Problem 103 - A Complete Resource Book in Chemistry for JEE Main

Question

The coordination number of \(\mathrm{Ni}^{2+}\) is 4 . \(\mathrm{NiCl}_{2}+\mathrm{KCN}\) (excess) \(\longrightarrow \mathrm{A}\) (Cyano complex) \(\mathrm{NiCl}_{2}+\) conc. \(\mathrm{HCl}\) (excess) \(\longrightarrow \mathrm{B}\) (chloro complex) The IUPAC name of \(\mathrm{A}\) and \(\mathrm{B}\) are (a) potassiumtetracyanonickelate(II), potassiumtetrachloronickelate (II) (b) tetracyanopotassiumnickelate (II), tetrachloropota-ssiumnickelate(II) (c) tetracyanonickel(II), tetrachloronickel(II) (d) potassium tetracyanonickel(II), potassium tetra-chloronickel(II)

Step-by-Step Solution

Answer

The IUPAC names are (d) potassium tetracyanonickel(II), potassium tetrachloronickel(II).

Step 1: Determine the Products

In the reaction of \( \mathrm{NiCl}_2 \) with excess \( \mathrm{KCN} \), the \( \mathrm{CN}^- \) ions replace the \( \mathrm{Cl}^- \) ions around the \( \mathrm{Ni}^{2+} \) ion to form the cyano complex. Similarly, when \( \mathrm{NiCl}_2 \) reacts with concentrated \( \mathrm{HCl} \), the \( \mathrm{Cl}^- \) ions form a chloro complex.

Step 2: Write the Formulas of Complexes A and B

The formula for the cyano complex A is \( \mathrm{K}_2[\mathrm{Ni(CN)}_4] \) because \( \mathrm{Ni}^{2+} \) coordinates with 4 \( \mathrm{CN}^- \) ligands. The formula for the chloro complex B is \( \mathrm{NiCl}_4^{2-} \) because \( \mathrm{Ni}^{2+} \) also coordinates with 4 \( \mathrm{Cl}^- \) ligands.

Step 3: Assign IUPAC Names to Complexes

For \( \mathrm{A} = [\mathrm{Ni(CN)}_4]^{2-} \), the IUPAC name is **potassium tetracyanonickelate(II)**, as potassium balances the charge. For \( \mathrm{B} = \mathrm{NiCl}_4^{2-} \), since \( \mathrm{Ni} \) does not require an additional cation to balance the charge, the IUPAC name is **potassium tetrachloronickel(II)**, assuming a potassium counter ion is present.

Key Concepts

Coordination Number

The coordination number is a crucial concept in coordination chemistry that indicates the number of ligands attached to the central metal ion. In the context of the given exercise, the coordination number of nickel(II) ion, denoted as \( \text{Ni}^{2+} \), is 4. This means there are four ligands, either \( \text{CN}^- \) (cyanide ions) or \( \text{Cl}^- \) (chloride ions), surrounding each nickel ion in the complexes formed. This number directly influences the geometry of the complex. For a coordination number of 4, the complex can adopt geometrical shapes, typically either a tetrahedral or square planar structure, depending on the specific metal and ligands involved. In many complexes of \( \text{Ni}^{2+} \), a square planar geometry is often observed due to the specific ligand interactions present.

IUPAC Nomenclature

Naming coordination compounds according to IUPAC conventions is vital for clear communication in chemistry. The names should convey the chemical composition and structural characteristics of the compound concisely. In the given exercise, we named two complexes:

• For the cyano complex A, the IUPAC name is "potassium tetracyanonickelate(II)," reflecting the presence of four cyano ligands around nickel, balanced by potassium ions.
• For the chloro complex B, the name "potassium tetrachloronickel(II)" indicates the four chloride ligands bound to nickel, with a presumption of potassium as counter ions to balance the charge.

The nomenclature follows the standard format: 1. Name the cation first, followed by the complex anion. 2. Use prefixes (di-, tri-, tetra-, etc.) to indicate the number of each type of ligand. 3. After the ligands, name the central metal atom and specify its oxidation state in Roman numerals.

Complex Ions

Complex ions are entities where a central metal ion is bonded to a surrounding array of molecules or ions known as ligands. These are typically formed through coordinate covalent bonds, where ligands donate at least one pair of electrons to the metal center. In the context of the original problem, nickel forms two distinct complex ions:

• One with cyanide ions \( \text{[Ni(CN)}_4\text{]}^{2-} \) in the cyano complex.
• Another with chloride ions \( \text{NiCl}_4^{2-} \) forming the chloro complex.

These complex ions play a crucial role in defining the physical and chemical properties of the compounds. The types of ligands and their coordination dictate attributes like color, magnetism, and solubility.

Ligands

Ligands are atoms, ions, or molecules that donate electron pairs to a central metal ion, forming complex structures in coordination chemistry. They play a fundamental role by stabilizing the metal and determining the properties of the complex ion. In the given exercise, we interact with two types of ligands:

• \( \text{CN}^- \) (cyanide ions), which are strong field ligands capable of stabilizing low-spin complexes.
• \( \text{Cl}^- \) (chloride ions), which are weak field ligands often leading to high-spin complexes.

Each ligand can donate an electron pair, forming coordinate covalent bonds with the nickel ion. The ligand-field strength influences the electronic configuration, affecting properties such as the magnetic behavior and the geometry of the complex ion.