Problem 13 - Chemistry in 30 days

Question

Among the following, the pair in which the two species are not isostructural is (a) \(\mathrm{IO}_{3}^{-}\)and \(\mathrm{XeO}_{3}\) (b) \(\mathrm{BH}_{4}^{-}\)and \(\mathrm{NH}_{4}^{+}\) (c) \(\mathrm{PF}_{6}^{-}\)and \(\mathrm{SF}_{6}\) (d) \(\mathrm{SiF}_{4}\) and \(\mathrm{SF}_{4}\)

Step-by-Step Solution

Answer

Option (d) \( \mathrm{SiF}_{4} \) and \( \mathrm{SF}_{4} \) are not isostructural.

Step 1: Define Isostructural Species

Isostructural species are species that have the same or similar structures. Typically, they possess the same number of atoms or ions in their molecular or ionic form and similar geometric arrangements.

Step 2: Analyze Option (a)

Consider \( \mathrm{IO}_{3}^{-} \) and \( \mathrm{XeO}_{3} \). Both have tetrahedral geometry and a total of four electron domains (three bonding pairs and one lone pair for \( \mathrm{IO}_{3}^{-} \), and four bonding pairs for \( \mathrm{XeO}_{3} \)). Thus, they can be considered isostructural.

Step 3: Analyze Option (b)

Examine \( \mathrm{BH}_{4}^{-} \) and \( \mathrm{NH}_{4}^{+} \). Both ions exhibit a tetrahedral geometry with four bonding pairs and no lone pairs, making them isostructural.

Step 4: Analyze Option (c)

Consider \( \mathrm{PF}_{6}^{-} \) and \( \mathrm{SF}_{6} \). Both have an octahedral geometry with six bonding pairs and no lone pairs. Hence, they are isostructural.

Step 5: Analyze Option (d)

Compare \( \mathrm{SiF}_{4} \) and \( \mathrm{SF}_{4} \). \( \mathrm{SiF}_{4} \) has a tetrahedral geometry with four bonding pairs and no lone pairs, whereas \( \mathrm{SF}_{4} \) has a seesaw shape with five electron domains (four bonding pairs and one lone pair). These structures are not similar; hence they are not isostructural.

Step 6: Conclusion

Based on the analyses, the pair that is not isostructural is \( \mathrm{SiF}_{4} \) and \( \mathrm{SF}_{4} \).

Key Concepts

Tetrahedral Geometry

Tetrahedral geometry is a symmetric arrangement that occurs when a central atom forms bonds with four other atoms positioned at the corners of an imaginary tetrahedron. This arrangement leads to bond angles of approximately 109.5 degrees. Understanding this concept is vital.

• The tetrahedral structure minimizes electron pair repulsion, leading to its distinct three-dimensional shape.
• A classic example is methane (\(\text{CH}_4\)), where a carbon atom is centrally bonded to four hydrogen atoms.
• In our exercise, both \(\mathrm{BH}_{4}^{-}\) and \(\mathrm{NH}_{4}^{+}\) display this geometry due to the presence of four single bonds and no lone electron pairs.

This geometry not only determines how atoms are arranged but also influences the molecule's properties, such as polarity and reactivity. As these examples show, molecules or ions with this structure can be isostructural if their spatial configurations and electron arrangements match.

Octahedral Geometry

Octahedral geometry is a symmetrical six-coordinate system. In this structure, the central atom forms connections with six other atoms or groups, placed at vertices of an octahedron.

• This structure generates equal angles of 90 degrees between any two bonds.
• Examples of this geometry include hexafluorophosphate (\(\mathrm{PF}_6^{-}\)) and sulfur hexafluoride (\(\mathrm{SF}_6\)). Both feature six bonds and lack lone pairs, making them isostructural.

Understanding octahedral geometry is crucial, especially when exploring compounds with extensive bonding interactions. Its high symmetry leads to unique properties, often reducing molecular polarity if identical bonds are present. This structure supports the formation of complex geometrical shapes, vital in areas such as coordination chemistry.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule, determined by the repulsions between electron pairs surrounding a central atom. Knowing the molecular geometry is essential for understanding the physical and chemical properties of substances.

• Different geometries lead to varying molecular shapes, influencing reactivity, polarity, and phase at room temperature.
• Geometries such as linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral provide insight into the molecule's behavior.
• In our exercise, the term 'isostructural' relates explicitly to similarities in these geometric configurations rather than purely the chemical formulae.

Molecular geometry significantly affects the molecule's pharmacological, catalytic, and spectroscopic properties. For instance, identifying the geometry helps predict reactivity patterns and how a molecule will interact with others. Studying model examples, such as those in our exercise, offers valuable insights into how changes in structure lead to differences in behavior.