The 2πx2 orbital shows that there's
pi bonding between C1 and C2, but there is also a small amount of pi bonding between C2 and
C3. The bonding in HC4N can thus be viewed as a resonance between the two
configurations shown to the left. The top configration dominates, however. The singly
occupied 3π orbitals are consistent with this: the largest amplitude is on C3, but
there's some amplitude on C1 as well.