In organometallic chemistry, ligands are groups or molecules that bond to a metal atom or ion, donating electrons. They strongly influence the stability, reactivity, and geometry of the complex.
Here are the common ligands grouped by type:
1. Neutral Ligands (L-type: donate 2e⁻)
- Carbon monoxide (CO) – strong field, common in stable complexes like Fe(CO)₅.
- Phosphines (PR₃) – e.g., PPh₃, PMe₃, tunable donor/acceptor properties.
- Amines (NR₃) – e.g., NH₃, Et₃N.
- Ethers (ROR’) – e.g., THF (tetrahydrofuran).
- Isocyanides (CNR) – similar to CO in bonding.
2. Anionic Ligands (X-type: donate 2e⁻ and carry -1 charge)
- Hydride (H⁻) – common in catalytic systems.
- Halides (Cl⁻, Br⁻, I⁻, F⁻) – simple, strongly bound.
- Alkyls (R⁻) – e.g., CH₃⁻, used in organometallic catalysis.
- Aryls (Ar⁻) – aromatic carbons bonded to metal.
- Alkoxides (RO⁻) – e.g., OMe⁻.
3. π-Bonding Ligands
- Alkenes (C=C) – e.g., Zeise’s salt [PtCl₃(C₂H₄)]⁻.
- Alkynes (C≡C) – bind through π-electrons.
- Cyclopentadienyl (Cp, C₅H₅⁻) – aromatic, 6e⁻ donor (common in metallocenes like ferrocene).
- Arene ligands (C₆H₆) – benzene and substituted arenes.
4. Multidentate (Chelating) Ligands
- Bidentate phosphines (e.g., dppe = 1,2-bis(diphenylphosphino)ethane).
- Bipyridine (bipy, 2,2′-bipyridine).
- Ethylene diamine (en).
- Cyclooctadiene (COD, C₈H₁₂) – binds through two double bonds.
5. Special/Other Ligands
- Nitrosyl (NO) – can bind in linear or bent fashion.
- Carbenes (CR₂) – Fischer or Schrock carbenes.
- Dihydrogen (H₂) – η²-H₂ complexes.
- Borohydride (BH₄⁻) – multi-hapto ligand.
Summary:
The most common ligands in organometallic chemistry are CO, phosphines, hydride, alkyls, halides, alkenes, alkynes, Cp (cyclopentadienyl), and arenes.