Transition elements: general properties, reactions, variable oxidation states, complex formation
Transition Elements: General Properties
🔬 Transition metals sit in the d‑block of the periodic table. They are known for their variable oxidation states, colourful compounds, and ability to form complexes with ligands.
⚛️ Think of them as chameleons – they can change their “colour” (oxidation state) depending on the environment.
Key Features
- Partially filled d‑orbitals give rise to multiple oxidation states.
- Often form coloured solutions due to d→d electronic transitions.
- Can act as catalysts because of their ability to change oxidation state easily.
- Form stable complexes with a variety of ligands (donor atoms).
Variable Oxidation States
Common Oxidation States of Selected Transition Metals
| Element | Common Oxidation States |
|---|---|
| Fe | $+2$, $+3$ |
| Cu | $+1$, $+2$ |
| Cr | $+2$, $+3$, $+6$ |
| Ni | $+2$, $+3$ |
Example: Copper
- In a basic solution, $\mathrm{Cu^{2+}}$ forms the blue $\mathrm{[Cu(H_2O)_6]^{2+}}$ ion.
- When reduced, it becomes the reddish $\mathrm{Cu^+}$ ion, often seen in the green solution of $\mathrm{Cu_2O}$.
- These changes are useful in redox titrations and in the production of copper salts.
Complex Formation
Ligand Types
- Monodentate: $\mathrm{NH_3}$, $\mathrm{H_2O}$
- Bidentate: $\mathrm{EDTA}$, $\mathrm{oxalate}$
- Polydentate: $\mathrm{EDTA}$ (six‑point), $\mathrm{PPh_3}$ (phosphine)
Stability Rules (18‑Electron Rule)
🧪 A stable complex often has a total of 18 valence electrons (including d‑electrons and ligand electrons). This is similar to the noble gas configuration.
Example Complexes
- $\mathrm{[Fe(CN)_6]^{4-}}$ – a deep blue complex used in electroplating.
- $\mathrm{[Cu(NH_3)_4]^{2+}}$ – a bright blue solution, classic example of ligand field theory.
- $\mathrm{[Co(NH_3)_6]^{3+}}$ – pink, shows how ligand choice affects colour.
Reactions Involving Transition Metals
Redox Reactions
Example: $\mathrm{Fe^{3+} + e^- \rightarrow Fe^{2+}}$ – the iron in a rusting process.
Ligand Exchange
When a ligand is replaced by another, the reaction can be fast or slow depending on the metal and ligands. Example: $\mathrm{[Co(NH_3)_5Cl]^{2+} + 2\,NH_3 \rightarrow [Co(NH_3)_7]^{3+} + Cl^-}$.
Examination Tips
Tip 1: Write oxidation numbers in brackets when dealing with redox equations, e.g., $\mathrm{[Fe^{3+}] + e^- \rightarrow Fe^{2+}}$.
Tip 2: Remember the 18‑electron rule for complex stability – it helps predict which complexes are likely to form.
Tip 3: Use colour changes as a quick check for oxidation state changes in qualitative tests.
📝 Practice drawing electron configurations: e.g., $\mathrm{Fe^{2+}}$ is $d^6$, $\mathrm{Fe^{3+}}$ is $d^5$. This helps in predicting magnetic properties.
📝 When given a complex formula, count the total electrons to check if it satisfies the 18‑electron rule.
Revision
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