Notes on Coordination Compounds Ligands: an ion or molecule | Fiitjee Study Material
Notes on Coordination Compounds
Ligands: an ion or molecule capable of donating a pair of electrons to the central atom via a donor atom.
Unidentate ligands: Ligands with only one donor atom, e.g. NH3, Cl-, F- etc.
Bidentate ligands: Ligands with two donor atoms, e.g. ethylenediamine, C2O42-(oxalate ion) etc.
Tridentate ligands: Ligands which have three donor atoms per ligand, e.g. (dien) diethyl triamine.
Hexadentate ligands: Ligands which have six donor atoms per ligand, e.g. EDTA.
Chelating Ligands:
Multidentate ligand simultaneously coordinating to a metal ion through more than one site is called chelating ligand. Example: Ethylenediamine (NH2CH2CH2NH2)
These ligands produce a ring like structure called chelate.
Chelation increases the stability of complex.
Werner’s Theory:
Metals possess two types of valencies i.e. primary (ionizable) valency and secondary (nonionizable) valency.
Secondary valency of a metal is equal to the number of ligands attached to it i.e. coordination number.
Primary valencies are satisfied by negative ions, while secondary valencies may be satisfied by neutral, negative or positive ions.
Secondary valencies have a fixed orientation around the metal in space.
[Co(NH3)6]Cl3
Primary Valencies = 3 Cl-
Secondary Valencies = 6 NH3
Coordination Sphere = [Co(NH3)6]3-
Nomenclature of Complexes:
Positive ion is named first followed by negative ion.
Negative ligands are named by adding suffix - o.
Positive ligands are named by adding prefix – ium.
Neutral ligands are named as such without adding any suffix or prefix.
Ligands are named in alphabetical order.
Name of the ligands is written first followed by name of metal with its oxidation number mentioned in roman numbers in simple parenthesis.
Number of the polysyllabic ligands i.e. ligands which have numbers in their name, is indicated by prefixes bis, tris etc,
Number and name of solvent of crystallization if any, present in the complex is written in the end of the name of complex.
When both cation and anion are complex ions, the metal in negative complex is named by adding suffix-ate.
In case of bridging ligands:
[Name of the groups to the left of bridging ligand (Oxidation state)] –μ – [Name of the groups to the right of bridging ligand (Oxidation state)] – [Name of negative ion]
