denticity
/ den-TISS-it-ee, from Latin dent- 'tooth' /
Picture a hand gripping a ball. One finger is a weak hold; a whole hand wrapping around grips far better. Ligands grip metals the same way: some touch at one point, others at several. Denticity (from the Latin for 'tooth') counts how many points — how many donor atoms — a single ligand uses to bite onto the metal.
A monodentate ligand (one tooth) binds through a single donor atom: water, ammonia, chloride, and cyanide are all monodentate. A bidentate ligand binds through two donor atoms at once, like a pincer — ethylenediamine grips through both nitrogens; oxalate and acetylacetonate through two oxygens. Tridentate, tetradentate, and so on up to polydentate (many teeth) ligands grip through three, four, or more donor atoms; EDTA is hexadentate, wrapping a metal through two nitrogens and four oxygen-bearing arms and occupying all six sites of an octahedron by itself. A ligand that grips through two or more donors, forming a ring that includes the metal, makes a chelate. Counting denticity is also how you translate ligand count into coordination number: a metal with three bidentate ligands has coordination number six.
Denticity matters because multidentate ligands form much more stable complexes than the same number of separate monodentate donors — the chelate effect. This is why EDTA is used to lock up unwanted metal ions in everything from food preservation to medicine, why biological metal sites use multidentate protein and porphyrin frameworks, and why denticity is one of the first things to identify when reading a complex. A subtlety: a ligand with several potential donor atoms does not always use them all — it may be forced to bind through fewer if geometry or competition prevents full wrapping.
Compare [Ni(NH3)6]2+, made of six monodentate ammonias, with [Ni(en)3]2+, made of three bidentate ethylenediamines. Both have coordination number six and bind through six nitrogens, but the chelating en version is far more stable in solution — a direct consequence of denticity.
Six nitrogen donors either way, but three bidentate ligands beat six monodentate ones for stability.
Denticity is the number of donor atoms one ligand actually uses to bind, not the number of donor atoms it possesses. A potentially bidentate ligand sometimes binds through only one atom if geometry or crowding prevents both from reaching the metal.