JEE Chemistry Coordination Compounds Notes Set 02

Valence Bond Theory

The metal gives empty orbitals that hybridise and accept ligand pairs. Coordination number 4 gives sp³ (tetrahedral) or dsp² (square planar). Coordination number 6 gives sp³d² (outer orbital, high spin) or d²sp³ (inner orbital, low spin). Strong field ligands such as CN⁻ and CO pair the d electrons; weak field ligands such as halides do not. Magnetic moment \( \mu=\sqrt{n(n+2)} \) BM, where n is the number of unpaired electrons.

Example. [Fe(CN)₆]⁴⁻: Fe²⁺ is d⁶, CN⁻ pairs the electrons, d²sp³, diamagnetic. [FeF₆]³⁻: Fe³⁺ is d⁵, sp³d², five unpaired electrons, \( \mu \approx 5.92 \) BM. [Ni(CN)₄]²⁻ is square planar and diamagnetic, while [NiCl₄]²⁻ is tetrahedral with two unpaired electrons.

Crystal Field Theory

1. Ligands are treated as point charges. In an octahedral field the d orbitals split into t_2g (lower, 3 orbitals) and e_g (upper, 2 orbitals) with gap \( \Delta_o \).
2. In a tetrahedral field the order is reversed (e below t₂) and \( \Delta_t=\frac{4}{9}\Delta_o \), so tetrahedral complexes are almost always high spin.
3. If Δ_o is less than the pairing energy P the complex is high spin; if Δ_o is greater than P it is low spin. This applies to d⁴ to d⁷ ions only.
4. CFSE (octahedral) = \( (-0.4\,x + 0.6\,y)\Delta_o \) plus pairing terms, where x electrons are in t_2g and y in e_g.

Spectrochemical Series

Weak to strong: I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < NO₂⁻ < CN⁻ < CO. The colour of a complex is due to d-d transitions; the wavelength absorbed depends on Δ, and the colour seen is the complement. Complexes with no d electrons (d⁰) or a full d¹⁰ shell are colourless.

Stability and Uses

1. Stability constant: higher K means a more stable complex; chelates are more stable than those of unidentate ligands (chelate effect).
2. Metal carbonyls such as Ni(CO)₄ (tetrahedral), Fe(CO)₅ (trigonal bipyramidal) and Cr(CO)₆ (octahedral) have the metal in zero oxidation state and show synergic bonding.
3. Uses: extraction of silver and gold (cyanide complexes), EDTA titration of hardness, Ni by dimethylglyoxime, chlorophyll (Mg), haemoglobin (Fe), vitamin B12 (Co), cisplatin in cancer therapy.

Key Facts to Remember

1. Strong field ligand and d⁴-d⁷ ion: low spin, fewer unpaired electrons.
2. Pd, Pt and Au(III) 4d/5d ions, and Ni²⁺ with strong ligands, favour square planar shape.
3. Larger Δ means shorter wavelength absorbed.
4. Ionisation in solution is found by the number of ions outside the square bracket, using AgNO₃ to precipitate free chloride.