Question

Suggest a chemical test for distinguishing between (a) \(\left[\mathrm{Ni}\left(\mathrm{SO}_{4}\right)(\mathrm{en})_{2}\right] \mathrm{Cl}_{2}\) and \(\left[\mathrm{NiCl}_{2}(\mathrm{en})_{2}\right] \mathrm{SO}_{4} ;\) (b) \(\left[\mathrm{NiI}_{2}(\mathrm{en})_{2}\right] \mathrm{Cl}_{2}\) and \(\left[\mathrm{NiCl}_{2}(\mathrm{en})_{2}\right] \mathrm{I}_{2}\).

Short answer

For (a), Barium Chloride test will form a white precipitate with free SO4, distinguishing \(\left[\mathrm{NiCl}_{2}(\mathrm{en})_{2}\right]\mathrm{SO}_{4}\). For (b), Silver Nitrate test will form a yellow precipitate with free I-, distinguishing \(\left[\mathrm{NiI}_{2}(\mathrm{en})_{2}\right]\mathrm{Cl}_{2}\).

Step-by-step solution

Understand the Complexes

Identify that both pairs of complexes consist of Nickel (II) coordination compounds, where 'en' stands for ethylenediamine, a bidentate ligand. Also note that the anions and cations are arranged differently in each pair.

Suggest a Test for (a)

For distinguishing between \(\left[\mathrm{Ni}\left(\mathrm{SO}_{4}\right)(\mathrm{en})_{2}\right]\mathrm{Cl}_{2}\) and \(\left[\mathrm{NiCl}_{2}(\mathrm{en})_{2}\right]\mathrm{SO}_{4}\), perform a test for sulfate ions; add Barium Chloride (BaCl2) to both solutions. The complex with the SO4 as a part of the coordination sphere will not form a precipitate, whereas the complex with the free SO4 anion will form a white precipitate of Barium Sulfate (BaSO4).

Suggest a Test for (b)

For distinguishing between \(\left[\mathrm{NiI}_{2}(\mathrm{en})_{2}\right]\mathrm{Cl}_{2}\) and \(\left[\mathrm{NiCl}_{2}(\mathrm{en})_{2}\right]\mathrm{I}_{2}\), perform a test for iodide ions; add Silver Nitrate (AgNO3) to both solutions. The complex with the I2 as a part of the coordination sphere will not react, whereas the complex with the free I- anion will form a yellow precipitate of Silver Iodide (AgI).

Key concepts

Coordination Chemistry

Coordination chemistry involves the study of compounds that have a central metal atom or ion, typically a transition metal, surrounded by molecules or anions known as ligands. These ligands can donate pairs of electrons to the metal, forming coordination bonds and resulting in complex structures known as coordination compounds or complexes. A key aspect to understand is the coordination number, which indicates the number of ligand donor atoms bonded to the central ion, and the geometric arrangement of the ligands around the metal.

In coordination compounds, ligands can be neutral molecules like water or ammonia, or ions like chloride or hydroxide. Bidentate ligands, such as ethylenediamine (en) in the given complexes, can form two bonds to the metal. This versatility allows for a robust field of chemistry where various properties like color, solubility, and reactivity are influenced by the nature of the metal-ligand interactions.

Test for Sulfate Ions

To identify sulfate ions (\text{SO}_4^{2-}) in a solution, a reliable and common method is the barium chloride test. In a solution containing sulfate ions, when barium chloride (\text{BaCl}_2) is added, a white precipitate of barium sulfate (\text{BaSO}_4) will form. This reaction occurs because \text{BaSO}_4 is insoluble in water and precipitates out of the solution.

The chemical equation for this reaction is:\[\text{Ba}^{2+}(aq) + \text{SO}_4^{2-}(aq) \rightarrow \text{BaSO}_4(s)\]
For a sulfate ion test to be accurate, the sulfate must be free and not part of a complex's coordination sphere, as the barium ions cannot displace ligands coordinated to a metal center.

Test for Iodide Ions

Testing for the presence of iodide ions (\text{I}^-) involves the addition of silver nitrate (\text{AgNO}_3) to the solution. If iodide ions are free in the solution, they will react with silver ions to form a yellow precipitate of silver iodide (\text{AgI}):
\[\text{Ag}^+(aq) + \text{I}^-(aq) \rightarrow \text{AgI}(s)\]

Sensitivity of the Test

Iodide ions form a precipitate with silver ions even at very low concentrations, making it a sensitive test for iodide detection. Similar to testing for sulfate ions, if the iodide ions are bound within a coordination compound, they will not be available to form a precipitate with silver ions. Therefore, this test distinguishes between free iodide ions and those sequestered within a complex.

Nickel (II) Complexes

Nickel (II) complexes, such as the ones in the given exercise, contain nickel in the +2 oxidation state. This is a common and stable oxidation state for nickel, facilitating the formation of a variety of coordination compounds with different ligands. Nickel (II) typically forms octahedral complexes when bound with six donor atoms from the ligands.

Importance in Coordination Chemistry

Nickel (II) ions are particularly interesting due to their preferences for certain ligands and geometries, which is essential in the synthesis of targeted compounds in medicinal and material chemistry. In the context of the exercise, Nickel (II) complexes with ethylenediamine as a bidentate ligand demonstrate how ligands can significantly alter the physical and chemical properties of a coordination compound, including its reactivity towards various chemical tests.