Phosphine Ligands

Phosphine Ligands (PR₃)

Electron Contribution: 2 electrons (neutral ligand).

Key Features:

• Common examples include PPh₃, PMe₃, PCy₃, and BINAP.
• These ligands are very bulky which makes it difficult for more than four PR₃ ligands to bind with a metal center at the same time. 
• Often form unsaturated complexes. 
• Versatile in steric and electronic tuning, widely used in catalysis.

Electronic Properties:

• Phosphines are σ-donors and can also engage in π-backbonding to some extent, depending on substituants and metal center.
• Donor strength is influence by the nature of the R groups (PR₃):
  • Electron-donating groups (eg. alkyl): increase basicity of phosphorus and enhance σ-donation.
  • Electron-withdrawing groups (eg. aryl with electonegative substituants): reduce σ-donation.

Steric Properties:

• The streic bulk of a phosphine ligand is quantified using the Tolman Cone Angle.
• Cone angle is measures the angle formed at the metal center by the outer edges of the R groups on the phosphorus.
• Bulky substituants can prevent undesired ligand association, improving the stability of the complex and increasing selectivity of the catalytic process.

Cone Angle Diagram

Synthesis:

• Phosphine complexes are commonly made through a ligand substitution process- exchanging one ligand for another on the metal center. 

Reactions-Catalysis:

• Phosphines often act as spectator ligands meaning they don't directly participate in the reaction, but are along for the ride. 
• Used in a number of industrial processes- cross-coupling reactions (eg, Suzuki, Heck, Stille) and hydrogenation reactions.