Problem 121 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify the coordination compounds (a) \(\mathrm{FeSO}_{4} \cdot\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} \cdot 6 \mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{Fe}(\mathrm{CN})_{2} \cdot 4 \mathrm{KCN}\) (c) \(\mathrm{KCl} \cdot \mathrm{MgCl}_{2} .6 \mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{CuSO}_{4} .4 \mathrm{NH}_{3}\)

Step-by-Step Solution

Answer

Compounds (b) and (d) are coordination compounds.

Step 1: Understanding Coordination Compounds

Coordination compounds consist of a central atom or ion, typically a metal, surrounded by a number of other molecules or ions called ligands. These compounds are distinct from simple ionic or molecular compounds because the ligands are attached to the central metal through coordinate covalent bonds.

Step 2: Analyze Compound (a)

The compound given is \(\mathrm{FeSO}_{4} \cdot\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} \cdot 6 \mathrm{H}_{2} \mathrm{O}\). It is commonly known as Mohr's salt. Since there are no ligands coordinating with the central metal atom Fe, it does not form a coordination compound.

Step 3: Analyze Compound (b)

The compound given is \(\mathrm{Fe}(\mathrm{CN})_{2} \cdot 4 \mathrm{KCN}\). Here, \(\mathrm{CN}^{-}\) acts as a ligand coordinating with the central \(\mathrm{Fe}\) atom. The compound forms a complex ion \([\mathrm{Fe} (\mathrm{CN})_{6}]^{4-}\). Therefore, this is a coordination compound.

Step 4: Analyze Compound (c)

The compound given is \(\mathrm{KCl} \cdot \mathrm{MgCl}_{2} .6 \mathrm{H}_{2} \mathrm{O}\). This compound is a combination of KCl, MgCl2, and water. It does not involve any complex ion formation and hence is not a coordination compound.

Step 5: Analyze Compound (d)

The compound given is \(\mathrm{CuSO}_{4} .4 \mathrm{NH}_{3}\). Here, \(\mathrm{NH}_{3}\) acts as a ligand coordinating around the central \(\mathrm{Cu}\) atom, forming the complex ion \([\mathrm{Cu}(\mathrm{NH}_{3})_{4}]^{2+}\). Thus, this is a coordination compound.

Key Concepts

Ligands

Ligands are key players in the formation of coordination compounds. A ligand is essentially a molecule or ion that donates a pair of electrons to form a bond with a central metal atom or ion. In simpler terms, ligands surround the metal and are attached to it like friends surrounding a person at a party.
Common examples of ligands include water ( H_2O ), ammonia ( NH_3 ), and cyanide ( CN^- ). Each of these ligands has a pair of electrons ready to share with the metal.
There are multiple types of ligands depending on the number of electron pairs they donate:

• Monodentate ligands: Donate one pair of electrons. Examples include H_2O and NH_3 .
• Bidentate ligands: Donates two pairs of electrons, like ethylenediamine ( C_2H_4(NH_2)_2 ).
• Polydentate ligands: Donates multiple electron pairs, such as EDTA, which can donate up to six pairs!

Ligands define the geometry and properties of a coordination complex, making them crucial to understanding how these compounds operate.

Central Metal Atom

The central metal atom is like the host at the party in a coordination compound scenario. It is the nucleus around which ligands gather to form a stable structure. Generally, the central metal atom is a transition metal such as iron (Fe), copper (Cu), or magnesium (Mg).
These metals are prime candidates for this role because they have empty orbitals that can accommodate additional electrons from the ligands. This makes them ideal for forming coordinate covalent bonds.
A few important roles of the central metal atom are:

• An anchor: It holds the ligands together, creating the entire complex structure.
• Electronic influence: Affects the electronic properties of the entire compound. For instance, iron can stabilize multiple oxidation states, influencing the overall chemistry.
• Color contributor: Transition metals often give colors to the compounds because of d-d electron transitions.

Understanding the role of the central metal atom helps us determine the stability and reactivity of the coordination compound.

Coordinate Covalent Bonds

Coordinate covalent bonds are the special connections in coordination compounds. Unlike regular covalent bonds, where each atom contributes one electron, in coordinate covalent bonds, one atom donates both electrons to form the bond.
This type of bonding is central to the creation of coordination compounds. Typically, the ligand donates the electron pair, which the metal atom "accepts" due to its empty orbitals.
Here's why they matter:

• Complex Stability: These bonds help establish a stable structure by holding ligands and the central metal together.
• Reactivity Control: The stability of these bonds can influence how the coordination compound reacts in chemical reactions.
• Solubility & Properties: Impact properties like solubility, reactivity, and conductivity.

Notably, because of the coordinate covalent bonds, coordination compounds exhibit unique chemical and physical properties, distinguishing them from normal ionic or covalent compounds.