Molecular orbitals of allylic cations, anions and radicals
In allylic species 3 orbital wave functions are interfering to form 3 molecular orbitals among which the pi antibonding molecular orbital will have the max energy. The first two mo can accomodate 4 electrons. Since pi antibonding are at a higher energy level the first 4 electrons will remain in first two mo only. Similarly in cation and radical electrons will remain in these orbitals hence the pi antibonding will always remain empty.