Dien

[Ekin 1C]

What makes dien a tridentate?

[Vasiliki G Dis1C]

If you draw out the structure, you see that the 3 nitrogens all have one lone pair. Since they have this lone pair, it makes them capable of bonding, making the overall ligand have 3 bonding sites. This makes the entire thing qualified as tridentate. The easiest way to determine this is to draw out the Lewis structure of the compounds (they aren't as hard or long to draw as they look!!) Hope this helps :)