Coordination Compounds
AP EAMCET – Chemistry
(All content is tailored for quick, exam‑ready revision.)
1. Simple Explanation
Coordination compounds (also called complexes) are molecules that contain a central metal atom or ion surrounded by molecules or ions called ligands.
- The metal is the central atom and the ligands donate one or more pairs of electrons to form coordinate covalent bonds.
- The assembly is called a coordination sphere.
- Coordination compounds are responsible for many natural processes (heme in blood, chlorophyll in plants) and for industrial catalysts.
2. Key Points
| Topic | Detail |
|---|---|
| Ligands | 1. Monodentate – one donor atom (e.g., NH₃, H₂O). 2. Bidentate – two donor atoms (e.g., ethylenediamine). 3. Polydentate – more than two (e.g., EDTA). |
| Coordination Number | Number of donor atoms attached to the metal. Common values: 4 (tetrahedral), 5 (square pyramidal/ trigonal bipyramidal), 6 (octahedral). |
| Geometry | Determined by coordination number & steric factors. Octahedral, tetrahedral, square planar, linear, trigonal bipyramidal. |
| Nomenclature | 1. Ligand first (alphabetical, ignoring prefixes di–, tri–). 2. Metal second (in +2, +3, …, +6). 3. Use prefixes (mono‑, di‑, tri‑, tetra‑) only if the ligand name starts with a vowel or when it would otherwise be ambiguous. 4. For anionic ligands add -o (e.g., chloride → chlorido). |
| Isomerism | 1. Structural – linkage, coordination, ionisation, solvate. 2. Stereochemical – geometric (cis/trans, fac/mer) and optical (enantiomers). |
| Stability | Chelate Effect – polydentate ligands form more stable complexes than equivalent monodentate ligands. |
| Spectrophotometry | d–d transitions in transition‑metal complexes give characteristic colours; used for qualitative and quantitative analysis. |
| Common Applications | Pigments, catalysts (e.g., Wilkinson’s catalyst), medicine (cis‑platin), water‑hardening agents, analytical reagents. |
3. Worked Example
Problem
Name the complex:
[
[\text{Co}(\text{NH}_3)_5\text{Cl}]^{2+}
]
Step‑by‑Step Solution
| Step | What to do | Result |
|---|---|---|
| 1 | Identify the ligands – NH₃ (neutral, monodentate) and Cl⁻ (anionic). | Ligands: ammine (monodentate) and chlorido (anionic) |
| 2 | Arrange ligands alphabetically – ammine comes before chlorido. | ammine, chlorido |
| 3 | Add prefixes – 5 ammine → penta‑ammine, 1 chlorido → chlorido (no prefix needed). | penta‑ammine‑chlorido |
| 4 | Write metal name – cobalt, with oxidation state +3 (since the complex has a 2+ charge and Cl⁻ contributes –1). | cobalt(III) |
| 5 | Put together – penta‑ammine‑chlorido‑cobalt(III) |
Answer: penta-ammine-chlorido-cobalt(III)
Tip: Always check the overall charge to deduce the metal oxidation state.
4. Quick Revision Summary
- Coordination sphere = metal + ligands.
- Ligand types: monodentate, bidentate, polydentate.
- Coordination number → most common geometries (tetrahedral, octahedral, square planar).
- Naming rules: ligand first (alphabetical), metal second (oxidation state).
- Isomerism: structural (linkage, ionisation) & stereochemical (cis/trans, fac/mer, optical).
- Chelate effect → polydentate ligands give more stable complexes.
- Spectrophotometry – d–d transitions give colours; use in analysis.
- Applications – pigments, catalysts, drugs, water‑hardening agents, analytical reagents.
Remember: For exams, practice naming a variety of complexes and solving isomerism problems. Sketch the geometry to avoid confusion, especially for high coordination numbers.