Hydride Ligands

Hydride Ligands (H-)

Electron Contribution: 2 electrons (anionic ligand).

Key Features:

  • Common in catalytic intermediates such as hydrogenation catalysts.
  • Example: [HFe(CO)4].

Electronic Properties:

  • Hydrides are strong σ-donors
  • Metal-hydride bond is a primarily covalent interaction formed via donation of a pair of electrons from hydride to empty metal orbital.
  • Metal-hydride bond strength primarily influenced by:
    • Metal oxidation state- higher oxidation state relates to stronger M-H bond.
    • Metal d-electron configuration: more d-electrons usually correlate with stronger M-H bond. 
    • Coordination enviormnent (eg. what other ligands are present).

Structure: 

  • Hydride ligands are typically terminal ligands but can also exists as bridging ligands between two metall centers. 

Synthesis of Metal-Hydride Complexes:

Direct Hydrogenation: Reaction between metal and hydrogen gas, often under heat and pressure.

  • M + H2 --> MH2.

Hydride Transfer Reaction: Reaction between metal complex and hydride donors such as LiAlH4 and NaBH4.

  • MLnX + LiAlH4 -->MLnH + biproducts.

Oxidative Addition: Low valent metal complex undergoes a reaction where oxidation number increases by two and two hydrogen atoms coordinate.

  • ML4 + H2 --> ML4H2 

Reactivity/Applications:

Oxidative Addition and Reductive Elimination:

  • Reductive elimination involves the release of H2 gas from the metal complex.
  • Oxidative addition involves the addition of H2 gas onto the metal complex. 

Migratory Insertion: Hydrides can migrate to coordinated ligands like CO or olefines to create new C-H bond. 

  • M-H + CO --> M-CHO

Catalytic Cycles: Hydride ligands are cruical intermediates in many catalytic cycles.

  • Hydrogenation: Wilkenson's Catalyst
  • Hydroformylation: Forming aldehydes form alkenes, CO, H2.
  • Hydrosilylation: Adding Si-H to unsaturated substrates.

Biological Relevance:

Although rare, metal-hydride intermediates are proposed in some enzymatic processes:

  • Hydrogenase: Enzyme that interconverts H2 with protons/electrons often involve nickel and iron hydrides.
  • Nitrogenase: Proposed to use hydride intermediates to convert N2 to NH3.

 

 

Image: Structure of [H2Fe(CO)4] with a labeled H- ligand.