Question

Which of the following orbital combinations does not form $\pi$ bond?
A.${p_x} + {p_x}$ sideways overlapping
B.${d_{{x^2} - {y^2}}} + {p_y}$ sideways overlapping
C.${d_{xy}} + {d_{xy}}$ sideways overlapping
D.${d_{yz}} + {p_y}$ sideways overlapping

Answer 1

$\pi$ bonds are the covalent bonds formed by the side to side overlapping of half- filled non hybridised atomic orbitals of an atom. These orbitals are known as pi orbitals and it is necessary that the pi orbitals lie in a parallel orientation in order to form a pi bond.

Complete step by step solution:
The bond formed by the sharing of electrons between two atoms is known as a covalent bond. The sharing occurs by the overlapping of different atomic orbitals. Head to head overlap of the atomic orbitals results in the formation of sigma bonds whereas, the lateral or side to side overlap of the orbitals results in the formation of pi bonds.
Considering option A, that is,${p_x} + {p_x}$ sideways overlapping.
The ${p_x}$ orbital lies on the x axis. Since both the orbitals are parallel to each other, pi bond is formed.
Considering option B, that is, ${d_{{x^2} - {y^2}}} + {p_y}$ sideways overlapping.
The ${d_{{x^2} - {y^2}}}$ and ${p_y}$ orbitals do not have a suitable orientation for sideways overlapping and thus pi bond is not formed.
Considering option C, that is, ${d_{xy}} + {d_{xy}}$ sideways overlapping.
Both the orbitals lie in the same plane parallel to each other and can form pi bonds.
Considering option D, that is ${d_{yz}} + {p_y}$ sideways overlapping.
Both the orbitals are parallel to the y axis and to each other and can form a pi bond.

Thus, the correct option to the question is B.

Note:
The sideways overlap of pi bonds occurs perpendicular to the inter- nuclear axis as the orbitals forming the pi bond lie parallel to each other and perpendicular to the inter- nuclear axis. In other words, the electron density of the pi bond is concentrated above and below the plane containing the nuclear axis. The extent of overlap in a pi bond is much less than that in sigma bond and thus, pi bond is weaker than sigma bond.