Question

In aqueous solution of 1 mole of $CoC{l_3} \cdot 4N{H_3}$ gives 1 mole of AgCl precipitate with excess $AgN{O_3}$, then what is the secondary valency of the metal?
(A) 3
(B) 4
(C) 5
(D) 6

Answer 1

Secondary valency of the central metal atom in the complex is the number of ligands that makes covalent coordinate bonds with the metal. 1 mole of $AgN{O_3}$ will form 1 mole of AgCl by reacting with 1 mole of chloride anions.

Complete step by step solution:
Here we are given a cobalt complex and we need to find the secondary valency of the cobalt in the given complex. First, let’s see what is meant by secondary valency.
-Secondary valency of the metal ion is the number of anions or neutral atoms bonded to it. In short, the number of ligands that are bonded to the central metal atom is the secondary valency of the metal atom.
-Here, we are given that 1 mole of $CoC{l_3} \cdot 4N{H_3}$ reacts with $AgN{O_3}$ to give 1 mole of AgCl.
-$AgN{O_3}$ has silver metal and it has a very high tendency to form silver halides as a product. So, if any ionic halide is there in the complex, then it will react with the $AgN{O_3}$ solution and will form AgCl.
-So, as 1 mole of AgCl is produced here from 1 mole of $CoC{l_3} \cdot 4N{H_3}$, the complex should have one chlorine atom in its ionic form. That means that one chlorine atom is not forming a covalent coordinate bond with the metal. Instead, it is forming an ionic bond with the cation.
-So, compound’s structure can also be written as $[CoC{l_2}{(N{H_3})_4}]Cl$
-Now, secondary valency is the number of ligands that are bonded with the metal ion with the covalent coordinate bond. Here, we can see that two chlorine atoms and four ammonia are bonded with a central metal ion.
-So, we can say that the secondary valency of the central metal atom in the given complex is 6.

Therefore, the correct answer is (D).

Note: Do not get confused between the primary and secondary valency of the metal. Primary valency of the metal is the number of negatively charged ions required to satisfy the positive charge of the metal atom.