal is bound to more than one kind of
donor groups, e.g., [Co(NH3)4Cl2]+

Naming of mononuclear coordination compounds:

 The cation is named first in both positively and negatively charged coordination entities.