Bonding In Coordination Complexes: Valence Bond Theory (VBT) Notes | Study Inorganic Chemistry - Chemistry

BONDING IN COORDINATION COMPLEXES

There are three theories proposed to explain the nature of bonding in coordination complexes.

• Valence Bond Theory (VBT)
• Crystal Field Theory (CFT)
• Ligand Field Theory (LFT) or Molecular Orbital Theory (MOT)

 Valence Bond Theory

This theory is mainly based on following assumptions:

• The central metal cation or atom makes available a no. of vacant s, p and or d orbitals equal to its coordination no. to form coordinate bonds with ligands.
• These vacant metal orbitals hybridised to form new set of equivalent hybrid orbitals. These hybrid orbitals have same geometry and energy.
• The ligands having lone pair of electrons overlaps with these hybrid orbitals to form a coordinate covalent bond.
  
   
• The hybridisation and geometry of the complex is dependent on the coordination no. and the no. of unpaired electrons (magnetic moment) of the metal ion or atom.
• Ligands are of two types:
  a. Strong field ligands or simply strong ligands. These are either carbon donor or nitroge n donor. For example, CO, CN^– etc.
  b. Weak field ligands or simply weak ligands. These are halogen (F^–, Cl^– etc.) and majorit y of oxygen donor ligands.
• Strong ligands have t he tendenc y to pair up d-electrons of metal atom or ion to provide necessary orbitals for hybridisation, if required. On the other hand, weak ligands do not have tendency to pair up d electrons.
• In octahedral complexes, the central metal cation is eit her d²sp³ or sp³d² hybridised. The complexes having d²sp³ hybridisation are called inner orbital complexes as they involve the inner d orbital (n-1) d orbital for hybridisation.
• The complexes having sp³d² orbitals are called outer orbital complexes as they involved and orbitals for hybridisation.
• In case of second and third series transit ion metal complexes, pairing takes place irrespective of the nature of ligands due to diffused nature of 4d and 5d orbitals. Hence, these transition metals only form inner orbital complexes.
• The complexes having one or more unpaired electrons are paramagnetic and the complexes which do not have any unpaired electrons are called diamagnetic complexes.

Here is the table which consists of geometries which are found in coordination complexes.

Hybridization schemes for the σ-bonding frameworks of different geometrical configuration of ligand donor atoms

Various higher level geometries are given below