Successive stepwise formation constants K in a series such as ML (n = 1, 2, ...) usually decrease as n increases. Exceptions to this rule occur when the geometry of the ML complexes is not the same for all members of the series. The classic example is the formation of the diamminesilver(I) complex [Ag(NH)] in aqueous solution.
In this case, K > K. The reason for this is that, in aqueous solution, the ion written as Ag actually exists as the four-coordinate tetrahedral aqua species [Ag(HO)]. The first step is then a substitution reaction involving the displacement of a bound water molecule by ammonia forming the tetrahedral complex [Ag(NH)(HO)]. In the second step, all the aqua ligands are lost and a linear, two-coordinate product [HN–Ag–NH] is formed. Examination of the thermodynamic data shows that the difference in entropy change is the main contributor to the difference in stability constants for the two complexation reactions.
Other examples exist where the change is from octahedral to tetrahedral, as in the formation of [CoCl] from [Co(HO)].