MP Board · Class 12 · Chemistry · Coordination CompoundsDiscuss Valence Bond Theory (VBT) for coordination compounds. Explain the formation of $Co(NH3)6^{3+}$ and $CoF6^{3-}$ complexes on the basis of VBT, indicating their hybridization, geometry, and magnetic properties.

Introduction to Valence Bond Theory (VBT)\nValence Bond Theory, proposed by Linus Pauling, explains the bonding in coordination compounds by focusing on the overlap of atomic orbitals between the central metal atom/ion and the ligands. The fundamental postulates and key aspects of this theory are detailed below:

• Coordination Number and Orbitals: The central metal 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 undergo hybridization (mixing of atomic orbitals of comparable energies) to yield a set of equivalent hybrid orbitals of definite geometry (such as octahedral, tetrahedral, or square planar).
• Overlap: Each ligand donates a pair of electrons into these hybridized empty orbitals of the central metal atom or ion.
• Magnetic Behavior: If all electrons in the metal ion are paired, the complex is diamagnetic. If unpaired electrons are present, the complex is paramagnetic.

Explanation of $[Co(NH_3)_6]^{3+}$ Complex

1. Oxidation State of Cobalt: Cobalt has an atomic number of $27$. Its electronic configuration is $[Ar] 3d^7 4s^2$. In $[Co(NH_3)_6]^{3+}$, cobalt is in the $+3$ oxidation state, resulting in a $3d^6$ configuration.
2. Electron Pairing: $NH_3$ is a strong field ligand. Under its influence, the electrons in the $3d$ orbitals are forced to pair up, leaving two empty $3d$ orbitals.
3. Hybridization: The available empty orbitals (two $3d$, one $4s$, and three $4p$) undergo $d^2sp^3$ hybridization to form six equivalent hybrid orbitals.
4. Geometry and Magnetism: These six hybrid orbitals accept electron pairs from six $NH_3$ molecules, resulting in an octahedral geometry. Since there are no unpaired electrons, the complex is diamagnetic and is referred to as an inner orbital or low spin complex.

Explanation of $[CoF_6]^{3-}$ Complex

1. Oxidation State and Configuration: Here also, cobalt is in the $+3$ oxidation state, with a $3d^6$ electronic configuration.
2. Weak Field Ligand Effect: Fluoride ($F^-$) is a weak field ligand and does not cause the pairing of $3d$ electrons.
3. Hybridization: To accommodate six pairs of electrons from $F^-$ ions, the outer $4d$ orbitals are used. The hybridization involved is $sp^3d^2$.
4. Geometry and Magnetism: The complex possesses an octahedral geometry. Because there are four unpaired electrons in the $3d$ level, the complex is strongly paramagnetic and is known as an outer orbital or high spin complex.