Question

In aqueous solution 1 mole of \[CoCl3.4NH3\] gives 1 mole of \[AgCl\] precipitate with excess\[AgNO3\], then what is the secondary valency of metal?
(a) 3
(b) 4
(c) 5
(d) 6

Answer 1

Hint: The number of secondary valencies equal to the number of ligand atoms coordinated to the metal i.e. coordination. Every metal has a fixed number of secondary valencies.

Complete solution step by step:
As per question, 1 mole of \[CoCl3.4NH3\] gives mole of \[AgCl\] that means one \[Cl\]formed a primary valency and represented as
\[[Co(NH3)4Cl2]Cl\]
In other terms, the complex \[[Co(NH3)4Cl2]Cl\] ionises to give \[[Co(NH3)4Cl2]+\]and\[Cl-\]. So there is only one \[Cl-\] that can be precipitated as \[AgCl\]. Therefore secondary valency of metal is 6.

Option (4) is the correct answer.

Additional information:
In coordination compound, metals exhibits two types of valencies – one is primary valency (also known as principle valency or ionisable valency) and other is secondary valency (also called as auxiliary valency or non-ionisable valency). The combining sphere consists of the power of a metal atom into two spheres of attraction-primary sphere and secondary sphere. The primary sphere satisfies the primary valency of the metal whereas secondary spheres satisfy the secondary valency of the metal.Secondary valency satisfied by either neutral molecules or negative ions or in rare cases by positive ions whereas on the other hand primary valency satisfied by negative ions.
According to the modern concept of coordination compounds, the species which satisfy the secondary valency are called ligands and the primary valency is the oxidation state of the central metal ion. Primary valency is non-directional whereas secondary valency has a fixed orientation in space around the metal. The number of species satisfying secondary valency gives a definite geometry to the coordination. When coordination compound undergoes ionisation in aqueous medium, the species which satisfy the secondary valency cannot be obtained in free state whereas the species which satisfy the primary valency can be obtained in their free state.

Note: The primary valency is the number of negative ions which are the same as charge on metal ions whereas secondary valency is the number of ligands as coordinated to the metal ion.