Question

If \(\mathrm{PtCl_2 \cdot 2NH_3}\) does not react with \(\mathrm{AgNO_3}\), what will be its formula?

Hint:

No free \(\mathrm{Cl^-}\) means both chlorides are inside the coordination sphere: \([\mathrm{Pt(NH_3)_2Cl_2}]\).

Answer 1

Step 1: Understanding the Concept:
In a coordination compound, ligands inside the square brackets (the coordination sphere) are bonded to the metal and do not ionise in water. Only the ions outside the bracket ionise. Silver nitrate gives a white precipitate of \(\mathrm{AgCl}\) only with chloride ions that are free in solution.

Step 2: What the observation tells us:
\(\mathrm{PtCl_2 \cdot 2NH_3}\) gives no precipitate with \(\mathrm{AgNO_3}\). So no chloride is free. Both \(\mathrm{Cl^-}\) ions are inside the coordination sphere, attached to platinum.

Step 3: Write the formula:
The two \(\mathrm{NH_3}\) molecules and the two \(\mathrm{Cl^-}\) ions all sit in the coordination sphere, so the coordination number of \(\mathrm{Pt}\) is 4. The oxidation state is \(0 + 0 + 2(-1) + x = 0\), so \(x = +2\).

Final Answer:
The formula is \([\mathrm{Pt(NH_3)_2Cl_2}]\).