Determining chelating ligands from structure

[Anne Cam 3A]

How can you tell if a "ring" is formed around the central metal atom as is characteristic of chelating ligands? Can this be determined from the Lewis structure or is the molecular shape necessary? In other words is the ring usually three dimensional? Porphyrins don't seem to do this according to the diagram in the course reader because they are square planar.

[Chem_Mod]

There is not a definite way to tell. Knowing which of the ligands from the course reader can chelate is sufficient, and what their denticity is.