Problem 122 - A Complete Resource Book in Chemistry for JEE Main

Question

The main draw back of valence bond theory is/are (a) It cannot explain magnetic properties of co-ordination compounds (b) It cannot distinguish between high spin and low spin complex (c) It does not provide an answer to the origin of characteristic colours of complex ions (d) It is a qualitative approach

Step-by-Step Solution

Answer

The main drawbacks of valence bond theory are all of the above options (a, b, c, and d).

Step 1: Identify the limitations

First, let's identify the possible limitations of the valence bond theory from the options given.

Step 2: Understand the Theory

Valence bond theory describes the bonding in terms of overlapping atomic orbitals but has several limitations. It does not provide fundamental insights into the magnetic properties of coordination compounds, cannot distinguish between high-spin and low-spin complexes, does not address the origin of characteristic colors of complex ions, and is fundamentally a qualitative approach.

Step 3: Match the Limitations to Options

Let's match the identified limitations to each of the given options: (a) It cannot explain magnetic properties of coordination compounds - This is a known drawback. (b) It cannot distinguish between high spin and low spin complex - This is accurate. (c) It does not provide an answer to the origin of characteristic colors of complex ions - This is a limitation. (d) It is a qualitative approach - True, it is qualitative.

Step 4: Final Conclusion

From the matching step, we can conclude that all the mentioned drawbacks (options a, b, c, and d) are indeed limitations of the valence bond theory.

Key Concepts

Coordination Compounds

Coordination compounds are chemical structures where a central metal atom is bonded to a surrounding array of molecules or ions, known as ligands. These complexes are fundamental in the study of chemistry for their diverse applications and intriguing properties.
Coordination compounds are important because they help explain many natural and synthetic processes, including enzyme function, mineral formation, and new material development.

• The central atom or ion is usually a transition metal. This metal serves as the hub around which ligands coordinate.
• Ligands have lone electron pairs that they donate to the metal center, forming coordinate covalent bonds.
• The arrangement and type of ligands give the complex its unique properties.

These structural arrangements influence the behavior and properties of the coordination compounds, such as stability, color, and reactivity, making them essential to understand in the field of chemistry.

Magnetic Properties

The magnetic properties of coordination compounds are closely linked to the arrangement and interaction of electrons around the central metal ion.
Valence Bond Theory struggles to fully explain this, leading to its limitation. When the electrons in the d orbitals of a metal ion are unpaired, the complex exhibits magnetic properties. This is known as paramagnetism.

• Paramagnetic substances are attracted into magnetic fields due to these unpaired electrons.
• Diamagnetic compounds, on the other hand, have all their electrons paired and are slightly repelled by magnetic fields.

Understanding these complementary behaviors helps chemists design materials with specific magnetic properties needed in applications like magnetic resonance imaging (MRI) and data storage technology.

High-Spin and Low-Spin Complexes

High-spin and low-spin complexes arise due to the way ligands split the d orbitals of the metal center in a coordination compound. The splitting can lead to different electronic configurations.
Ligands cause a crystal field splitting, resulting in two sets of orbitals with different energies. Depending on the strength of the ligands and the size of the splitting, coordination compounds can be classified as high-spin or low-spin:

• High-spin complexes have a smaller splitting between orbitals and electrons occupy the higher energy orbitals keeping them "unpaired." Take note of how this often leads to paramagnetic nature.
• Low-spin complexes result from larger splitting, where electrons tend to pair up in lower energy orbitals, often leading to diamagnetism.

The nature of the ligand, known as the 'ligand field strength,' plays a crucial role, dictating whether a complex becomes high-spin or low-spin, significantly affecting its properties.

Characteristic Colors of Complex Ions

The colors exhibited by complex ions are one of their most fascinating features and depend directly on the electronic transitions within the complex.
When light strikes a complex ion, electrons can absorb specific wavelengths, promoting electron transitions between different energy levels of the d orbitals. As a result, this absorption leads to the appearance of color, which is the complementary color of the light absorbed.

• The color depends on the energy gap between split d orbitals, which is influenced by the ligand's field strength.
• Stronger ligand fields lead to a larger difference in energy, often resulting in distinct color changes.
• The specific ligand and its arrangement around the metal ion determine which wavelengths are absorbed.

Thus, when we observe the beautiful colors of these compounds, we are essentially witnessing the crystal field interactions at play, providing a visual representation of more complex chemical interactions.