Hypervalent molecule

Hypervalent molecule refers to a type of chemical molecule that contains one or more atoms with more than eight electrons in their valence shell. This concept challenges the traditional Lewis structure theory, which suggests that the octet rule is a universal guideline for molecule formation. Hypervalent molecules are prevalent in the third period of the periodic table and beyond, where elements have d orbitals available for bonding. These molecules play a crucial role in various chemical reactions and are fundamental in the fields of organic chemistry, inorganic chemistry, and pharmaceutical chemistry.

Overview

The term "hypervalent" was first introduced by Jeremy I. Musher in 1969 to describe molecules that exceed the octet rule. Hypervalent molecules typically involve elements from the third period or higher, including phosphorus, sulfur, and iodine. For example, phosphorus pentachloride (PCl₅) and sulfur hexafluoride (SF₆) are classic examples of hypervalent molecules.

Formation

Hypervalent molecules form through the expansion of the valence shell, utilizing d orbitals to accommodate more than eight electrons. This expansion is possible due to the increased energy level of the third period elements, which allows for the involvement of d orbitals in bonding. The formation of hypervalent molecules is influenced by factors such as the electronegativity of the involved atoms and the molecule's geometric configuration.

Characteristics

Hypervalent molecules exhibit several distinctive characteristics:

• They often have a central atom bonded to more than four atoms.
• These molecules can display various geometrical shapes, such as trigonal bipyramidal and octahedral.
• The bonds in hypervalent molecules can have partial ionic and covalent character, contributing to their stability.
• Hypervalent molecules are typically more reactive than their non-hypervalent counterparts due to the presence of highly polarizable electrons.

Applications

Hypervalent molecules are utilized in a wide range of applications, including:

• Catalysis, where they act as catalysts in organic synthesis reactions.
• Material science, in the development of new materials with unique properties.
• Pharmaceuticals, where they are used in the synthesis of drugs and active pharmaceutical ingredients (APIs).

Controversy

The concept of hypervalency has been a subject of debate among chemists. Critics argue that the d orbitals play a minimal role in the bonding of these molecules, suggesting that the term "hypervalent" might be misleading. Instead, they propose alternative explanations based on electron delocalization and resonance structures.

See Also

• Octet rule
• Valence shell electron pair repulsion theory (VSEPR)
• Molecular orbital theory
• Covalent bond