chelate
/ KEE-late, from Greek chele 'claw' /
The word comes from the Greek chele, the claw of a crab. Picture a crab's claw closing around a pebble from two sides at once — that double grip is much harder to shake loose than a single poke. A chelate is a complex in which a single ligand grips the metal from two or more sides at once, pinning it inside a ring.
Precisely, a chelate forms when a polydentate ligand binds a metal through two or more of its donor atoms, so that the metal and the ligand's connecting backbone together close a ring. Ethylenediamine, with two nitrogen donors joined by a two-carbon bridge, clamps a metal into a five-membered ring (M-N-C-C-N) — a classic chelate. Oxalate and acetylacetonate make similar rings through oxygen donors. The ligand doing the gripping is called a chelating ligand or chelating agent, and the loop it closes is the chelate ring; five- and six-membered rings are the most stable, while three- or four-membered rings are too strained to be common. The number of such rings can be large: EDTA wraps a metal in five fused chelate rings at once.
Chelates are far more stable than otherwise-similar complexes built from separate monodentate ligands — the chelate effect, which is mostly an entropy story. This makes chelating agents enormously useful: EDTA softens water and locks up toxic metals, chelation therapy removes lead or iron from the body, and nature's own chelates — heme, chlorophyll, and metalloenzyme sites — hold their metals firmly in place. When such a chelate ring lacks any internal mirror symmetry, the whole complex can become chiral and show optical isomerism.
In [Co(en)3]3+ three ethylenediamine ligands each close a five-membered M-N-C-C-N ring around the cobalt. The cobalt is locked inside three claws at once, and the whole propeller-like arrangement has no mirror plane, so the complex is chiral and exists as two non-superimposable mirror images.
Three chelate rings lock cobalt in a propeller shape that comes in left- and right-handed forms.
Chelation needs the donor atoms spaced so the ring is comfortable — usually five or six members. Donors too close together would force a strained three- or four-membered ring and generally will not chelate; they may instead bind two different metals or just one donor.