the production of thymoquinone from thymol and carvacrol

left to right. This is justifiable since it will be increasingly difficult to remove the third
electron from the d orbital. The lower oxidation states are usually found in ionic
compounds and higher oxidation states tend to be involved in covalent compounds
(Solomons 1988).
3.3.2. Coordination Number
Coordination number is the ability to bind to Lewis bases (ligands) to form
coordination complex ions. In other words, coordination complexes are species in which
a central metal is attached to ligand by coordinate covalent bonds. The central metal is a
Lewis acid (electropositive). A Ligand is a Lewis base (electronegative). The total
number of metal-ligand bonds is called the coordination number. Coordination number
varies from two to eight and it depends on the size, charge, and electron configuration
of the transition metal. Many metals show more than one coordination number
(Solomons 1988).
Within a ligand, the metal ion is directly bonded to the donor atom. A coordinate
covalent bond is a covalent bond in which one atom (i.e., the donor atom) supplies both
electrons. This type of bonding is different from a normal covalent bond in which each
atom supplies one electron. If the coordination complex carries a net charge, the
complex is called a complex ion. Compounds that contain a coordination complex are
called coordination compounds (Brunel et al. 1998).
3.4. Types of Ligands
Ligands are classified as monodentate, bidantate, polydentate ligands.
Monodentate ligands bond using the electron pairs of a single atom. Bidentate ligands
bond using the electron pairs of two atoms. Polydentate ligands bond using the electron
pairs of many atoms. This group includes bidentate. Polydentate ligands are also known
as chelating agent (Keim et al. 2002).
Monodentate (one tooth) ligand can only form one bond with the metal ion such
-
as H2O, CN- , NH3 , NO2 , SCN- , OH- , Cl- , etc. in Figure 3.2. Bidentate ligand can form
two bonds to a metal such as Ethylenediamine, (H 2 N-CH 2 - CH2-NH 2 ), oxalate in Figure
3.3. Polydentate ligands (chelating ligands) such as EDTA(ethylenediaminetetraacetate)
15