Problem 157 - A Complete Resource Book in Chemistry for JEE Main

Question

The coordination number of a central metal atom in a complex is determined by (a) the number of only anionic ligands bonded to the metal ion (b) the number of ligands around a metal ion bonded pi-bonds (c) the number of ligands around a metal ion bonded by sigma and pi-bonds (d) the number of ligands around a metal ion bonded by sigma bonds

Step-by-Step Solution

Answer

The correct answer is (d), the number of ligands around a metal ion bonded by sigma bonds.

Step 1: Understanding the Problem

We need to identify what a coordination number is in the context of a metal complex. The coordination number refers to the number of ligand donor atoms directly bonded to the central metal atom through sigma bonds primarily.

Step 2: Analyzing the Options

Let's evaluate each choice to determine which one accurately describes the coordination number: (a) This option considers only anionic ligands, which is not accurate since the coordination number counts all ligands regardless of their charge. (b) Pi-bonding comes into play in certain complexes, but coordination number primarily involves counting the direct sigma bonds. (c) This includes both sigma and pi bonds considering bonded ligands, which complicates the primary definition. (d) This option correctly identifies that the coordination number is determined by the number of ligands bonded to the metal ion through sigma bonds.

Step 3: Selecting the Correct Option

Based on the analysis, option (d) is correctly aligned with the definition of coordination number as it reflects the key feature: the number of sigma bonds formed between ligand donor atoms and the central metal atom.

Key Concepts

Central Metal Atom

In a metal complex, the central metal atom plays a crucial role as it is the main site where bonding with ligands occurs. This atom can be an element from the d-block of the periodic table, like transition metals. These elements are known for their ability to form various compounds due to their capability to bear different oxidation states.
A central metal atom is the focal point where

• Ligand coordination happens
• Electronic properties of the complex are defined
• Overall stability and geometry of the complex are determined

The coordination number, which signifies the number of ligand donor atoms directly attached to this atom, provides insights into the shape and reactivity of the complex. Depending on how this central entity bonds with ligands, the resultant structure can vary greatly, showcasing geometry styles like tetrahedral, octahedral, square planar, etc.

Metal Complex

A metal complex refers to a structured formation where a central metal atom is surrounded by molecules or ions known as ligands. These complexes are central to areas like coordination chemistry and bioinorganic chemistry. The ligands act as donors, donating electron pairs to the metal atom.
The nature of metal complexes is determined by several factors, such as:

• The type of metal
• The oxidation state of the metal
• The type and number of ligands

One of the main types of interactions observed in these complexes is through coordination bonds. Understanding these bonds helps predict the electronic properties and potential uses of the complexes. Metal complexes are pivotal in many biological and industrial processes, acting as catalysts or components of biological molecules like hemoglobin.

Sigma Bonds

Sigma bonds represent the basic form of a covalent bond in chemical structures. In the context of coordination chemistry, sigma bonds form the backbone of metal-ligand bonding. They are the primary way through which ligands are bonded to the central metal atom in a metal complex, determining the coordination number.
Characteristics of sigma bonds include:

• Forming through the end-to-end overlap of atomic orbitals
• Allowing free rotation of bonded atoms around the bond axis
• Being stronger and more stable than other types of covalent bonds, like pi bonds

Sigma bonds create a single covalent link that is not easily broken, which is why they are an important consideration in the stability and structure of a metal complex. Understanding sigma bonds is integral to comprehending how metal complexes form and react, particularly the contribution each ligand makes toward the overall configuration and function of the complex.