MOT: Heteronuclear Diatomic Molecules
Molecular Orbital Theory (MOT) explains the bonding in molecules where atoms are different (Heteronuclear). Unlike homonuclear molecules ($O_2, N_2$), where atomic orbitals (AOs) are at the same energy level, heteronuclear molecules have AOs at different energy levels due to differences in Electronegativity.
1. Key Principles for Heteronuclear Species
Energy Difference
The atomic orbitals of the more electronegative atom are lower in energy than those of the less electronegative atom.
- Bonding MOs: Are closer in energy to the atomic orbitals of the more electronegative atom (more "character" of the electronegative atom).
- Antibonding MOs: Are closer in energy to the atomic orbitals of the less electronegative atom.
2. Carbon Monoxide (CO)
Total electrons = $6 (C) + 8 (O) = 14$. It is isoelectronic with $N_2$.
Configuration: $\sigma_{1s}^2 \sigma^*_{1s}{}^2 \sigma_{2s}^2 \sigma^*_{2s}{}^2 \pi_{2p}^4 \sigma_{2p}^2$
Key Properties:
- Diamagnetic: No unpaired electrons.
- HOMO: The Highest Occupied Molecular Orbital is $\sigma_{2p}$ (largely carbon character), which makes CO a good $\sigma$-donor ligand in metal carbonyls.
3. Nitric Oxide (NO)
Total electrons = $7 (N) + 8 (O) = 15$. It is an odd-electron molecule.
Configuration: $\sigma_{1s}^2 \sigma^*_{1s}{}^2 \sigma_{2s}^2 \sigma^*_{2s}{}^2 \sigma_{2p}^2 \pi_{2p}^4 \pi^*_{2p}{}^1$
Magnetic Nature: Paramagnetic due to one unpaired electron in $\pi^*$ orbital.
Comparison: $NO, NO^+, NO^-$
| Species | Total Electrons | Bond Order | Magnetic Nature |
|---|---|---|---|
| $NO^+$ | 14 (Isoelectronic with $N_2$) | 3.0 | Diamagnetic |
| $NO$ | 15 | 2.5 | Paramagnetic |
| $NO^-$ | 16 (Isoelectronic with $O_2$) | 2.0 | Paramagnetic |
Stability Order: $NO^+ > NO > NO^-$
4. Hydrogen Fluoride (HF)
This illustrates bonding between atoms with very large energy differences.
- Hydrogen: $1s$ orbital.
- Fluorine: $2s, 2p$ orbitals (Very low energy).
The $H(1s)$ orbital overlaps effectively only with the $F(2p_z)$ orbital (assuming z-axis is internuclear) because they are closest in energy and symmetry.
1. One bonding $\sigma$ MO (Combination of $H_{1s} + F_{2p}$).
2. Non-bonding orbitals: The $F(2s)$ and $F(2p_x, 2p_y)$ orbitals remain essentially non-bonding (Lone pairs).
5. Bond Length vs Bond Order
Inverse Relationship
$$ \text{Bond Length} \propto \frac{1}{\text{Bond Order}} $$
For NO species:
Bond Order: $NO^+ (3) > NO (2.5) > NO^- (2)$
Bond Length: $NO^+ < NO < NO^-$
MOT Quiz
Test your concepts on Molecular Orbitals. 10 MCQs with explanations.
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