formation constant
For a mononuclear binary complex, if a central atom (central group) M (the 'metal') and a ligand L have been defined, then in the following expressions Kn is the stepwise formation constant, and β n is the cumulative formation constant for the complex MLn. They can both be referred to as stability constants (stepwise and cumulative). \[K_{n}=K(\text{ML}_{n- 1}+\text{L}\rightleftharpoons \text{ML}_{n})\] \[\beta_{n} = K(\text{M}+n\text{L}\rightleftharpoons \text{ML}_{n})\]
Orange Book, 2nd ed., p. 11 (