3 moles of metal complex with formula Co(en)$_2$Cl$_3$ gives 3 moles of silver chloride on treatment with excess of silver nitrate. The secondary valency of Co in the complex is ________. (Round off to the Nearest Integer).
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Secondary valency is the same as the coordination number. To find it, identify all the ligands within the coordination sphere and sum up the number of bonds they form (e.g., monodentate=1, bidentate=2, etc.).
The reaction with silver nitrate (AgNO$_3$) precipitates chloride ions (Cl$^-$) that are outside the coordination sphere (i.e., counter-ions).
The problem states that 3 moles of the complex Co(en)$_2$Cl$_3$ produce 3 moles of AgCl precipitate.
This implies a 1:1 molar ratio: 1 mole of the complex yields 1 mole of AgCl.
This means there is one chloride ion acting as a counter-ion outside the coordination sphere.
Therefore, the correct formula for the complex is [Co(en)$_2$Cl$_2$]Cl.
The species inside the square brackets is the complex ion, and the Cl outside is the counter-ion.
The secondary valency of the central metal ion is its coordination number. The coordination number is the total number of coordinate bonds formed by the ligands with the central metal ion.
In the complex ion [Co(en)$_2$Cl$_2$]$^+$, the ligands are:
- 'en', which is ethylenediamine (H$_2$NCH$_2$CH$_2$NH$_2$). It is a bidentate ligand, meaning it forms two coordinate bonds. There are two 'en' ligands.
- 'Cl', which is chloro. It is a monodentate ligand, meaning it forms one coordinate bond. There are two 'Cl' ligands inside the sphere.
Coordination Number = (Number of 'en' ligands $\times$ denticity of 'en') + (Number of 'Cl' ligands $\times$ denticity of 'Cl')
Coordination Number = (2 $\times$ 2) + (2 $\times$ 1) = 4 + 2 = 6.
Thus, the secondary valency of Cobalt (Co) is 6.
