Lewis Base Ligands -- Halides Common Misconception: The halides are anionic ligands, so they are NOT electron-withdrawing ligands. In organic chemistry the halogens can be considered neutral ligands and do drain electron density from whatever they are attached to. But here they are anionic and are perfectly happy with that charge. Their electronegativity makes the halides “poor” donor ligands. As one moves from F  to I , the donor ability increases as the electro­negativity drops.

Oxygen Donors

Sulfur Donors

Sulfides (and thiolates) are extremely effective bridging ligands and play a critically important role as such in bioinorganic chemistry:

Nitrogenase The nitrogenase enzyme catalyzes the extremely difficult reaction: N 2 + 8H + + 8e- 2NH 3 + H 2 The conversion of atmospheric N 2 into NH 3 is a marvel of catalysis and provides the plant with its own fertilizer. In the Haber catalytic process for converting N 2 + 3H 2 into NH 3, over 400ºC and 400 atm of H 2 /N 2 pressure are required along with a heterogeneous Fe catalyst. Nitrogenase does this at room temperature and pressure!!

Nitrogen Donors In general, alkylated amines are not particularly good ligands. This is mainly due to the relatively short N-C bond distances and the stereoelectronic problems generated from this. Chelating amines have fewer steric problems and are better ligands for transition metal centers. Perhaps the most famous neutral nitrogen donor ligand is bipyridine or bipyridyl, almost universally abbreviated bipy.

Inorganic Amides The lone pairs in an amide are about 2eV higher in energy than in OR . This makes an amide a considerably stronger donor.

Alkyl-Imido (nitrene) Ligand

Anionic 6e- donor coordination via three lower nitrogen centers Moderate donor Steric effects adjusted via R groups Tris(pyrazolyl)borate Ligand The tris(pyrazolyl)borate (Tp) ligand has become extremely popular in the last decade and is sometimes called the inorganic Cp ligand. It is anionic and is a 6e- donor when all three nitrogens are coordinated to a metal center, just like Cp -.

A Few Biologically Important N-Ligands