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

(a) potassiumtetracyanonickelate(II), potassiumtetrachloronickelate(II).

Step 1: Understand the reaction with KCN

The reaction of \(\text{NiCl}_2\) with an excess of \(\text{KCN}\) forms a cyano complex where the \(\text{Ni}^{2+}\) ion coordinates with the cyano groups. This forms \(\text{K}_2[\text{Ni(CN)}_4]\). The coordination number of 4 for \(\text{Ni}^{2+}\) is fulfilled by the four cyanide ligands.

Step 2: Identify compound A

Compound A is identified as \(\text{K}_2[\text{Ni(CN)}_4]\). The IUPAC name of this compound is potassium tetracyanonickelate(II). Here, 'tetracyano' refers to the four cyanide ions coordinating to nickel, and 'nickelate' indicates that it is a nickel complex.

Step 3: Understand the reaction with conc. HCl

The reaction of \(\text{NiCl}_2\) with concentrated HCl (in excess) forms a chloro complex where the \(\text{Ni}^{2+}\) ion coordinates with chlorine atoms. This forms \(\text{NiCl}_4^{2-}\), a tetrachloro species.

Step 4: Identify compound B

Compound B is identified as \([\text{NiCl}_4]^{2-}\). When combined with potassium ions from \(\text{KCl}\) (formed in the reaction), it forms \(\text{K}_2[\text{NiCl}_4]\). The IUPAC name of this compound is potassium tetrachloronickelate(II). "Tetrachloro" refers to the four chloride ions coordinated to nickel.

Step 5: Match options with the IUPAC names identified

The correct IUPAC names for compound A (potassium tetracyanonickelate(II)) and compound B (potassium tetrachloronickelate(II)) must now be matched with the provided options. Thus, the correct option is (a) potassiumtetracyanonickelate(II), potassiumtetrachloronickelate(II).

Key Concepts

IUPAC nomenclature

IUPAC nomenclature is a systematic method of naming chemical compounds as recommended by the International Union of Pure and Applied Chemistry (IUPAC). It provides a standardized way to name compounds so that chemists can communicate with precision and clarity. When naming coordination compounds, there are a few rules to follow:

• Ligands are named first, in alphabetical order, with any prefixes indicating the number of each kind of ligand (e.g., di-, tri-, tetra- for two, three, four, respectively).
• Anionic ligands often have their names ending in 'o', such as chlorido for Cl^- and cyanido for CN^-.
• The central atom/ion follows, with its oxidation state indicated by Roman numerals in parentheses.
• If the complex is an anion, the metal often ends with the suffix '-ate', and its Latin name may be used if available.

In the case of our exercise, compound A is \[\text{K}_2[\text{Ni(CN)}_4]\], whose IUPAC name is potassium tetracyanonickelate(II). Here, 'tetracyano' refers to the presence of four cyanide ligands, and 'nickelate' indicates it's a nickel complex in ionic form. Compound B, \[\text{K}_2[\text{NiCl}_4]\], is potassium tetrachloronickelate(II), where 'tetrachloro' means four chloride ligands bound to nickel.

Nickel complexes

Nickel complexes are coordination compounds that feature nickel as the central metal, coordinating with various ligands. These ligands can be neutral molecules like water or ammonia, or ions such as cyanide (CN^-) and chloride (Cl^-).

• The nickel ion in its +2 oxidation state is noted as \[\text{Ni}^{2+}\]. It commonly exhibits a coordination number of 4 or 6 but also can adapt to other numbers depending on the ligands present and their steric hindrance.
• Nickel forms a variety of geometries based on the coordination number and ligand type, including tetrahedral and square planar geometries, useful in applications like catalysis and organic synthesis.

Our study involves two nickel complexes: \[\text{K}_2[\text{Ni(CN)}_4]\] (from reacting NiCl₂ with KCN) and \[\text{K}_2[\text{NiCl}_4]\] (from reacting NiCl₂ with HCl in excess). Both complexes highlight the versatility of nickel in forming different compounds through ligand exchange, depending on the reacting acids or bases.

Coordination number

The coordination number is a key concept in coordination chemistry that refers to the total number of ligand atoms that are directly bonded to the central metal ion. This number largely determines the geometry and stability of the complex.

• In octahedral complexes, the coordination number is typically 6, such as in \[\text{[Fe(H}_2\text{O)}_6]^{3+}\]. Conversely, tetrahedral or square planar complexes often have a coordination number of 4, which we see with \[\text{Ni}^{2+}\] in this exercise.
• The choice of ligands can significantly affect the coordination geometry. For example, cyanide is a strong field ligand and tends to form square planar complexes with nickel, as seen in \[\text{K}_2[\text{Ni(CN)}_4]\].
• Understanding coordination numbers helps predict the physical and chemical properties of the compound, like solubility and electronic interaction.

In the provided examples, the coordination number is consistently 4, resulting in stable nickel cyano and chloro complexes, demonstrating nickel's adaptability in varying coordination environments.