Crystal Field Theory & Metal Carbonyls
Module 3 | CBSE Class 12 Chemistry | Coordination Compounds
1. Introduction to Crystal Field Theory (CFT)
Valence Bond Theory (VBT) had limitations; it couldn't explain the colour of complexes or definitively distinguish between weak and strong ligands. To overcome this, the Crystal Field Theory (CFT) was introduced.
- CFT considers the metal-ligand bond to be purely ionic arising from electrostatic interactions.
- Ligands are treated as point charges (in case of anions like Cl-) or point dipoles (in case of neutral molecules like NH3 or H2O).
- The five d-orbitals in an isolated gaseous metal atom/ion are degenerate (have the same energy).
- When ligands approach the metal ion to form a complex, this degeneracy is destroyed due to repulsion between ligand electrons and metal d-electrons. This results in splitting of d-orbitals.
2. Crystal Field Splitting in Octahedral Complexes
In an octahedral complex, the metal is at the centre of an octahedron, and six ligands approach along the three axes (x, y, and z axes).
- The d-orbitals which lie along the axes (dx2-y2 and dz2) experience greater repulsion from the approaching ligands. Their energy is raised. These are called the eg set of orbitals.
- The d-orbitals which lie between the axes (dxy, dyz, and dzx) experience lesser repulsion. Their energy is lowered relative to the average energy. These are called the t2g set of orbitals.
The energy of the two eg orbitals increases by (3/5)Δo (or +0.6 Δo), and the energy of the three t2g orbitals decreases by (2/5)Δo (or -0.4 Δo) relative to the barycentre (average energy level).
2.1 Strong Field vs Weak Field Ligands (The role of Pairing Energy, P)
When filling electrons into these split d-orbitals, the first three electrons go into the lower t2g orbitals singly (Hund's rule). For the fourth electron, two possibilities arise:
- It could enter the t2g level and pair up, requiring Pairing Energy (P).
- It could jump to the higher eg level, requiring Splitting Energy (Δo).
| Condition | Field Type | Electron Filling (4th e-) | Configuration | Spin Type |
|---|---|---|---|---|
| Δo < P | Weak Field Ligands | Jumps to eg orbital. | t2g3 eg1 | High Spin Complex |
| Δo > P | Strong Field Ligands | Pairs up in t2g orbital. | t2g4 eg0 | Low Spin Complex |
3. Crystal Field Splitting in Tetrahedral Complexes
In tetrahedral complexes, four ligands approach the central metal ion. None of the d-orbitals point directly exactly towards the ligands, but the orbitals lying between the axes (dxy, dyz, dzx) are closer to the approaching ligands than those along the axes.
1. The energy of the t2 set (dxy, dyz, dzx) is raised.
2. The energy of the e set (dx2-y2, dz2) is lowered.
(Note: the 'g' subscript is dropped in tetrahedral complexes because they lack a center of symmetry/inversion).
Since the splitting energy in a tetrahedral field (Δt) is always considerably less than the pairing energy (P), tetrahedral complexes are almost always high spin complexes.
4. The Spectrochemical Series
The crystal field splitting depends upon the field produced by the ligand. Ligands can be arranged in a series in the order of increasing field strength. This is an experimentally determined series based on the absorption of light by complexes.
Oxygen donors (H2O, ox) are intermediate.
Nitrogen and Carbon donors (NH3, en, CN-, CO) are strong field ligands, producing large Δo and forming low spin complexes.
5. Colour in Coordination Compounds
The colour of coordination compounds is readily explained by CFT using the concept of d-d transition.
When white light passes through a complex, the metal ion absorbs light of a specific frequency from the visible region. This energy is used to excite an electron from the lower energy d-orbital (e.g., t2g) to a higher energy d-orbital (e.g., eg).
NCERT Example: The complex [Ti(H2O)6]3+ absorbs light in the yellow-green region. Thus, the transmitted light appears purple (violet).
