Factors which govern stability of the complex

Two distinct factors govern how stable a given coordination complex will be. (a) The charge-to-size ratio of the metal ion: a HIGHER charge-to-size ratio produces a MORE stable complex, because a smaller, more concentrated positive charge attracts a ligand's donated electron pair more strongly. For divalent (2+) first-row transition-metal ions specifically, this produces the well-known empirical stability ordering $\text{Cu}^{2\oplus}>\text{Ni}^{2\oplus}>\text{Co}^{2\oplus}>\text{Fe}^{2\oplus}>\text{Mn}^{2\oplus}>\text{Cd}^{2\oplus}$, called the Irving-Williams order. As a worked illustration of the underlying size effect: Cu2+ and Cd2+ carry the exact same +2 charge, yet Cu2+ (ionic radius 69 pm) has a noticeably higher charge-to-size ratio than the larger Cd2+ (ionic radius 97 pm), so Cu2+ consistently forms more stable complexes than Cd2+ does. (b) The nature (basicity) of the ligand: stability also depends on how readily the ligand can donate its lone pair of electrons to the metal ion, i.e. how strong a Lewis base the ligand is -- ligands that are STRONGER Lewis bases form MORE stable complexes with a given metal ion. This second factor is exactly why $[\text{Ag(CN)}_2]^-$ ($K=5.5\times10^{18}$) is so much more stable than the analogous $[\text{Ag(NH}_3)_2]^{\oplus}$ ($K=1.6\times10^{7}$): …