Coordination compounds have magnetic and optical properties. They also show colour. To explain these properties shown by coordination compounds H Bethe presented his Crystal Field Theory in 1929.
The main points of this theory are as follows:
1. The nature of bonding in the complexes is electrostatic i.e. the metal ligand bond is supposed to be ionic.
2.Anionic ligands are supposed negative points or point charges & neutral ligands are taken as point dipoles.
3.CFT is based on the concept of splitting of d-orbitals.
Partition of d-orbitals: The five d-orbitals are divided into two sets: t2g (dxy, dyz and dxz) and eg(dx2-y2 and dz2)
I.The crystal field splitting in octahedral coordination entities: In an octahedral coordination entity, there is repulsion between the electrons of d-orbitals of the central atom or ion and the electrons of the ligands.
The repulsion is more when d-orbitals are directed towards ligands, than when they are away from ligands. Since, eg orbitals which are along axes i.e. in the direction of ligands face more repulsion which finally leads the raising of energy of orbitals and the energy of t2g orbitals is lowered. Due to this, d-orbitals are splitted as follows:
The splitting of degenerate orbitals due to the presence of ligands is called crystal field splitting.
Crystal field splitting energy or crystal field stablising energy(CFSE): The energy required for the separation of d-orbitals is called CFSE. It is denoted by ∆o.
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.