Coordination Compounds – Study Notes (PSU‑GATE Chemistry)
1. Simple Explanation
A coordination compound (or complex) consists of a central metal ion (usually a transition metal) surrounded by ligands (donor molecules or ions) that are bound through coordinate covalent bonds.
The metal–ligand assembly is called a coordination complex.
Key aspects:
| Concept | What it means |
|---|---|
| Coordination number | Number of ligand donor atoms bound to the metal |
| Geometry | Spatial arrangement of the ligands (tetrahedral, square‑planar, octahedral, etc.) |
| Ligand | Any ion or molecule that donates at least one pair of electrons |
| Chelating ligand | A ligand that can bind through two or more donor atoms (e.g., ethylenediamine) |
| Chelate effect | Greater stability of complexes with chelating ligands compared to equivalent monodentate ligands |
2. Key Points
| Topic | Important Details |
|---|---|
| Types of Ligands | - Monodentate (e.g., NH₃, Cl⁻) - Bidentate (e.g., ethylenediamine) - Polydentate (chelates) (e.g., EDTA) - Bridging (connect two metals) |
| Coordination Number (CN) | 2, 3, 4, 5, 6 (most common). CN = number of donor atoms. |
| Geometries | - CN=4: Tetrahedral or Square‑planar (rare) - CN=5: Trigonal‑bipyramidal or Square‑pyramidal - CN=6: Octahedral (most common) |
| Spectrochemical Series | Order of ligands by the strength of the field they produce: |
F⁻ < Cl⁻ < Br⁻ < NO₃⁻ < I⁻ < H₂O < SCN⁻(S) < NCS⁻(N) < py < 2‑CH₃CN < NO₂⁻ < NH₃ < en < 1,10‑phenanthroline < CN⁻ < CO |
|
| Crystal Field Theory (CFT) | Splitting of d‑orbitals in a ligand field. Octahedral splitting Δ₀, tetrahedral splitting Δₜ (≈4/9 Δ₀). |
| Ligand Field Stabilization Energy (LFSE) | Energy gained due to electron occupancy in lower‑energy orbitals. Helps explain magnetic and spectral properties. |
| High‑Spin vs Low‑Spin | Depends on ligand field strength and electron configuration. |
| Trans Effect | In square‑planar complexes, ligands trans to strong σ‑donors are labile. |
| Chelate Effect | Chelating ligands form more stable complexes because of entropic advantage and multiple bonds. |
| Naming (IUPAC) | 1. List ligands alphabetically (ignore prefixes like di‑, tri‑). 2. Give metal name with oxidation state in Roman numerals. |
| Oxidation State Determination | Sum of ligand charges + metal charge = 0. Substitute known charges. |
| Electronic Configuration | Determine d-electron count: 10 – (oxidation state) for group 3–12 metals. |
| Spectrochemical / Color | d‑d transitions; ligand field strength influences color. |
3. Worked Example
Complex: [Cr(NH₃)₆]Cl₃
Step 1: Determine Oxidation State of Cr
- NH₃ is a neutral ligand → charge 0.
- Complex ion
[Cr(NH₃)₆]³⁺(overall +3). - Let oxidation state of Cr = x.
(x + 6(0) = +3 \Rightarrow x = +3).
Cr(III).
Step 2: Count d‑electrons
- Group 6 metal (Cr) has 6 valence electrons.
- Oxidation state +3 removes 3 electrons → 6 – 3 = 3 d‑electrons.
Electronic configuration:t₂g³ e_g⁰.
Step 3: Predict Geometry & Spin
- Octahedral coordination (CN=6).
- Cr(III) (d³) → strong field but for d³ all electrons occupy t₂g, so low‑spin (no unpaired electrons).
- Magnetic moment = 0 μ_B (diamagnetic).
Step 4: Spectral Properties
- Δ₀ (octahedral splitting) depends on ligand. For NH₃, Δ₀ ≈ 12,000 cm⁻¹.
- d³ → one possible transition: (t_{2g}^3 \rightarrow t_{2g}^2 e_g^1).
- Gives violet or blue color.
Step 5: Naming
- Ligands: ammonia (no prefix).
- Metal: chromium(III).
- IUPAC name: hexamminenchromium(III) chloride.
Summary of Properties
| Property | Value |
|---|---|
| Oxidation state | +3 |
| d‑electrons | 3 |
| Geometry | Octahedral |
| Spin | Low‑spin |
| Magnetic moment | 0 μ_B |
| Color | Violet/blue |
| Stability | High (strong field ligand)** |
4. Quick Revision Summary
| Topic | Key Take‑away |
|---|---|
| Ligands | Monodentate → single donor; Chelates → multiple donors; Bridging → connect metals |
| Coordination number | Number of donor atoms; dictates geometry |
| Geometry | Octahedral (CN=6), tetrahedral (CN=4), square‑planar (CN=4), trigonal‑bipyramidal (CN=5), etc. |
| Spectrochemical series | Determines field strength → influences spin state |
| CFT & LFSE | Splitting of d‑orbitals; low‑ |