Ethylenediamine

Ethylenediamine (en)

Electron Contribution: 4 electrons (neutral bidentate ligand).

Key Features:

• A chelating ligand, often used in coordination chemistry.
• Example: [Ni(en)₃]²⁺.

Introduction:

Ethylenediamine (en, H₂N–CH₂–CH₂–NH₂) is a classic bidentate chelating ligand in coordination and organometallic chemistry. Due to its ability to form stable five-membered chelate rings with metal centers, ethylenediamine and its derivatives are widely used in the synthesis of coordination complexes, catalysts, and bioinorganic models.

Structure and Bonding:

Ethylenediamine is a flexible, bidentate ligand that typically coordinates to metals through its two nitrogen donor atoms. The chelate ring formed (M–N–C–C–N) adopts a gauche conformation, minimizing steric strain while maximizing orbital overlap with the metal center.

Coordination Modes: 

• Bidentate Chelation: The most common mode, where both nitrogen atoms bind to the same metal center.
• Bridge Mode: In some polymetal complexes, en ligands can bind to two different metal centers.
• Monodentate Coordination: Possible but rare mode of binding possible if electronic and steric factors prevent bidentate coordination.

Electronic and Steric Effects:

• Donor Strength: Ethylenediamine ligands are strong σ-donors but weak π-acceptors. This makes en ligands good at stabilizing metals in medium and high oxidation states. 
• Steric Influence: The ligand is relatively small, allowing for high coordination numbers, but substitutions on the nitrogen or ethylene backbone can introduce steric hinderance. 

Applications in Organometallic Chemisty:

Hydrogenation: En complexes are effective in homogeneous hydrogenation, especially with late transition metals. 

• [Ru(en)₂(PPh₃)H]⁺:
  • Catalyzes alkene hydrogenation under mild conditions.
  • The en ligand prevent dissociation and stabilizes the Ru(II) intermediate. 
• [Ir(en)COD]⁺:
  • Used in asymetric transfer hydrogenation of ketones.
  • Chiral en derivatives enduce enantoselectivity.

C-C Coupling Reactions: En-ligated metals participate in cross-coupling and C–H activation reactions.

• [Pd(en)Cl₂]: Heck Coupling
  • Stabilizes Pd(0) intermediates, preventing aggragation into inactive nanoparticles. 
  • More thermally stable than monodentate amine ligands. 
• [Ni(en)Br₂]: Negishi Coupling
  • Enables alkyl-alkyl cross coupling (traditionally challenging due to β-hydride elimination).
  • The chelate effect suppresses unwanted side reactions. 

Asymetric Synthesis: Chiral derivatives like trans-1,2-diaminocyclohexane allow for enantioselective reactions to occur, typically 

Redox Active Complexes: 

• Ethylenediamine stabilizes metals in multiple oxidation states (e.g., Co(III)/Co(II) in classical Werner complexes).

Proposed Image: Tris-chelate complex of [Ni(en)₃]²⁺ with bidentate en ligands clearly shown.