Coordination Chemistry

1. Isomerism in Complexes

Geometrical Isomerism: Cis/Trans for square planar ($MA_2B_2$) and octahedral ($MA_4B_2$). Optical Isomerism: Common in octahedral complexes with chelate ligands like 'en'.

2. Crystal Field Theory (CFT)

Splitting of d-orbitals in octahedral ($t_{2g}$ and $e_g$) and tetrahedral fields. The magnitude of splitting ($\Delta_o$) depends on the ligand strength (Spectrochemical Series).

3. Werner's Theory and Key Terms

A metal shows a primary valency (oxidation state, satisfied by ions, ionisable) and a secondary valency (coordination number, satisfied by ligands, non-ionisable). For $[Co(NH_3)_6]Cl_3$, all three $Cl^-$ precipitate with $AgNO_3$; for $[CoCl(NH_3)_5]Cl_2$, only two do.

• Denticity: monodentate ($NH_3$, $Cl^-$), bidentate (en, oxalate), hexadentate (EDTA$^{4-}$).
• Ambidentate ligands can bind through either of two atoms: $NO_2^-$/$ONO^-$, $SCN^-$/$NCS^-$. This gives linkage isomerism.
• Chelates (ring-forming ligands) are more stable than similar complexes with monodentate ligands.

4. IUPAC Naming

Name ligands alphabetically before the metal, then the oxidation state in Roman numerals. Anionic complexes take the suffix "-ate" (ferrate, cuprate, argentate). Example: $K_3[Fe(CN)_6]$ is potassium hexacyanidoferrate(III).

5. Valence Bond Theory and Magnetism

• $[Co(NH_3)_6]^{3+}$: $d^2sp^3$, inner orbital, diamagnetic.
• $[CoF_6]^{3-}$: $sp^3d^2$, outer orbital, paramagnetic (4 unpaired).
• $[Ni(CN)_4]^{2-}$: $dsp^2$, square planar, diamagnetic. $[NiCl_4]^{2-}$: $sp^3$, tetrahedral, paramagnetic.

6. Spectrochemical Series and CFSE

$I^- < Br^- < SCN^- < Cl^- < F^- < OH^- < C_2O_4^{2-} < H_2O < NCS^- < NH_3 < en < CN^- < CO$. If $\Delta_o > P$ (pairing energy), electrons pair in $t_{2g}$ (low spin). Tetrahedral splitting is smaller, $\Delta_t \approx \frac{4}{9}\Delta_o$, so tetrahedral complexes are almost always high spin. The colour observed is complementary to the light absorbed.

Interactive diagram: open this page in a browser to use it.

7. Metal Carbonyls

The M–CO bond has σ-donation from CO to the metal and π-back-donation from metal d orbitals into CO's empty π* orbital. This synergic bonding strengthens the M–C bond and weakens the C–O bond.

Practice questions

1. The IUPAC name of K₄[Fe(CN)₆] is:

   1. Potassium hexacyanoiron(II)
   2. Potassium hexacyanidoferrate(II)
   3. Potassium hexacyanidoferrate(III)
   4. Potassium ferrocyanide(IV)
   Answer

   B. Fe is +2 here, and an anionic complex takes "-ate": hexacyanidoferrate(II).

2. Which of the following is an ambidentate ligand?

   1. NH₃
   2. NO₂⁻
   3. en
   4. H₂O
   Answer

   B. NO₂⁻ can bind via N (nitrito-N) or O (nitrito-O).

3. [Ni(CN)₄]²⁻ is:

   1. Tetrahedral and paramagnetic
   2. Square planar and diamagnetic
   3. Octahedral and paramagnetic
   4. Tetrahedral and diamagnetic
   Answer

   B. CN⁻ is a strong field ligand; Ni(II) 3d⁸ pairs up and uses dsp² hybridisation.

4. The number of ions produced in solution by [Co(NH₃)₅Cl]Cl₂ is:

   1. 2
   2. 3
   3. 4
   4. 6
   Answer

   B. It ionises into [Co(NH₃)₅Cl]²⁺ and 2Cl⁻, giving 3 ions.

5. Which ligand produces the largest crystal field splitting?

   1. Cl⁻
   2. H₂O
   3. NH₃
   4. CO
   Answer

   D. CO is at the strong-field end of the spectrochemical series due to π-back-bonding.