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. 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) potassium tetracyanonickelate(II), potassium tetrachloronickelate(II)

Step 1: Understand complex formation with KCN

Nickel(II) chloride (\(\text{NiCl}_2\)) in the presence of excess potassium cyanide (\(\text{KCN}\)) forms a complex ion by replacing the chloride ions with cyanide ions to produce \(\text{K}_2[\text{Ni(CN)}_4]\), a cyanide complex. Since the coordination number is 4, four cyanide ions coordinate with the nickel ion.

Step 2: Write the chemical formula for cyano complex

The cyano complex formed is \(\text{K}_2[\text{Ni(CN)}_4]\). The complex ion \([\text{Ni(CN)}_4]^{2-}\) is coordinated by four cyanide ions \((\text{CN}^-\)), and the potassium ions balance the charge of the complex.

Step 3: Assign IUPAC name to cyano complex

The IUPAC name for \(\text{K}_2[\text{Ni(CN)}_4]\) is potassium tetracyanonickelate(II). The nickel is in a -2 complex ion state, thus contributing to the overall neutral charge when combined with two potassium ions.

Step 4: Understand complex formation with conc. HCl

Nickel(II) chloride (\(\text{NiCl}_2\)) in the presence of excess concentrated hydrochloric acid (conc. HCl) forms a chloro complex where chloride ions act as ligands. The resulting complex is \(\text{K}_2[\text{NiCl}_4]\). The chloride ions replace the usual coordination water molecules due to excess HCl.

Step 5: Write the chemical formula for chloro complex

The chloro complex formed is \(\text{K}_2[\text{NiCl}_4]\), where four chloride ions \((\text{Cl}^-\)) coordinate with the nickel ion, resulting in the complex ion \([\text{NiCl}_4]^{2-}\).

Step 6: Assign IUPAC name to chloro complex

The IUPAC name for \(\text{K}_2[\text{NiCl}_4]\) is potassium tetrachloronickelate(II). Here again, the nickel is in a -2 complex ion state.

Step 7: Compare against options to determine correct answer

After deriving the IUPAC names, compare them to the given options. The correct names are potassium tetracyanonickelate(II) for \(\text{A}\) and potassium tetrachloronickelate(II) for \(\text{B}\), leading to option (a) as the answer.

Key Concepts

Complex Ion Formation

Complex ion formation is a central concept in coordination chemistry. It involves the interaction between metal ions and molecules or ions called ligands. These ligands surround the metal ion through coordinate covalent bonds. In this interaction, metal ions typically have empty orbitals that accept electron pairs from ligands.
In the given exercise, we observe nickel(II) chloride ( \(\text{NiCl}_2\) ) reacting with excess potassium cyanide ( \(\text{KCN}\) ) or concentrated hydrochloric acid (conc. HCl). Each scenario leads to complex formation.
When \(\text{NiCl}_2\) reacts with \(\text{KCN}\) , the chloride ions in nickel chloride are replaced with cyanide ions. This produces a complex ion, \([\text{Ni(CN)}_4]^{2-}\) . In the presence of conc. HCl, chloride ions replace potential ligands in the nickel coordination environment, resulting in \([\text{NiCl}_4]^{2-}\) . Whenever such reactions occur, we see specific geometric and electronic considerations inherent in complex ion formation.

Coordination Number

The coordination number is the number of ligand atoms that are bonded directly to a central metal ion. This varies depending on several factors, including the size and charge of the ligands and the metal ion itself. Typically, coordination numbers range from 2 to 12.
In this exercise, we observe that the coordination number for nickel (\(\text{Ni}^{2+}\)) is 4. Since the nickel ion forms a 2- bond with the \([\text{Ni(CN)}_4]^{2-}\) and \([\text{NiCl}_4]^{2-}\) complexes, four ligands bond with the nickel for each complex respectively.
Such a coordination number often leads to a square planar or tetrahedral spatial arrangement of the ligand atoms around the metal ion, which is common for metal ions like nickel in coordination compounds.

IUPAC Nomenclature

Coordination complexes are named according to the International Union of Pure and Applied Chemistry (IUPAC) nomenclature. This systematic approach helps in clearly describing the composition and structure of coordination entities.
Names are constructed by listing the ligands in alphabetical order followed by the name of the central metal and its oxidation state in roman numerals.
For complexes such as \(\text{K}_2[\text{Ni(CN)}_4]\) and \(\text{K}_2[\text{NiCl}_4]\) , the respective names are derived as follows:

• In the cyano complex, "potassium" is the counter ion, "tetracyano-" denotes the four cyanide ligands, and "nickelate(II)" suggests the oxidation state of nickel.
• For the chloro complex, "potassium" is again the counter ion, "tetrachloro-" indicates four chloride ligands, and "nickelate(II)" details the oxidation state.

This practice ensures that any chemist can interpret the formula and know the components and structure of the compound.

Ligands

Ligands are ions or molecules that bind to a central metal atom to form a coordination complex. They are crucial in determining the properties and reactivity of the metal complexes. Ligands can be neutral molecules like water or ammonia, or anionic like chloride (\(\text{Cl}^-\)) and cyanide (\(\text{CN}^-\)).
In our exercise, the ligands involved are chloride ions in one context and cyanide ions in another. Cyanide ion is particularly known as a strong field ligand because it can result in pairing of the electrons in the metal's d orbitals. Chloride ions are weaker ligands compared to cyanide ions.

• Cyanide ions in \([\text{Ni(CN)}_4]^{2-}\) lead to a stable complex that reflects high affinity of nickel for such ligands, forming a square planar geometry.
• Chloride ions in \([\text{NiCl}_4]^{2-}\) form weaker interactions, often resulting in more tetrahedral geometries depending on the surrounding conditions and environment.

Thus, the choice and nature of ligands are vital in dictating the structure and stability of the formed complexes.