Problem 58 - A Complete Resource Book in Chemistry for JEE Main

Question

The coordination number of \(\mathrm{Fe}(\mathrm{II})\) in oxyhaemoglobin is (a) 10 (b) 8 (c) 6 (d) 4

Step-by-Step Solution

Answer

The coordination number of Fe(II) in oxyhaemoglobin is 6.

Step 1: Understanding Coordination Number

The coordination number of an atom in a molecule or crystal is the number of atoms, ions, or molecules that it is directly bonded to.

Step 2: Identify the Complex

In the case of oxyhaemoglobin, the \( ext{Fe(II)}\) ion is part of a heme group.

Step 3: Count the Bonding Interactions

In the heme group of oxyhaemoglobin, \( ext{Fe(II)}\) forms four bonds with nitrogen atoms in the plane of the porphyrin ring. It also forms one bond with the histidine residue from the protein and one additional bond with the oxygen \(O_2\) molecule.

Step 4: Calculate the Coordination Number

Adding these interactions together, \( ext{Fe(II)}\) in oxyhaemoglobin is bonded to a total of six atoms or groups.

Key Concepts

Coordination Number

The coordination number is a fundamental concept in coordination chemistry. It defines the number of bonds an atom has with surrounding atoms, ions, or molecules. For metals, this typically refers to the number of donor atoms bonded directly to the central metal ion.
Understanding the coordination number is crucial since it influences the geometry and therefore the properties of the complex. In many biological and inorganic systems, the coordination number can hint at the overall efficiency and role of the complex.
In coordination complexes:

• Higher coordination numbers often lead to more stable complexes.
• The coordination number dictates the shape around the central atom, ranging from geometric structures like tetrahedral, square planar, or octahedral.
• Complexes with different coordination numbers may exhibit various physical and chemical behaviors, suggesting why precise calculation of the coordination number matters in complex biochemistry.

Oxyhaemoglobin

Oxyhaemoglobin is an essential component of the human respiratory system. It is a form of hemoglobin bound to oxygen, responsible for the transportation of oxygen using red blood cells.
In this complex, iron is present in the ferrous state ( Fe(II)) and binds with oxygen (O_2), resulting in an octahedral arrangement around the iron atom.
The heme group, which is part of the larger hemoglobin protein:

• Contains a central Fe(II) ion.
• Forms a planar porphyrin ring, involving four nitrogen atoms in coordination with the iron.
• Facilitates further bonding with one histidine residue from the hemoglobin protein and the oxygen molecule, achieving a coordination number of six.

This ability to bind oxygen is vital for metabolic processes, emphasizing the biochemical significance of oxyhaemoglobin in maintaining life processes.

Fe(II) Complexes

Fe(II) complexes are a significant topic within coordination chemistry, especially due to their widespread presence in biological and industrial systems.
Iron, in the +2 oxidation state, tends to form complexes easily due to its moderate electron affinity. The chemistry of Fe(II) relies heavily on:

• Its ability to participate in electron transfer due to its accessible oxidation state.
• Formation of colorful complexes due to d-d electronic transitions.
• The tendency to form both high-spin and low-spin complexes, affecting their magnetic properties.

In biological systems, Fe(II) complexes such as those found in hemoglobin are crucial for life functions:This state allows for reversible bonding with dioxygen, crucial for the transport of oxygen in the bloodstream.
Their coordination geometry often affects how these complexes interact with other biological molecules, directly influencing their reactivity and role in various metabolic pathways.