denticity
/ den-TISS-ih-tee /
Think of how many fingers a hand uses to hold an object: one fingertip, a pinch of two, or a full five-finger grip. Denticity counts the same thing for a ligand — how many points it uses to attach to a single metal ion at once.
Formally, denticity is the number of donor atoms in one ligand that bind to the same metal centre. A ligand using one point is monodentate, two points is bidentate, and so on up to many points; the Latin root for 'tooth' gives the word, as if the ligand bites the metal with several teeth.
It matters because higher denticity generally means a more stable complex, thanks to the chelate effect. This is precisely why EDTA, a six-point ligand, makes such durable complexes and is so useful as a titrant. The caveat: denticity counts only the points actually bonded in a given complex, which can be fewer than a ligand's maximum.
Water is monodentate (one point), ethylenediamine is bidentate (two points), and EDTA is hexadentate (six points) — and that climb in denticity is matched by a steep climb in complex stability.
More binding points generally means a more stable complex.
Denticity describes how many points one ligand uses; it should not be confused with coordination number, which counts the total bonds the metal makes to all its ligands together.