Notes On Introduction - CBSE Class 12 Chemistry
Complex compounds are a special class of compounds in which the metal atoms (or) ions are bound to a number of anions (or) neutral molecules. In modern terminology, these compounds are called coordination compounds. Coordination compounds (or) complex compounds are a type of addition compounds. When two (or) more stable compounds are allowed to combine in stoichiometric ratios, crystalline compounds are formed. These are known addition (or) molecular compounds. On the basis of nature, addition (or) molecular compounds are divided into two categories. They are double salts and coordination (or) complex compounds. Double salts are molecular compounds that are stable in the solid state, but break down into individual constituents when dissolved in water. Ex: An aqueous solution of potash alum will give the tests for K+, Al +3, and SO4-2 K2SO4.Al2(SO4)3.24H2O →  2K+  +  2Al+3  +  4SO4-2  +  24H2O On the other hand, coordination compounds are molecular compounds that retain their identity even when dissolved in water. Ex: When potassium ferrocyanide is dissolved in water, it does not give the usual tests for Fe+2 and CN-1, indicating that, [Fe (CN)6]-4 does not dissociate into Fe+2 and CN-1. Since, these ions are present in the form of a new ion, called a complex ion. Compounds that have complex ions are called complex compounds. As the central metal ion in the complex ion forms dative (or) coordinate covalent bonds with the species surrounding it, complex ions are also known as coordinate ions and hence the corresponding compounds are known as coordinate compounds. Transition metals show a greater tendency to form complex compounds, due to the relatively smaller size of the metal ions, high ionic charges and the availability of vacant d -orbitals for bond formation. Complex compounds find extensive use in electroplating, metallurgical processes, textile dyeing and medicinal chemistry. And also as industrial catalysts and analytical reagents.

#### Summary

Complex compounds are a special class of compounds in which the metal atoms (or) ions are bound to a number of anions (or) neutral molecules. In modern terminology, these compounds are called coordination compounds. Coordination compounds (or) complex compounds are a type of addition compounds. When two (or) more stable compounds are allowed to combine in stoichiometric ratios, crystalline compounds are formed. These are known addition (or) molecular compounds. On the basis of nature, addition (or) molecular compounds are divided into two categories. They are double salts and coordination (or) complex compounds. Double salts are molecular compounds that are stable in the solid state, but break down into individual constituents when dissolved in water. Ex: An aqueous solution of potash alum will give the tests for K+, Al +3, and SO4-2 K2SO4.Al2(SO4)3.24H2O →  2K+  +  2Al+3  +  4SO4-2  +  24H2O On the other hand, coordination compounds are molecular compounds that retain their identity even when dissolved in water. Ex: When potassium ferrocyanide is dissolved in water, it does not give the usual tests for Fe+2 and CN-1, indicating that, [Fe (CN)6]-4 does not dissociate into Fe+2 and CN-1. Since, these ions are present in the form of a new ion, called a complex ion. Compounds that have complex ions are called complex compounds. As the central metal ion in the complex ion forms dative (or) coordinate covalent bonds with the species surrounding it, complex ions are also known as coordinate ions and hence the corresponding compounds are known as coordinate compounds. Transition metals show a greater tendency to form complex compounds, due to the relatively smaller size of the metal ions, high ionic charges and the availability of vacant d -orbitals for bond formation. Complex compounds find extensive use in electroplating, metallurgical processes, textile dyeing and medicinal chemistry. And also as industrial catalysts and analytical reagents.