CBSE · Class 12 · Chemistry · Coordination CompoundsDiscuss Valence Bond Theory (VBT) for coordination compounds with its main postulates and limitations. Also, explain the geometry and magnetic behavior of $Co(F6)^{3-}$ and $Co(NH3)6^{3-}$ complexes on the basis of VBT.

Valence Bond Theory (VBT) for Coordination Compounds

\nValence Bond Theory was proposed by Linus Pauling to explain the formation, magnetic properties, and geometrical structures of coordination compounds. The theory focuses on the overlap of atomic orbitals between the central metal atom/ion and the ligands.

Main Postulates of VBT

• Coordination Number and Orbitals: The central metal atom or ion makes available a number of empty orbitals equal to its coordination number for the formation of coordinate bonds with ligands.
• Hybridization: These empty orbitals (usually s, p, and d orbitals) undergo hybridization (mixing and recasting) to form a set of equivalent hybrid orbitals with definite directional geometry (e.g., $sp^3$ for tetrahedral, $d^2sp^3$ or $sp^3d^2$ for octahedral).
• Coordinate Bond Formation: Each ligand donates a pair of electrons into these empty hybrid orbitals of the central metal atom.
• Inner and Outer Orbital Complexes: Depending on whether the d-orbitals used in hybridization belong to the inner $(n-1)d$ shell or outer $nd$ shell, complexes are classified as inner orbital (low spin) or outer orbital (high spin) complexes.
• Magnetic Properties: The presence of unpaired electrons imparts paramagnetic character, while all paired electrons result in diamagnetic character.

Application to $[Co(F_6)]^{3-}$ and $[Co(NH_3)_6]^{3-}$

1. $[Co(F_6)]^{3-}$ Complex:

   • Oxidation state of Co is $+3$.
   • Electronic configuration of $Co^{3+}$ is $[Ar] 3d^6$.
   • $F^-$ is a weak field ligand and is unable to cause pairing of electrons in the $3d$ orbitals.
   • Hybridization involved is $sp^3d^2$ using outer $4s, 4p,$ and $4d$ orbitals.
   • Geometry: Octahedral.
   • Magnetic Behavior: Contains 4 unpaired electrons, hence it is paramagnetic and forms a high-spin complex.

2. $[Co(NH_3)_6]^{3-}$ Complex:

   • Oxidation state of Co is $+3$.
   • Electronic configuration of $Co^{3+}$ is $[Ar] 3d^6$.
   • $NH_3$ is a relatively strong field ligand and forces the electrons in $3d$ orbitals to pair up.
   • Hybridization involved is $d^2sp^3$ using inner $3d, 4s,$ and $4p$ orbitals.
   • Geometry: Octahedral.
   • Magnetic Behavior: Contains 0 unpaired electrons, hence it is diamagnetic and forms a low-spin complex.

Limitations of VBT

• It involves a number of assumptions and does not provide a quantitative explanation of the magnetic data.
• It does not explain the color exhibited by coordination compounds.
• It fails to give a quantitative interpretation of the thermodynamic or kinetic stabilities of coordination compounds.
• It does not distinguish between weak and strong field ligands properly without experimental backing.