Complexation Equilibria

Introduction

Many metal ions form complexes with ligands that can donate electrons to empty orbitals on the metal. The equilibrium between the metal ion and the ligands is always written with the formation of the complex being the forward direction. For example:

Co²⁺ _(aq) + 6 NH_{3 (aq)} Co(NH₃)₆²⁺_(aq)

The equilibrium constant expression for a complex is written following the same rules as for any other equilibrium. The equilibrium constant is called the formation constant, K_f. The K_f expression for the above equilibrium is:

     [Co(NH3)62+]
Kf = ------------
     [Co2+][NH3]6

K_f Values for Some Complexes

Formula	Name	Ksp
AgCl2-	dichlorosilver(I)	1.8x105
Ag(CN)2-	dicyanosilver(I)	2x1020
Ag(NH3)2+	diamminesilver(I)	1.7x107
Co(NH3)62+	hexaamminecobalt(II)	1x105
Co(NH3)63+	hexaamminecobalt(III)	1x1023
Co(NH3)5Cl2+	pentaamminechlorocobalt(III)	2x1028