Isomerism in Transition Metal Compounds

Transition metal complexes exhibit a wide variety of isomerism due to different modes of coordination of ligands and the possible geometries around the central metal ion. These isomers may have distinct physical and chemical properties.

Types of Isomerism

Structural Isomerism:

• Ionization Isomerism: Exchange between a ligand in the coordination sphere and an anion outside it.
  Example: [Co(NH₃)₅SO₄]Br vs [Co(NH₃)₅Br]SO₄.
• Hydrate (Solvate) Isomerism: Exchange between water molecules inside and outside the coordination sphere.
  Example: [Cr(H₂O)₆]Cl₃ vs [Cr(H₂O)₅Cl]Cl₂·H₂O.
• Linkage Isomerism: Arises when an ambidentate ligand binds differently.
  Example: [Co(NH₃)₅NO₂]²⁺ (N-bound nitro) vs [Co(NH₃)₅ONO]²⁺ (O-bound nitrito).
• Coordination Isomerism: Present in compounds containing both cationic and anionic complexes.
  Example: [Co(NH₃)₆][Cr(CN)₆] vs [Cr(NH₃)₆][Co(CN)₆].
• Polymerization Isomerism: Compounds with the same empirical formula but different molecular weights.

Stereoisomerism:

• Geometrical Isomerism: Found in square planar and octahedral complexes.
  Example: [Pt(NH₃)₂Cl₂] has cis and trans forms.
  Example: [Co(NH₃)₄Cl₂]⁺ has cis and trans forms.
• Optical Isomerism: Occurs in chiral complexes without a plane of symmetry.
  Example: [Co(en)₃]³⁺ exists as d and l enantiomers.

Metal Carbonyls: Synthesis and Structure

Introduction

Metal carbonyls are complexes containing carbon monoxide (CO) as the ligand. They are important in organometallic chemistry and catalysis.

Synthesis of Metal Carbonyls

• Direct Combination: Reaction of metal with CO under high pressure and temperature.
  Ni + 4CO → Ni(CO)₄
• Reduction Method: Using reducing agents in presence of CO.
  FeCl₂ + CO + reducing agent → Fe(CO)₅
• Substitution Reaction: Ligands replaced by CO.
  [Mn(CO)₅Br] + NaBH₄ → HMn(CO)₅

Bonding in Metal Carbonyls (Synergic Bonding)

• σ-donation: from filled CO 5σ orbital to empty metal orbital.
• π-back bonding: from filled metal d-orbitals to empty π^* orbitals of CO.
• Synergic effect stabilizes the complex and weakens the C–O bond (observed as lower ?_CO in IR spectra).

Structure of Common Metal Carbonyls

• Ni(CO)₄: Tetrahedral, 18-electron complex, diamagnetic.
• Fe(CO)₅: Trigonal bipyramidal geometry.
• Cr(CO)₆: Octahedral, 18-electron complex.
• Mn₂(CO)₁₀: Dimeric structure with metal–metal bond.

Applications of Metal Carbonyls

• Used in catalysis (hydroformylation, carbonylation).
• Source of high-purity metals (Mond’s process for Ni).
• Precursors for synthesis of other organometallic compounds.