A chelate complex is formed when a multidentate ligand (a ligand with two or more donor atoms) binds to a single central metal atom or ion at two or more points simultaneously. This creates a stable ring-like structure called a chelate ring.
The oxalate ion (${\mathit{C}}_2{\mathit{O}}_4^{2-}$) is a bidentate ligand. It has two oxygen atoms that can donate lone pairs to the metal center, forming a five-membered ring which is highly stable.
Structure: ${\left[\mathit{O}-\mathit{C}\left(=\mathit{O}\right)-\mathit{C}\left(=\mathit{O}\right)-\mathit{O}\right]}^{2-}$
Chelation: Both negatively charged oxygen atoms bind to the same metal ion.