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 +