**5.2 Complexation**

It is a process in which two or more species are associated and they form a complex. When the complexes of metal ions and the ligands are formed in such a way that the single metal atom enjoys the central position then it is called a mononuclear complexes. When more than one metal ion is present in the centre they form a polynuclear complex [38]. During the formation of polynuclear complexes, on the basis of number of binding ligands involved the metal atom may hold a positive, negative, or neutral charge. Several studies have confirmed that the formation of complexes by the mononuclear ligands is more desirable than polynuclear ligands because the latter contains multiple ligands results multiple species binding.
