Compounds in which transition metals make several anions or neutral molecules are called coordination compounds. Ex: Chlorofyll, haemoglobin, vitamin B-12 etc.
Werner’s theory for coordination compounds: In 1898, a swiss chemist propounded his theory of coordination compounds. The main points of this theory are as follows:
1. Metals show two types of linkages or valences- primary and secondary in coordination compounds.
2. The primary valences are normally ionisable and are satisfied by anions(negative ions).
3. The secondary valences are nonionisable.
4. The ions/groups bound by the secondary linkages to the metal have characteristic spatial arrangements corresponding to different coordination numbers.
Isomerism in coordination compounds: Compounds having same molecular formula but different structures are called and this phenomenon is called isomerism.
Coordination compounds show two types isomerism: 1. Structural isomerism and 2. Stereo isomerism
I.Structural isomerism : Isomerism due the difference in the structures of compounds is called structural isomerism. This isomerism is of four types:
2. Stereo or space isomerism : The isomerism due to the different relative positions of the ligands is known as stereo isomerism. This isomerism is of two types :
I. Geometrical isomerism : The isomerism in which isomers differ in the spatial distribution of atoms or groups about the central atom is known as geometrical isomerism. It is also known as cis-trans isomerism.