Question

Compound with no sigma bond?

Could suggest a compound with no sigma bond only with the pi bond?

Answer 1

That will never happen. The first chemical bond to be formed has to be via direct overlap of atomic orbitals, and must be a `sigma` bond. Since `pi` bonds are via side-on overlap, which is inherently less overlap, `pi` bonds are weaker than `sigma` bonds. Therefore, the first bond to be formed, which has to sufficiently stabilize the resultant molecule, is preferentially a `sigma` bond.