Bonding in Coordination Compounds
Werner’ s Theory Metals exhibit two types of valencies in the formation of complexes. These are primary valencies and secondary valencies.
1. Primary valencies correspond to oxidation number( ON) of the metal and are satisfied by anions. These are ionisable and non-directional.
2. Secondary valencies correspond to coordination number( CN) of the metal atom and are satisfied by ligands. These are non-ionisable and directional. Hence, geometry is decided by these valencies.
Valence Bond Theory( VBT)
This theory was proposed by L. Pauling in 1930 s. According to this theory, when a complex is formed, the metal ion / atom provides empty orbitals to the surrounding ligands. Coordination number shows the number of such empty orbitals, i. e., number of empty orbitals is equal to the coordination number. These empty orbitals hybridised before participation in bonding and the nature of hybridisation depends on the nature of metal and on the nature of approaching ligand.
Inner orbital complexes or outer orbital complexes
When outer d-orbital are used in bonding, the complexes are called outer orbital complexes. They are formed due to weak field ligands or high spin
ligands and hybridisation is sp 3 d 2. They have octahedral shape.
When d-orbitals of( n – 1) shell are used, these are known as inner orbital complex, they are formed due to strong field ligands or low spin ligands
and hybridisation is d 2 sp 3. They are also octahedral in shape. 1. 6 – ligands( unidentate), octahedral entity.
( i) Inner orbital complex [ Co( NH3) 6 ] 3 +