the production of thymoquinone from thymol and carvacrol

it surrounds the metal, forms very stable complex ions (Arends et al. 2001) as seen in
Figure 3.4.
Any ion or molecule with a pair of nonbonding electrons can be a ligand. Many
ligands are described as monodentate (literally, "one-toothed") because they "bite" the
metal in only one place. Typical monodentate ligands are given in the figure below
(Figgis 1966, Keim et al. 2002).
Figure 3.2. Typical monodentate ligands.
(Source: Keim et al. 2002)
Other ligands can attach to the metal more than one place. Ethylenediamine (en)
is a typical bidentate ligand.
Figure 3.3. Bidentate ligand, ethylenediamine.
(Source: Keim et al. 2002)
Each end of this molecule contains a pair of nonbonding electrons that can form
a covalent bond to a metal ion. Ethylenediamine is also an example of a chelating
ligand. The term chelate comes from a Greek stem meaning "claw." It is used to
describe ligands that can grab the metal in two or more places, the way a claw would.
Linking ethylene- diamine fragments gives tridentate ligands and tetradentate ligands,
16