Coordination Compounds

1. Introduction

Coordination compounds contain metal ions bonded to ligands through coordinate bonds. They are vital in biological systems, medicine, and industrial catalysis.

2. Werner's Theory

Alfred Werner proposed that metal ions have two types of valencies:

• Primary valency: Ionizable, corresponds to oxidation state.
• Secondary valency: Non-ionizable, corresponds to coordination number.

Secondary valencies are directed in space, giving specific geometries.

3. Ligands

Anions or neutral molecules that donate electron pairs to the metal ion.

Unidentate: One donor atom (NH₃, H₂O, Cl⁻, CN⁻). Bidentate: Two donor atoms (en, ox²⁻). Polydentate: Multiple donor atoms (EDTA is hexadentate). Ambidentate: Two possible donor atoms (NO₂⁻/ONO⁻, SCN⁻/NCS⁻).

4. IUPAC Nomenclature

1. Name ligands in alphabetical order (ignoring prefixes).
2. Use prefixes di-, tri-, tetra-, penta-, hexa- for simple ligands.
3. Use bis-, tris-, tetrakis- for complex ligand names.
4. Name the metal with its oxidation state in Roman numerals in parentheses.
5. For anionic complexes, add -ate suffix to the metal name.

Example: [Co(NH₃)₆]Cl₃ — Hexaamminecobalt(III) chloride. K₃[Fe(CN)₆] — Potassium hexacyanoferrate(III).

5. Isomerism

5.1 Structural Isomerism

Ionization isomerism: [Co(NH₃)₅Br]SO₄ vs [Co(NH₃)₅SO₄]Br. Linkage isomerism: [Co(NH₃)₅NO₂]²⁺ vs [Co(NH₃)₅ONO]²⁺. Coordination isomerism: [Co(NH₃)₆][Cr(CN)₆] vs [Cr(NH₃)₆][Co(CN)₆].

5.2 Stereoisomerism

Geometrical: cis-trans (square planar) or fac-mer (octahedral). Optical: Non-superimposable mirror images (chiral complexes).

6. Isomerism in Coordination Compounds — Extended

6.1 Geometrical Isomerism in Square Planar Complexes

For [Ma₂b₂] (M = metal, a and b are monodentate ligands):

• Cis-isomer: Same ligands adjacent (90° apart).
• Trans-isomer: Same ligands opposite (180° apart).

Example: [Pt(NH₃)₂Cl₂] — cisplatin (cis, anticancer) and transplatin (trans, inactive).

6.2 Geometrical Isomerism in Octahedral Complexes

For [Ma₄b₂]:

• Cis: b ligands at 90°.
• Trans: b ligands at 180°.

For [Ma₃b₃]:

• fac (facial): Three identical ligands occupy one face of the octahedron (all at 90° to each other).
• mer (meridional): Three identical ligands lie in a plane including the metal.

6.3 Optical Isomerism

Optical isomers are non-superimposable mirror images (enantiomers). They rotate plane-polarised light in opposite directions.

Example: [Co(en)₃]³⁺ shows optical isomerism (Δ and Λ isomers).

'A complex must lack a plane of symmetry to be optically active. The presence of chelating ligands often creates chirality.'

7. Bonding Theories

6.1 Valence Bond Theory (VBT)

Metal ion uses hybrid orbitals to accept electron pairs from ligands.

• Coordination number 4: sp³ (tetrahedral) or dsp² (square planar).
• Coordination number 6: d²sp³ (inner orbital) or sp³d² (outer orbital).

6.2 Crystal Field Theory (CFT)

Ligands create an electrostatic field that splits the d-orbitals into t₂g and e_g sets.

Crystal field splitting energy (Δ₀) depends on the nature of the ligand.

• Strong field ligands (CN⁻, CO): large Δ₀, low spin complexes.
• Weak field ligands (I⁻, Br⁻, F⁻): small Δ₀, high spin complexes.

Spectrochemical series: I⁻ < Br⁻ < S²⁻ < SCN⁻ < Cl⁻ < F⁻ < OH⁻ < ox²⁻ < H₂O < NCS⁻ < NH₃ < en < CN⁻ < CO.

7. Applications

Biological: Haemoglobin (Fe²⁺), Chlorophyll (Mg²⁺), Vitamin B12 (Co³⁺). Industrial: Extraction of metals, catalysis, water softening.

8. Worked Problems

Problem 1: Write the IUPAC name of [Pt(NH₃)₂Cl₂] and draw its geometrical isomers. Solution: Diamminedichloridoplatinum(II). Cis-isomer (cisplatin) and trans-isomer.

Problem 2: Determine the magnetic moment of [Fe(CN)₆]⁴⁻. Solution: Fe²⁺ = 3d⁶. CN⁻ is strong field, so low spin: t₂g⁶, e_g⁰. No unpaired electrons. μ = 0 BM.

9. Common Mistakes

'Students often forget that the oxidation state of the metal is written in Roman numerals in parentheses after the metal name, not as a subscript.'

10. ISC Exam Focus

11. Self-Test Questions

1. Write the IUPAC names of: (i) K₃[Al(C₂O₄)₃] (ii) [Ni(CO)₄].
2. Distinguish between a weak field and a strong field ligand. How do they affect the spin state of a complex?
3. Explain geometrical isomerism in octahedral complexes with examples.
4. Calculate the magnetic moment of [Ni(CN)₄]²⁻ and [NiCl₄]²⁻. Account for the difference.
5. What is the spectrochemical series? List five ligands in increasing order of field strength.