formation constant
Some ligands grab a metal ion and hold on for dear life; others barely manage a loose handshake. The formation constant is the number that scores that grip: it tells you how strongly and how completely a metal ion and a ligand join to form a complex. A big formation constant means a tight, stable complex that forms almost completely; a small one means the partnership keeps falling apart.
Formally, it is the equilibrium constant, written Kf, for a metal ion combining with one or more ligands to make a coordination complex. It is built like any equilibrium constant: the concentration of the complex divided by the concentrations of the free metal and free ligand. Because such complexes are usually very stable, formation constants are often enormous numbers, sometimes 10^15 or larger.
Formation constants are the engine of complexometric titrations and of the chemistry that hides interfering ions out of the way. A large Kf is exactly what lets EDTA sweep up a metal ion sharply and completely at the end point. The practical caveat: the formation constant tabulated for ideal conditions is rarely the value you experience in the flask. Side reactions like protonation eat into it, so analysts use a conditional formation constant that reflects the actual pH and competing equilibria.
The EDTA complex with calcium has a formation constant of about 5 x 10^10 — huge, which is why a calcium-EDTA titration goes essentially to completion and gives a sharp end point.
A large Kf means a complex that forms almost completely.
Formation constant and stability constant are two names for the same quantity. Its inverse is the dissociation (or instability) constant, which describes the complex coming apart — a strong complex has a large formation constant and a tiny dissociation constant.