Question

Explain valence bond theory in coordination compounds.

Hint:

Always determine the metal oxidation state, then electronic configuration, then see whether pairing of d-electrons occurs.
This predicts the hybridisation and geometry of the coordination compound.

Answer 1

Step 1: Basic idea.
According to Valence Bond Theory (VBT), the central metal ion in a coordination compound makes available a certain number of vacant orbitals (hybrid orbitals) for the formation of coordinate covalent bonds with ligands.
The ligands donate lone pairs of electrons into these vacant orbitals.

Step 2: Hybridisation concept.
- The type of hybridisation depends on the coordination number of the complex.
- For example:
- Coordination number 6 $\Rightarrow$ octahedral complexes $\Rightarrow$ \(d^2sp^3\) (inner orbital) or \(sp^3d^2\) (outer orbital).
- Coordination number 4 $\Rightarrow$ tetrahedral complexes $\Rightarrow$ \(sp^3\).
- Coordination number 4 $\Rightarrow$ square planar complexes $\Rightarrow$ \(dsp^2\).

Step 3: Example.
\([Co(NH_3)_6]^{3+}\): Cobalt in +3 oxidation state has 3d$^6$.
Pairing occurs to give empty \(d\)-orbitals.
Hybridisation is \(d^2sp^3\), leading to an octahedral geometry.
\[ \boxed{\text{VBT explains geometry, magnetic properties and bonding in coordination complexes.}} \]