21-07-2012, 11:58 AM
Bonding in Coordination Compounds
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Crystal Field Theory
Proposed by Brethe and Van Vleck later on Oegel as popularised it for Inorganic chemist.
Postulates of CFT
Complex is a combination of central ion surrounded by other ions or molecules with electrical dipoles
Ligands were considered as Point dipoles or Dipoles (Neutral ligands).
Covalent bonding was not considered but the bonding b/w central metal cation and ligand arise due to the electrostatic attraction b/w the nucleus of the metal cations.
The interaction b/w the e- of cations and ligands is repulsive. It is these repulsive forces that are responsible for splitting of d-orbitals of the metal cations.
The bond b/w metal and surrounding ligand is purely ionic.
Strong Field and Weak Field Ligands
The magnitude of ∆o varies from stronger to weaker ligands
Weak field ligands/ High Spin Complexes are those which exert a weaker field on the central metal cation (Lower splitting power) i.e having lower ∆o b/w eg and t2g hence the two sets can be consider as degenerate and distribution of e- will take place as per Hunds` Rule.
Strong fields ligands/ Low Spin complexes are those which exert a strong field on the central metal ion and hence (Higher splitting power)i.e having higher ∆o value b/w eg and t2g and hence do not obey Hund`s Rule.
Thus stronger ligands( CN-) give larger value of ∆o and weaker field ligands (F-) give smaller value of ∆o .
Applications of coordination compounds
Colour of transition metal complexes
Number of unpaired electrons and magnetic properties
Coordination compounds in living systems
Coordination compounds in medicine