Hi, I have a question regarding metal complexes formed with diene, cyclobutadiene and allylic system. I don't understand why in a diene C1-C2 bond and C3-C4 bonds lengthen due to pi-antibonding while C2-C3 bond length shortens due to pi ponding. In cyclobutadiene, I don't believe there is bond lengthening due to pi-antibonding (something about the nodes) and I have no clue what happens to allylic system. If someone could help me understand this it would be great. Thanks.