Question

How complex compounds are formed?

Answer 1

Complex compounds have a central metal atom/ion and are surrounded by ligands, which can be charged or neutral. They bind tightly to the central atom and the complex practically doesn't dissociate in solution.

Complete answer: Compounds formed due to combination of two or more simple stable salts, which retain their identity in solid as well as in dissolved state, are called coordination compounds. These are the backbone of modern inorganic and bio-inorganic chemistry and chemical industry. Now, we need to know about additional compounds. Compounds made up of two or more stable compounds by crystallization in a fixed stoichiometric ratio are called addition compounds . For example:$KCl+MgC{{l}_{2}}+6{{H}_{2}}O\to KCl.MgC{{l}_{2}}.6{{H}_{2}}O$. Addition compounds are of two types:
1. Those which lose their identity in solution (double salt):
$FeS{{O}_{4}}.{{(N{{H}_{4}})}_{2}}S{{O}_{4}}.6{{H}_{2}}O\to F{{e}^{2+}}+2N{{H}_{4}}^{+}+2S{{O}_{4}}^{2-}+6{{H}_{2}}O$
2. Those which retain their identity in solution (coordination or complex compounds):
$Fe{{(CN)}_{2}}+4KCN\to {{K}_{4}}[Fe{{(CN)}_{6}}]\to 4{{K}^{+}}+{{[Fe{{(CN)}_{6}}]}^{4-}}$
The terms related to complex coordination compounds are:
1. Coordination entity: It constitutes a central metal atom or ion bonded to a fixed number of ions or molecules with coordinate bonds
2. Central atom/ ion: In coordination entity, the atom/ion to which a fixed number of ions/groups are attached in a definite geometrical arrangement around it, is called central atom/ion.
3. Ligands: The donor atoms, molecules or anions which donate a pair of electrons to the metal atom/ion are called ligands.
4. Coordination number: It may be defined as the number of coordinate bonds formed with central atom/ion by the ligands.
In a nutshell, these complex compounds have a central metal atom/ion and are surrounded by ligands, which can be charged or neutral. This is how complex compounds are formed.

Note: Coordination number of central atom/ion is determined only by number of sigma bonds formed by ligands with central metal atom/ion. Pi bonds, if formed, are not counted for this purpose.