What covalent bond is the longest?
The longest covalent bond I can find is the bismuth-iodine single bond.
The order of bond lengths is single > double > triple.
The largest atoms should form the longest covalent bonds. So we look at atoms in the lower right corner of the Periodic Table.
The most likely candidates are Pb, Bi, and I.
The experimental bond lengths are:
Bi-I = 281 pm; Pb-I = 279 pm; I-I = 266.5 pm.
So the polar covalent Bi-I bond is the longest covalent measured so far.