TABLE OF CONTENTS Complex formation is important in two ways. It may produce a species that has more useful characteristics for a particular chemical separation method, and alternatively, the concentrations of particular species can be diminished to levels below those at which they interfere in reactions designed to separate other molecules or ions.
Practically every metal forms complex ions of some kind. Some metals form more numerous and more stable complexes than others, but in the reactions of analytical chemistry, the possibility of complex formation must always be considered. At least two sets of equilibria are involved in the process of complex formation. There is competition between solvent molecules and the ligand for the metal ion and simultaneous competition between protons and the metal ion for the ligand. Metal ions in aqueous solution possess solvent molecules in their primary solvation shell. Attraction between them is weak usually, and the number of solvent molecules immediately surrounding each metal ion is variable. However, in the transition-metal ions and higher-valent metal ions, definite complexes such as 
and 
exist in aqueous solutions. For this reason complex formation in aqueous solutions is really a replacement process in which solvent molecules in the coordination sheath surrounding a metal ion are replaced stepwise by other ligands.
Reaction rates must also be considered. The addition of aqueous ammonia to a solution of a silver salt will convert the hydrated silver ion into an ammonia complex...
