chelate
/ KEE-late /
Think of grabbing a ball with one finger versus with your whole hand. One finger and it slips away; a full grip and it stays put. A chelate is a metal held by the 'whole hand' — a single molecule that reaches around the metal with several arms at once.
Formally, a chelate is a complex in which one molecule (a chelating agent) binds a metal ion at two or more points simultaneously, forming a ring that includes the metal. The word comes from the Greek for a crab's claw, capturing the picture of being gripped from several sides.
Chelates matter because that multi-point grip makes them far more stable than complexes held by single-point binders — a fact called the chelate effect. This is exactly why EDTA, which grips at six points, is such a powerful and reliable titrant, and why chelates are used to lock up unwanted metals in foods, medicines, and detergents.
When EDTA binds a calcium ion, it wraps around with all six of its donor atoms to form a single stable chelate ring; this is far harder to pull apart than six separate water molecules clinging to the same calcium.
The multi-armed grip of a chelate is much sturdier than many single bonds.
A chelate is a kind of coordination complex — specifically one where a single ligand attaches at two or more points. Not all complexes are chelates, but all chelates are complexes.