Which among the following metals shows 6 secondary valencies?

(A) 1 mole of $PdCl_2.4NH_3$ reacts with excess of $AgNO_3$ to give 2 moles of $AgCl$ precipitated
(B) 1 mole of $NiCl_2.6H_2O$ reacts with excess of $AgNO_3$ to give 2 moles of $AgCl$ precipitated
(C) 1 mole of $PtCl_4.2HCl$ does not give reaction with $AgNO_3$
(D) 1 mole of $CoCl_3.4NH_3$ reacts with excess of $AgNO_3$ to gives 1 mole $AgCl$ precipitated
(E) 1 mole of $PtCl_2.2NH_3$ does not give reaction with $AgNO_3$

Choose the correct answer from the options given below:

(B), (C), (D) only

The correct answer is Option (1) → (B), (C), (D) only

To determine which metal shows 6 secondary valencies among the given options, let's break down each one in detail, focusing on coordination numbers and how they relate to secondary valencies:

Understanding Secondary Valencies

In coordination chemistry, the term "secondary valency" refers to the number of ligand bonds a metal can form in a complex. It indicates how many coordination sites a metal ion can occupy, which is often determined by its oxidation state and the nature of its ligands.

Analyzing Each Option

(A) 1 mole of PdCl₂·4NH₃

Palladium (Pd) typically forms complexes with coordination numbers of 2, 4, or 6, but in this case, with 4 ammonia ligands (which are monodentate), the coordination number is 4. Therefore, this complex does not show 6 secondary valencies.

(B) 1 mole of NiCl₂·6H₂O

Nickel (Ni) commonly exhibits a coordination number of 6, especially in octahedral complexes. In this complex, with 6 water molecules (which are also monodentate), Ni has 6 secondary valencies. When reacted with AgNO₃, it produces 2 moles of AgCl, indicating the presence of chloride ions, further supporting that Ni can accommodate 6 ligands.

(C) 1 mole of PtCl₄·2HCl

Platinum (Pt) in this oxidation state can typically form square planar complexes, resulting in a coordination number of 4. The fact that this complex does not react with AgNO₃ suggests that all the chloride ions are bonded to the platinum, thus confirming a coordination number of 4 and not showing 6 secondary valencies.

(D) 1 mole of CoCl₃·4NH₃

Cobalt (Co) can exhibit different coordination numbers. In this case, CoCl₃ can potentially coordinate with 6 ligands; however, since it reacts with excess AgNO₃ to yield only 1 mole of AgCl, it implies that only one chloride is available for reaction, suggesting that the remaining ligands are part of the complex. This may indicate a coordination number less than 6, although cobalt can exhibit 6 in other complexes.

(E) 1 mole of PtCl₂·2NH₃

This complex also suggests a coordination number of 4 because Pt generally forms 4-coordinate complexes in this state. It does not react with AgNO₃, indicating that both chloride ions are tightly bound and not available for reaction. Thus, it does not show 6 secondary valencies.

Conclusion

Based on this detailed analysis:

Nickel (B) clearly demonstrates 6 secondary valencies.

Cobalt (D) is somewhat ambiguous as it could exhibit a coordination number of 6 in some contexts, but in this case, it appears limited.

Palladium (A), Platinum (C), and Platinum (E) do not exhibit 6 secondary valencies.

Therefore, the correct answer is (B), (C), (D) only. This option highlights nickel as the most straightforward example of a metal with 6 secondary valencies