4.6.3. Hybridisation of Elements involving d Orbitals

The elements present in the third period contain d orbitals in addition to s and p orbitals. The energy of the 3d orbitals are comparable to the energy of the 3s and 3p orbitals. The energy of 3d orbitals are also comparable to those of 4s and 4p orbitals.

In the periodic table, elements in the third period (like sodium, magnesium, and aluminum) have not only s and p orbitals but also d orbitals. This means they can form hybrid orbitals that include these d orbitals. The significance of the d orbitals here is that their energy levels are similar to s and p orbitals, allowing them to mix or hybridize, which can lead to the formation of more complex bonding patterns.

Think of hybridisation like mixing colors in art. Just as combining blue (s orbitals) and yellow (p orbitals) creates green (a new hybrid orbital), including other colors (d orbitals) allows for a wider range of colors and shades, making the results more diverse and vibrant.

The important hybridisation schemes involving s, p and d orbitals are summarised below: (i) Formation of PCl5 (sp3d hybridisation): (ii) Formation of SF6 (sp3d2 hybridisation):

Hybridisation involving d orbitals includes specific types such as sp3d and sp3d2. For example, in PCl5 (phosphorus pentachloride), phosphorus undergoes sp3d hybridisation, combining one s, three p, and one d orbital to form five sp3d hybrid orbitals that are arranged in a trigonal bipyramidal shape. Similarly, in SF6 (sulfur hexafluoride), sulfur uses sp3d2 hybridisation, creating six equivalent hybrid orbitals arranged in an octahedral geometry, leading to the formation of six S-F sigma bonds.

Imagine organizing a team project: you can have different roles (orbitals) to contribute. If you have only a few roles (s and p), your options are limited (like less color mixing). But when d orbitals come into play, it's like adding more roles to the team. In PCl5, where you have five roles, everyone works together in specific ways (trigonal bipyramid) to achieve the project's goals efficiently, while in SF6, six roles create an octahedral structure, ensuring all areas are covered effectively.

These hybrid orbitals overlap with p orbitals of chlorine to form five P-Cl sigma bonds. The geometry of the PCl5 molecule is trigonal bipyramidal, where bond angles are 120° between the equatorial positions and 90° between equatorial and axial positions.

In hybridisation, the shape of the molecule is determined by how these new hybrid orbitals are oriented in space. For PCl5, the sp3d hybrid orbitals arrange themselves to minimize electron pair repulsion, resulting in the unique trigonal bipyramidal shape. This means that in a PCl5 molecule, three chlorine atoms sit in a plane forming a 'belt' around phosphorus, while the other two chlorine atoms are positioned above and below this plane, leading to two distinct bond angles (120° and 90°).

If you think of a party arrangement as a spatial problem, the host (P) wants to minimize awkwardness (repulsion) among guests (Cl). Therefore, the host puts three guests around a round table (equatorial positions) and stands two guests (the axial ones) at the edges of the table to maintain a balance, leading to an organized and comfortable interaction.

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