Some of the first complexes discovered by Werner in the 1890...

Some of the first complexes discovered by Werner in the 1890 s had the empirical formulas given below. Also given are the number of chloride ions per formula unit precipitated by the addition of $\mathrm{Ag}^{+}(a q)$. Explain these observations. $$ \begin{array}{lc} \hline \begin{array}{l} \text { Empirical } \\ \text { formula } \end{array} & \begin{array}{l} \text { Number of Cl- per formula } \\ \text { unit precipitated by } \mathbf{A g}^{+}(a q) \end{array} \\ \hline \mathrm{PtCl}_{4} \cdot 6 \mathrm{NH}_{3} & 4 \\ \mathrm{PtCl}_{4} \cdot 5 \mathrm{NH}_{3} & 3 \\ \mathrm{PtCl}_{4} \cdot 4 \mathrm{NH}_{3} & 2 \\ \mathrm{PtCl}_{4} \cdot 3 \mathrm{NH}_{3} & 1 \\ \mathrm{PtCl}_{4} \cdot 2 \mathrm{NH}_{3} & 0 \\ \hline \end{array} $$

Some of the first complexes discovered by Werner in the 1890 s had the empirical formulas given below. Also given are the number of chloride ions per formula unit precipitated by the addition of $\mathrm{Ag}^{+}(a q)$. Explain these observations.
$$
\begin{array}{lc}
\hline \begin{array}{l}
\text { Empirical } \\
\text { formula }
\end{array} & \begin{array}{l}
\text { Number of Cl- per formula } \\
\text { unit precipitated by } \mathbf{A g}^{+}(a q)
\end{array} \\
\hline \mathrm{PtCl}_{4} \cdot 6 \mathrm{NH}_{3} & 4 \\
\mathrm{PtCl}_{4} \cdot 5 \mathrm{NH}_{3} & 3 \\
\mathrm{PtCl}_{4} \cdot 4 \mathrm{NH}_{3} & 2 \\
\mathrm{PtCl}_{4} \cdot 3 \mathrm{NH}_{3} & 1 \\
\mathrm{PtCl}_{4} \cdot 2 \mathrm{NH}_{3} & 0 \\
\hline
\end{array}
$$

Key Concepts

Werner's Coordination Theory

This theory, developed by Alfred Werner, established that coordination compounds consist of a metal ion surrounded by ligands that form the primary (inner) coordination sphere, while ions outside the coordination sphere act as counter ions. Werner's theory fundamentally distinguished between the metal-ligand bonds responsible for the compound's structure and the ionic interactions that influence its chemical behavior.

Coordination Sphere versus Counter Ions

In coordination compounds, ligands directly bonded to the central metal ion constitute the coordination sphere, while ions not directly coordinated to the metal are present as counter ions. The number of free chloride ions precipitated by Ag+ reveals how many chlorides are not bound to the metal, reflecting the distinct roles of ligands in the coordination sphere versus counter ions.

Precipitation Reactions for Structure Determination

The use of Ag+ to precipitate chloride ions is a classical chemical test that distinguishes which chloride ions exist as free counter ions and which are coordinated to the metal. When Ag+ is added, only the chloride ions not bound to the metal form an insoluble precipitate (AgCl), thereby providing insight into the compound's structure and the arrangement of ligands around the metal center.

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