Valence Bond Theory (Coordination) | Class 12 Chemistry

VBT for Coordination Compounds

Hybridization, Geometry & Magnetic Properties

1. Main Postulates of VBT

Applied to coordination compounds by Linus Pauling, VBT explains bonding as the overlap between empty hybrid orbitals of the metal ion and filled orbitals of ligands.

Key Steps:

1. Identify the oxidation state of the Central Metal Ion.
2. Write the electronic configuration ($d$-electrons).
3. Check Ligand Strength:
   Strong Field Ligand (SFL): Pairs up inner electrons (e.g., $CO, CN^-, NH_3, en$).
   Weak Field Ligand (WFL): No pairing (e.g., $F^-, Cl^-, H_2O$).
4. Hybridize empty orbitals to accommodate ligand electrons.

2. Hybridization & Geometry

Coordination No.	Hybridization	Geometry	Type
4	$sp^3$	Tetrahedral	-
4	$dsp^2$	Square Planar	Inner Orbital
6	$d^2sp^3$	Octahedral	Inner Orbital (Low Spin)
6	$sp^3d^2$	Octahedral	Outer Orbital (High Spin)

3. Magnetic Properties

Determined by the number of unpaired electrons ($n$).

• Paramagnetic: Has unpaired electrons ($n > 0$). Attracted by magnetic field.
• Diamagnetic: No unpaired electrons ($n = 0$). Repelled by magnetic field.

$$ \mu = \sqrt{n(n+2)} \text{ B.M.} $$

Where $\mu$ is the Spin-only Magnetic Moment in Bohr Magnetons (B.M.).

4. Important Examples

A. $[Co(NH_3)_6]^{3+}$ (Octahedral)

• Co Oxidation State: +3 ($3d^6$).
• Ligand: $NH_3$ is Strong Field. Pairs up $3d$ electrons.
• Two inner $3d$ orbitals become empty.
• Hybridization: $d^2sp^3$ (Inner Orbital Complex).
• Magnetic: Diamagnetic ($n=0$).

B. $[CoF_6]^{3-}$ (Octahedral)

• Co Oxidation State: +3 ($3d^6$).
• Ligand: $F^-$ is Weak Field. No pairing.
• Inner $3d$ occupied. Uses outer $4d$ orbitals.
• Hybridization: $sp^3d^2$ (Outer Orbital Complex).
• Magnetic: Paramagnetic (High Spin, $n=4$).

C. $[Ni(CN)_4]^{2-}$ vs $[NiCl_4]^{2-}$ (Coordination No. 4)

$[Ni(CN)_4]^{2-}$: $Ni^{2+} (3d^8)$. $CN^-$ causes pairing. One d-orbital vacates. $dsp^2$ (Square Planar, Diamagnetic).

$[NiCl_4]^{2-}$: $Ni^{2+} (3d^8)$. $Cl^-$ no pairing. Uses $4s, 4p$. $sp^3$ (Tetrahedral, Paramagnetic, $n=2$).

5. Limitations of VBT

• It involves a number of assumptions.
• It does not explain the color of coordination compounds.
• It does not give a quantitative interpretation of magnetic data or thermodynamic stability.
• It does not distinguish clearly between weak and strong ligands (this is better explained by Crystal Field Theory).

Practice Quiz

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