CBSE Class 12 Chemistry, Coordination Compounds – 6, Crystal Field Theory

Summary

The crystal field theory, proposed by Hans, Beth, and Vanle, explains the bonding and properties of metal-ligand complexes. It assumes purely electrostatic interactions between metal ions and ligands. The theory describes how the approach of ligands leads to the splitting of metal ion d orbitals, creating different energy levels based on geometry (octahedral or tetrahedral). This splitting, influenced by ligand strength, determines electron configurations and the spin states of complexes. Strong field ligands cause low-spin configurations, while weak field ligands lead to high-spin complexes. The spectrochemical series ranks ligands by their field strength.

Takeaways

• 😀 The Crystal Field Theory (CFT) provides a more satisfactory explanation for bonding and properties of complexes compared to Valence Bond Theory.
• 😀 CFT assumes that the interaction between metal ions and ligands is purely electrostatic, with metal-ligand bonds being 100% ionic in nature.
• 😀 In CFT, ligands are treated as point charges (negative) or dipoles (neutral), and bonding may be ion-ion or ion-dipole interactions.
• 😀 In an isolated metal atom or ion, the 5d orbitals are degenerate, meaning they all have the same energy. This degeneracy is lost upon interaction with ligands.
• 😀 The presence of ligands causes the d orbitals to experience repulsion, lifting their degeneracy and splitting the orbitals into two groups with different energies.
• 😀 The splitting of the d orbitals in octahedral complexes depends on the number and arrangement of ligands around the central metal ion.
• 😀 In octahedral complexes, ligands approach the metal ion in a spherical manner, raising the energy of all d orbitals equally, known as the 'Barry center'.
• 😀 The d orbitals split into two sets: the higher-energy Eg set (dx²-y² and dz²) and the lower-energy T2g set (dxy, dxz, dyz), due to the repulsion from ligands.
• 😀 Crystal field splitting in octahedral complexes is represented by Δo, with the Eg orbitals having higher energy (+2/5 Δo) and the T2g orbitals having lower energy (-3/5 Δo).
• 😀 Ligands are categorized as weak or strong field ligands based on how much they cause crystal field splitting. This is reflected in the spectrochemical series.
• 😀 The arrangement of electrons in the metal's d orbitals in octahedral complexes depends on the crystal field splitting energy (Δo) and pairing energy. Weak field ligands lead to high-spin complexes, while strong field ligands result in low-spin complexes.

Q & A

What is the main idea behind Crystal Field Theory?

Crystal Field Theory (CFT) explains the bonding and properties of metal-ligand complexes by focusing on the electrostatic interactions between metal ions and ligands. It suggests that metal-ligand bonds are ionic in nature, with ligands acting as point charges or dipoles.

Who proposed the Crystal Field Theory?

The Crystal Field Theory was proposed by Hans, Beth, and Vanle.

What assumption does the Crystal Field Theory make about metal-ligand bonding?

The theory assumes that metal-ligand bonding is purely electrostatic and that these bonds are 100% ionic in nature.

How does the approach of ligands affect the energy of d orbitals in a metal ion?

As ligands approach the metal ion, their negative charge repels the electrons in the metal's d orbitals, which raises the energy of these orbitals and breaks their degeneracy, leading to a splitting of the d orbitals into two energy levels.

What is the significance of the 'Eg' and 'T2g' orbital sets in an octahedral complex?

In an octahedral complex, the 'Eg' set of orbitals (dx² - y², dz²) experience greater repulsion from the ligands and thus have higher energy. The 'T2g' set (dxy, dxz, dyz) experiences less repulsion and has lower energy.

What does the symbol delta_O represent in Crystal Field Theory?

Delta_O represents the energy difference between the two sets of split d orbitals (Eg and T2g) in an octahedral geometry. It is also referred to as octahedral crystal field splitting.

What are weak field and strong field ligands in the context of Crystal Field Theory?

Weak field ligands cause small crystal field splitting, while strong field ligands cause a large crystal field splitting. The strength of the ligand field influences the electron configuration and spin of the metal ion.

What is the spectrochemical series?

The spectrochemical series is a list of ligands ranked according to their field strength, from weak field ligands to strong field ligands. It is based on the absorption of light by complexes formed with different ligands.

How does the crystal field splitting affect electron configuration in octahedral complexes with d⁴ configuration?

In an octahedral complex with a d⁴ configuration, the fourth electron can either pair in the lower-energy T2g orbitals or occupy the higher-energy Eg orbitals. The choice depends on the relative magnitudes of the crystal field splitting energy (delta_O) and the pairing energy (B).

How does crystal field splitting differ between octahedral and tetrahedral complexes?

In octahedral complexes, the T2g orbitals have lower energy, while the Eg orbitals have higher energy. In tetrahedral complexes, this order is reversed, with T2g orbitals having higher energy than Eg orbitals. Additionally, tetrahedral splitting (delta_T) is smaller than octahedral splitting (delta_O).