Conditions for Colour:
If there are no ligands (e.g., anhydrous CuSO4), crystal field splitting does not occur, and the substance is colourless. Hydrated CuSO4·5H2O is blue because water acts as a ligand causing splitting.
6. Bonding in Metal Carbonyls (Synergic Bonding)
Homoleptic carbonyls (compounds containing only carbonyl ligands, CO) are formed by most transition metals. E.g., [Ni(CO)4], [Fe(CO)5].
The metal-carbon bond in metal carbonyls possesses both σ (sigma) and π (pi) character. This is known as Synergic Bonding, which drastically strengthens the bond.
- σ bond formation: It is formed by the donation of a lone pair of electrons from the carbonyl carbon into a vacant orbital of the metal.
- π bond formation (Back-bonding): It is formed by the donation of a pair of electrons from a filled d-orbital of the metal into the vacant antibonding π* orbital of carbon monoxide.
This "give and take" mechanism creates a synergic effect, making metal carbonyl complexes highly stable.
7. Importance and Applications of Coordination Compounds
Coordination compounds are widely present in nature and have massive industrial and medical importance.
- Qualitative and Quantitative Analysis: Formation of coloured complexes is used to detect ions. (e.g., Ni2+ is detected using DMG).
- Hardness of Water: Hardness is estimated by simple titration with Na2EDTA. The Ca2+ and Mg2+ ions form stable complexes with EDTA.
- Metallurgy: Extraction of silver and gold uses the MacArthur-Forrest cyanide process. Gold combines with cyanide to form [Au(CN)2]-.
- Biological Systems:
- Chlorophyll is a coordination compound of Magnesium.
- Haemoglobin is a coordination compound of Iron.
- Vitamin B12 (cyanocobalamin) is a coordination compound of Cobalt.
- Medicine:
- Cis-platin [Pt(NH3)2Cl2] is highly effective in the treatment of cancer.
- Excess of copper and iron are removed by chelating ligands D-penicillamine and desferrioxamine B.
- EDTA is used in the treatment of lead poisoning.
8. Previous Year Questions (PYQs) & Exhaustive Question Bank
Part A: Conceptual (1-2 Marks)
Q1. What is the spectrochemical series? Explain the difference between a weak field ligand and a strong field ligand.
A strong field ligand produces a large crystal field splitting energy (Δo > P), forcing electrons to pair up in the inner t2g orbitals, forming low-spin complexes. A weak field ligand produces a small splitting energy (Δo < P), allowing electrons to enter the higher eg orbitals without pairing, forming high-spin complexes.
Q2. Why are tetrahedral complexes generally high spin?
Q3. Aqueous copper sulphate solution (blue in colour) gives a green precipitate with aqueous potassium fluoride. Explain.
Part B: Synergic Bonding & Applications (2-3 Marks)
Q4. Explain synergic bonding in metal carbonyls with a diagrammatic description.
1. σ Bond: The carbonyl (CO) molecule donates a lone pair of electrons from its carbon atom into an empty d-orbital of the metal.
2. π Bond (Back-bonding): Simultaneously, the metal atom donates electrons from its filled d-orbital back into the empty antibonding π* molecular orbital of the CO molecule.
This "give and take" mechanism strengthens the bond between the metal and the ligand, a phenomenon known as the synergic effect.
Q5. State the role of coordination compounds in:
(i) Biological systems.
(ii) Medicinal chemistry.
(i) Biological systems: Coordination compounds are vital for life. Chlorophyll, necessary for photosynthesis, is a complex of Mg. Haemoglobin, the oxygen carrier in blood, is a complex of Fe. Vitamin B12, essential to prevent pernicious anaemia, is a complex of Co.
(ii) Medicinal chemistry: The complex cis-platin, [Pt(NH3)2Cl2], is used as an anti-tumour agent to treat cancer. Chelating agents like EDTA are used to treat lead poisoning by forming soluble, excreteable complexes with the toxic metal.
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