Stability of the coordination compounds

The stability of a coordination compound reflects how strongly its metal-ligand interactions hold it together, and this can be measured quantitatively through the complex's stability (formation) constant, K. Since the metal-ligand interaction is itself a Lewis acid-Lewis base interaction (the metal ion accepting an electron pair, the ligand donating one -- as introduced in section 9.1), a STRONGER acid-base interaction produces a MORE stable complex. For the general complex-formation equilibrium $M^{a\oplus}+nL^{x-} \rightleftharpoons [ML_n]^{a\oplus+nx}$ (where a, x and (a+nx) are the charges on the free metal ion, the free ligand, and the resulting complex ion respectively), the equilibrium constant is written $K=\dfrac{[ML_n]^{a\oplus+nx}}{[M^{a\oplus}][L^{x-}]^n}$. A LARGER value of K means the equilibrium sits further toward the complex ion side, i.e. a thermodynamically MORE stable complex. Three worked equilibria illustrate the huge range these constants can span: $\text{Ag}^{\oplus}+2\text{CN}^-\rightleftharpoons[\text{Ag(CN)}_2]^-$, $K=5.5\times10^{18}$; $\text{Cu}^{2\oplus}+4\text{CN}^-\rightleftharpoons[\text{Cu(CN)}_4]^{2-}$, $K=2.0\times10^{27}$; and $\text{Co}^{3\oplus}+6\text{NH}_3\rightleftharpoons[\text{Co(NH}_3)_6]^{3\oplus}$, $K=5.0\times10^{33}$. …