Question

Which of the following statement is correct? (a) \(\mathrm{Al}\) forms \(\left[\mathrm{AlF}_{6}\right]^{3-}\) ion but \(\mathrm{B}\) does not form \(\left[\mathrm{BF}_{6}\right]^{3-}\) ion. (b) Boron and aluminium halides behave as Lewis acids. (c) The \(\mathrm{p} \pi-\mathrm{p} \pi\) back bonding occurs in the halides of boron and not in those of aluminium. (d) All of these

Short answer

Option (d) 'All of these' is correct as all statements (a), (b), and (c) are accurate.

Step-by-step solution

Understand the Coordination Ability

Aluminum (Al) and boron (B) are both group 13 elements but differ in size and electronic configuration. Aluminum can expand its coordination number to 6 due to the availability of d-orbitals. This allows aluminum to form complex ions like \([\text{AlF}_6]^{3-}\). Boron, on the other hand, does not have available d-orbitals for such expansion and therefore cannot form \([\text{BF}_6]^{3-}\) ions.

Lewis Acidity of Halides

Both boron and aluminum halides are capable of accepting electron pairs, thus behaving as Lewis acids. They are electron-deficient and tend to acquire extra electrons to complete their octets.

Explore Back Bonding

In boron halides like BF3, back bonding can occur between the filled p-orbitals of the halide and the empty p-orbital of boron, creating \(p\pi - p\pi\) bonding. Aluminum ions do not exhibit \(p\pi - p\pi\) back bonding significantly due to larger size and the potential involvement of d-orbitals which changes the mechanism of bonding.

Apply Conclusions to Options

Option (a) is correct as \(\text{Al}\) forms \([\text{AlF}_6]^{3-}\) and \(\text{B}\) does not form \([\text{BF}_6]^{3-}\). Option (b) is correct because both elements act as Lewis acids. Option (c) is correct as \(p\pi - p\pi\) back bonding occurs prominently in boron halides but not in aluminum halides due to structural and orbital differences. Therefore, option (d) 'All of these' is the correct choice.

Key concepts

Lewis Acidity

Lewis acidity refers to the ability of a compound to accept electron pairs. In coordination chemistry, Lewis acids play a pivotal role as they can bind to electron-rich molecules, known as Lewis bases. This interaction is crucial for forming complex ions.

Boron and aluminum, being Group 13 elements, naturally exhibit Lewis acidity. They have three electrons in their outer shell, which makes them electron-deficient and eager to accept electron pairs to complete their octet. This is why both boron and aluminum halides behave as Lewis acids. Specifically, boron trihalides like BF extsubscript{3} can effectively act as Lewis acids, as they seek electron pairs to fill their outer electron shell.

Understanding Lewis acidity is fundamental to grasping how compounds can interact to form more complex structures, which is a cornerstone of coordination chemistry. This electron pair accepting feature highlights the importance of orbital structure in determining the reactivity and formation of compounds.

Ppi-Ppi Back Bonding

Ppi-Ppi back bonding is a fascinating concept in chemistry that occurs primarily in certain types of compounds. This type of bonding involves the overlap of p orbitals between atoms, allowing electrons to be shared in a manner that stabilizes the molecule.

In boron halides, such as BF extsubscript{3}, p extpi-p extpi back bonding can readily occur. This phenomenon happens when the filled p orbitals of a halide overlap with the empty p orbitals of boron. This type of bond is significant because it enhances the stability of boron halides. They often have additional electron density shared between the boron atom and the surrounding halide atoms.

However, aluminum halides do not exhibit this kind of bonding as prominently. Due to aluminum's larger atomic size and the presence of d-orbitals, the bonding mechanism changes, and the p extpi-p extpi overlap is less effective. Understanding p extpi-p extpi back bonding gives insight into the molecular structure and stability of complex molecules, significantly impacting their chemical behavior.

Group 13 Elements

The Group 13 elements, often known as the Boron group, are fascinating due to their unique properties and behavior in chemical reactions. This group consists of boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (Tl). Each element in this group has three electrons in their outermost shell, contributing to their similar chemical characteristics.

Despite these similarities, significant differences exist, particularly between the smaller boron and the larger aluminum. For instance, while boron does not have available d-orbitals, aluminum can use these orbitals to expand its coordination number. This ability allows aluminum to form complex ions such as \[ [\text{AlF}_6]^{3-} \] that are not possible for boron.

The presence or absence of d-orbitals can affect various properties, such as complex ion formation and types of chemical bonds, like p extpi-p extpi back bonding, significantly. Understanding these group characteristics provides a foundation for exploring the diverse chemistry of each element, offering insights into their applications and significance in the field.