Q.Which of the following molecule contains no π bond?

Within valence bond theory, the specific GEOMETRY of atomic orbital overlap determines whether a covalent bond is classified as sigma or pi.

A sigma (σ) bond results from HEAD-ON (axial) overlap, where the two orbitals overlap directly along the line joining the two nuclei — also called axial overlap. Any overlap involving an s orbital (s-s or s-p) is always sigma, because the spherically-symmetric s orbital overlaps head-on regardless of approach direction; two p orbitals overlapping ALONG the molecular axis (e.g. pₓ-pₓ, if x is the molecular axis) are likewise sigma.

A pi (π) bond results from SIDEWAYS (lateral) overlap, where two p orbitals lie parallel to one another but perpendicular to the internuclear axis, and overlap above/below (or in front of/behind) that axis rather than head-on (e.g. p_y-p_y or p_z-p_z, if x is the molecular axis). …