Chapter 5 / 5.5 Bonding in Coordination Compounds

Bonding in Coordination Compounds 🤝

1. Why do we need new bonding ideas?

Werner’s early model could not explain why only some metals form complexes or why these substances have such clear magnetic, optical and directional properties. Modern views—Valence Bond Theory (VBT), Crystal-Field Theory (CFT), Ligand-Field Theory and Molecular-Orbital Theory—step in to answer these questions. :contentReference[oaicite:17]{index=17}

2. Valence Bond Theory (VBT) 😀

• Hybridisation toolkit
  4 (sp³) → tetrahedral • 4 (dsp²) → square-planar • 5 (sp³d) → trigonal-bipyramidal • 6 (sp³d² or d²sp³) → octahedral. :contentReference[oaicite:18]{index=18}
• Inner vs outer orbital sets
  [Co(NH₃)₆]³⁺ uses \(d^{2}sp^{3}\) inner orbitals → octahedral, diamagnetic (all electrons paired). [CoF₆]^3- uses \(sp^{3}d^{2}\) outer orbitals → octahedral, paramagnetic (4 unpaired electrons). :contentReference[oaicite:19]{index=19}
• Four-coordinate highlights
  [NiCl₄]^2- adopts sp³ tetrahedral and keeps two unpaired electrons (green). [Ni(CN)₄]^2- adopts dsp² square-planar with all electrons paired (colourless). :contentReference[oaicite:20]{index=20}

Magnetic clues 🧲

Measured magnetic moments let us count unpaired electrons and confirm shape. Example: a spin-only value of 5.9 BM for [MnBr₄]^2- proves a tetrahedral geometry (five unpaired d-electrons). :contentReference[oaicite:21]{index=21}

Limits of VBT 😕

• Relies on several assumptions.
• Gives only qualitative magnetic predictions.
• Cannot account for colour, stability trends or the weak/strong nature of ligands. :contentReference[oaicite:22]{index=22}

3. Crystal-Field Theory (CFT) ✨

CFT treats ligands as point charges (or dipoles) that split the degenerate metal d orbitals. :contentReference[oaicite:23]{index=23}

Octahedral splitting

The orbitals pointing straight at ligands—\(d_{x^{2}-y^{2}}\) and \(d_{z^{2}}\)—rise by \(+\frac{3}{5}\Delta_o\) (the \(e_g\) set). The three orbitals in-between axes—\(d_{xy}\), \(d_{xz}\), \(d_{yz}\)—drop by \(-\frac{2}{5}\Delta_o\) (the \(t_{2g}\) set). :contentReference[oaicite:24]{index=24}

Tetrahedral splitting

Here the pattern inverts and shrinks: \( \Delta_t = \tfrac{4}{9}\Delta_o \). Because the gap is smaller, electrons rarely pair up—low-spin tetrahedral compounds are uncommon. :contentReference[oaicite:25]{index=25}

High-spin vs low-spin ⚖️

If \( \Delta_o < P \) (pairing energy) ➜ electrons stay unpaired → high spin. If \( \Delta_o > P \) ➜ pairing is cheaper → low spin. :contentReference[oaicite:26]{index=26}

Spectrochemical series (field strength grows →)

I^– < Br^– < SCN^– < Cl^– < S^2- < F^– < OH^– < C₂O₄^2- < H₂O < NCS^– < edta^4- < NH₃ < en < CN^– < CO. :contentReference[oaicite:27]{index=27}

Limitations 🧐

• Anions should give the strongest fields by pure electrostatics, yet they appear near the weak end of the real series.
• The model ignores covalent metal–ligand bonding.

More advanced Ligand-Field and MO approaches patch these gaps. :contentReference[oaicite:28]{index=28}

4. Colour in Complexes 🌈

When visible light promotes an electron from \(t_{2g}\) to \(e_g\), the absorbed colour is removed; we see the complementary hue. Example: [Ti(H₂O)₆]³⁺ absorbs blue-green (~498 nm) and looks violet. :contentReference[oaicite:29]{index=29}

:contentReference[oaicite:30]{index=30}

Ligand changes tweak the gap—and the colour! Adding ethane-1,2-diamine (en) step-wise to [Ni(H₂O)₆]²⁺ turns the solution from green → pale-blue → purple → violet. :contentReference[oaicite:31]{index=31}

5. A quick look at Metal Carbonyls 🏭

• Ni(CO)₄ – tetrahedral
• Fe(CO)₅ – trigonal-bipyramidal
• Cr(CO)₆ – octahedral
• Mn₂(CO)₁₀ – Mn–Mn bonded, two square pyramids
• Co₂(CO)₈ – Co–Co bonded, two CO bridges

:contentReference[oaicite:32]{index=32}

High-Yield NEET Takeaways ⚡

1. Predict geometry & magnetism quickly—recognise inner vs outer orbital hybridisation patterns.
2. Master \( \Delta_o \) vs \( P \)—this tells you high-spin or low-spin in a flash.
3. Spectrochemical series—know the order; it explains both colour and spin state questions.
4. d–d transitions = colour—complementary hues often feature in exam visuals.
5. Hybridisation chart—sp³, dsp², sp³d², d²sp³ are instant geometry clues.

🥳 Happy learning & good luck with your prep!