Ammonia Ligands

Ammonia Ligands (NH3)

Electron Contribution: 2 electrons (neutral ligand).

Key Features:

  • Simple and small ligand, commonly seen in Werner complexes.
  • Example: [Co(NH3)6]3+.

Electronic Structure and Bonding: Amine ligands coordinate via the nonbonding lone pair on nitrogen, donating 2 electrons to empty metal orbital.

  • σ-donor Behavior: purely σ-donors with no signifigant π-backbonding character.

 

  • Basicity and Donor Strength: The basicity of an amine correlates to its donor ability.
  • Electron donating substituants (e.g. alkyl groups) increase donor strength. 
  • Aromatic amines and bulky tertiary amines exhibit weaker coordination due to resonance and steric hinderance. 

 

Common Ligand Types:

Type General Formula Example Basicity/Donor Strength
Ammonia NH₃ NH₃ High
Primary amine RNH₂ MeNH₂, EtNH₂ High
Secondary amine R₂NH Me₂NH Moderate
Tertiary amine R₃N Et₃N, pyridine Lower (bulky)
Aromatic amines ArNH₂ or Ar₃N Aniline, DMAP Weaker due to resonance
Chelating polyamines (RNHCH₂CH₂)ₙNR₂ diethylenetriamine (dien) Variable
  • Due to the high donor strength of ammonia and primary amines, they are not very synthetically useful in the context of organic chemistry because they are so difficult to remove and react.

Structural Roles in Complexes:

Monodentate Coordination:

  • Most simple amines terminally bind as monodentate ligands, occupying one coordination site. 

Chelating and Multidentate Amines:

  • Polydentate amines (e.g. ethylenediamine) form chelating rings, increasing complex stability via the chelation effect. 

Reactivity of Amine Ligands:

Ligand Substitution:

  • Amine ligands can be displaced by ligands which are stronger σ-donors or can participate in π-backbonding.
  • This can happen with CO, CN-, or chelating ligands.

Applications:

Catalysis:

  • Used in some catalysis reactions as a stabilizing ligand in hydrogenation reactions. 
  • Some cross-coupling reaction might also use amine ligands. 

Medicine:

  • Amine ligands are present in one of the most common chemotherapy drugs, cis-platin.
  • Used to increase the water solubility of [PtCl2(NH3)2].
  • Amines can help improve solubility, specificity, and stability of metal based drugs.

Issues:

Amine ligands are not often seen in organometallic chemistry because they get outshined by other ligands, namely phosphines. Amine ligands are purely σ-donors with no π-backbonding capabilities. As a result, amine ligands are not able to stabilize metal centers with low oxidation states as effectively as ligands like phosphines or carbonyls. 

Amine ligands are also susceptible to protonation leading to a weakened M-N bond which limits the ligands use in acidic media.

Phosphine ligands are more robust and can be used in more situations than amine ligands can, so typically phosphine ligands are used over amine ligands. 

 

Proposed Image: Octahedral structure of [Co(NH3)6]3+ showing ligand coordination.