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Molecular Orbital Theory and Transition Metal Complex Analysis
This article provides detailed explanations on molecular orbital interactions, π-bonding d-orbitals, the 18-electron rule, ferromagnetism, and magnetic moments of metal complexes.
1. M–dπ → π*(NO) Interaction
A transition metal with an empty σ-orbital and two π-d orbitals (containing 3 electrons) interacts with nitrogen monoxide (NO), which has 1 electron in its π* orbital.
- Electrons involved: 3 from the metal d-orbitals + 1 from NO
- Forms a filled bonding interaction
- Increases stability of the metal–NO complex through back-donation
2. Metal d-Orbitals in π-Bonding
π-bonding requires side-on overlap between ligand π orbitals and metal d-orbitals. The orbitals most commonly involved are:
- dxz
- dyz
3. Applying the 18-Electron Rule
Using the electron contributions from ligands and metals, we identify the central metals and predict geometry:
- [M(NO)4(CO)2]+: NO = 3e⁻ × 4, CO = 2e⁻ × 2 → 16e⁻; Metal = Sc+; Geometry = Octahedral
- M(NO)2(CO)2: Total 10e⁻ from ligands → Metal = Fe (0); Geometry = Tetrahedral/Square Planar
- [M(CO)6]+: CO = 12e⁻; Metal = Mn+; Geometry = Octahedral
4. What is Ferromagnetism?
Ferromagnetism is the phenomenon where atomic magnetic moments align parallel to each other, even without an external field. This creates a permanent magnetic effect, seen in elements like Fe, Co, and Ni.
5. Magnetic Moment and Term Symbols
MnCl₆⁴⁻: Mn²⁺ → d⁵ → high spin
- Unpaired electrons = 5
- μs = √[5(5+2)] = √35 ≈ 5.92 BM
- Term Symbol = ⁶S
Cr₂O₃: Cr³⁺ → d³
- Unpaired electrons = 3
- μs = √[3(3+2)] = √15 ≈ 3.87 BM
- Term Symbol = ⁴F
Fe₂O₃: Fe³⁺ → d⁵
- Unpaired electrons = 5
- μs = √[5(5+2)] = √35 ≈ 5.92 BM
- Term Symbol = ⁶S