Coordination Compounds
1.0 Basics
1.0 Basics
In the chapter transition metals we learnt that they form large number of complex compound in which the metal are bounded to number of anions or neutral molecules. In modern terminology such compounds are called co-ordination compounds.
Co-ordination chemistry is a branch of inorganic chemistry which deals with study of co-ordinates compounds.
Role of co-ordination compounds in various fields: The co-ordination compound is an important part of modern inorganic chemistry. New concepts of chemical bonding and molecular structure have provided insight into the functioning of vital compounds of biological system. It also find in applications in electroplating, textile dyeing and medicinal chemistry.
Co-ordination sphere: The central metal atom/ion and the ligand attached to it are enclosed in square bracket and is collectively termed as the coordination sphere. Ionisable groups are written outside the bracket and are called counter ions.
Example: ${K_4}\left[ {Fe{{(CN)}_6}} \right]$ is the coordination compound. ${\left[ {Fe{{(CN)}_6}} \right]^{4 - }}$ is the co-ordination sphere.
- If the coordination sphere is an anion then it is called Anionic complex.
- If the coordination sphere is an cation then it is called Cationic complex.
- If the coordination sphere is an Neutral then it is called Neutral complex.
Co-ordination entity: A coordination entity constitutes a central metal or ion bounded to a fixed number of ions or molecule.
Example: $\left[ {CoC{l_3}{{(N{H_3})}_3}} \right]$ is a coordination entity in which Cobalt ion $(Co)$ is surrounded by 3 Ammonia $(NH_3)$ molecule and 3 Chloride $(Cl^-)$ ions.
Some other co-ordination compounds are,
- $\left[ {Ni{{(CO)}_4}} \right]$
- $\left[ {PtC{l_2}{{(N{H_3})}_2}} \right]$
- ${\left[ {Fe(C{N_6}} \right]^{4 - }}$
- ${\left[ {Co{{(N{H_3})}_6}} \right]^{3 + }}$
Central atom or ion: In any coordination entity, the atom or ion to which a fixed number of ions or groups are bounded around it in a geometrical arrangement, it is called the central atom or ion.
- In other word, metal atom which accepts the electron pair from ligands are called as central atom.
Name of the central atom: The name of central metal atom depend on the charge on the complex.
- If the complex ion is anion then the name of the central metal atom ends with "ate" .The oxidation number of central metal atom should be written in brackets in Roman letters.
- If the complex ion is cation or neutral then the name of central metal atom is to be written as such and the oxidation number of central metal atom should be written in Parenthesis $\left[ {\ } \right]$ and in Roman letters.
Co-ordinations Number: A total number of ligand attached to central ion is known as coordination number of that ion.
If all the ligands attached are monodentate then coordination number is equal to number of ligands attached to central atom/ion.
Example:
- $\left[ {Co{{\left( {en} \right)}_3}} \right]:$ number of ligands attached are $3$ but the number of donor atoms are $6$ so coordination number is $6$.
- ${\left[ {PtC{l_6}} \right]^{2 - }}$: The co-ordination number of $Pt$ is $6$.
- ${\left[ {Ni{{(N{H_3})}_4}} \right]^{2 + }}$: The co-ordination number of $Ni$ is $4$.
Ligands: The ion or molecule bound to center atom/ion in the coordination entity is called Ligand.
- Based On Denticity: The number of dative bonds formed by ligand at the time with central metal atom is called denticity.
Depending upon the number donor sites, the ligand and denticity can be classified into:
| S. No | Name of the Ligand | Example |
| 1. | Monodentate | ${F^ - },C{l^ - },N{O_2},{H_2}O$ |
| 2. | Bidentate | ${\text{oxalate}} = {C_2}O_4^{2 - },{\text{ }}{H_2}NC{H_2}N{H_2} = {\text{ethyl 1,2 - diamine}}$ |
| 3. | Tridentate | $N{H_2}C{H_2}C{H_2}NHC{H_2}C{H_2}N{H_2} = {\text{di-ethylene-tri-amine}}$ |
| 4. | Tetradentate | $N{H_2}C{H_2}C{H_2}NHC{H_2}C{H_2}NHC{H_2}C{H_2}N{H_2} = {\text{tri - ethylene - tri - amine}}$ |
| 5. | Pentadentate | ${{(^ - }OOCC{H_2})_2}NHC{H_2}C{H_2}NH(C{H_2}CO{O^ - }) = {\text{ethyl - di - amine - tri - acetate ion}}$ |
| 6. | Hexadentate | Ethylenediaminetetraacetate ion $\left[ {EDT{A^{4 - }}} \right]$ is an important hexadentate ligand. It can bind through the two nitrogen and four oxygen atoms to central metal ion |
- Based on charge, ligands are of five types:
Chelate Ligand: When a di-or polydentate ligand uses its two or more donor atoms to bind a single metal ion, it is said to be a Chelate ligand.
Neutral Ligand: A ligand contains no charge is called Neutral Ligand. Example: $N{H_3},{H_2}O$
Positive Ligand: A ligand which contains positive charge is called Positive Ligand. Example: $N{O^ + },N{H_2} - NH_3^ + $
Negative Ligand: A ligand which cantains negative charge is called Negative Ligand. Example: $C{N^ - },N{O_2}^ - $
Ambidentate Ligand: Ligand which can ligate (ability to form bond) through two different atoms is called Ambidentate ligand. Example: $NO_2^ - ,SC{N^ - }$ .$NO_2^ - $ ion can coordinate either through nitrogen or similarly, $SC{N^ - }$ ion can coordinate through the sulphur or nitrogen.
Coordination Polyhedral: The spatial arrangement of ligand which is directly attached to central metal atom of coordination polyhedron around the central atom or ion.
