Coordination Compounds
4.0 Werner's Co-ordination Theory
4.0 Werner's Co-ordination Theory
According to Werner theory, every co-ordination compound consists of at least one central metal atom.
The central metal atom has two types of valencies :
1. Primary valency (${1^0}$)
2. Secondary valency (${2^0}$)
Primary Valency (${1^0}$): Primary valencies are those in which a metal normally exhibits in the formations of its simple salts. These are satisfied by only anions. These are represented by dotted lines between atom and anion. It is non-ionisable valency.
Example: The complex $\left[ {Co{{(N{H_3})}_6}} \right]C{l_3}$ actually exist as ${\left[ {Co{{(N{H_3})}_6}} \right]^{3 + }}$ and 3$C{l^ - }$. Thus, the primary valency is $3$ as there are three ionic bonds.
Secondary Valency (${2^0}$): Secondary valency is satisfied by neutral molecule or negative ions. These are satisfied by electron pair donors, the ions or the neutral species. This is represented by thick lines. It is non-ionisable valency. The secondary valency is equal to the coordination number and fixed for a meta. It gives geometry or different characteristic spatial arrangement of the complex.
Example:
| S.No. | Co-ordination number | Shape |
| 1. | 2 | Linear |
| 2. | 3 | Trigonal planar |
| 3. | 4 | Square planar |
| 4. | 5 | Trigonal bipyramidal (or) Square pyramidal |
| 5. | 6 | Octahedral |
- Whenever the complex is formed, the secondary valency should always be satisfied. This is proved with the following equations.
1. $CoC{l_3}.6N{H_3} + AgN{O_3} \to 3AgCl$
2. $CoC{l_3}.5N{H_3} + AgN{O_3} \to 2AgCl,$
3. $CoC{l_3}.4N{H_3} + AgN{O_3} \to 1AgCl,$
- Werner deduced that in $CoC{l_3}.6N{H_3}$ three chlorine act as a primary valency and six ammonia as secondary valencies. In modern terms the complex is written $\left[ {Co{{(N{H_3})}_6}} \right]C{l_3}$. The three chlorine are ionic and hence are precipitated indicating the presence of the two chlorine in the outer sphere i.e., in order to fulfill the secondary valency one chlorine from outer surface drift into inner surface sphere. Similar is the case with third complex. So actual structure becomes $\left[ {Co{{(N{H_3})}_6}} \right]C{l_3}$, $\left[ {Co(N{H_3}){}_5Cl} \right]C{l_2}$, $\left[ {Co{{(N{H_3})}_4}C{l_2}} \right]Cl$
Common points by $1{}^0$ and ${2^0}$ valency:
- Some of the negative ions can be satisfied by both $1{}^0$ and ${2^0}$ valencies and these are non-ionisable.
- Each metal has fixed number of secondary valencies which can be also referred to as coordination number.
- The ions attached to primary valencies possess ionizing nature where the ions attached to the secondary valencies do not ionize when the complex is dissolved in a solvent.
- Potassium ferrocyanide ions which satisfy $1{}^0$ and ${2^0}$ valency.
- Ferrocyanide ion satisfy $1{}^0$ valency and $SCN{}^ - $ ions satisfy ${2^0}$ valency.
