Question

Which of the following is more stable complex and why? ${\left[ {Co{{\left( {N{H_3}} \right)}_6}} \right]^{3 + }}$and ${\left[ {Co{{\left( {en} \right)}_3}} \right]^{3 + }}$

Answer 1

A complex ion consists of a metal ion at its centre and a number of other ions or molecules surrounding it. These ions or molecules are attached to the central metal ion through coordinate bonds.

Complete answer
Thermodynamic stability of a complex is its tendency to occur under the equilibrium conditions. It actually determines the magnitude to which the complex can be either formed or get converted to another complex at equilibrium. Thermodynamically it is favoured that a complex having monodentate ligand tends to react with either polydentate or bidentate ligands in order to form a chelate complex, as this is driven by entropy. Thus, bidentate or polydentate are considered to be more stable. In the given question, ${\left[ {Co{{\left( {en} \right)}_3}} \right]^{3 + }}$is a metal chelate owing to the presence of bidentate ligands i.e. ethylene diamine ligand. Therefore, ${\left[ {Co{{\left( {en} \right)}_3}} \right]^{3 + }}$is more stable than ${\left[ {Co{{\left( {N{H_3}} \right)}_6}} \right]^{3 + }}$which contain only monodentate ligands.

Note:
Alternatively, the complex ion stability can also be explained through the concept of CFSE i.e. crystal field stabilization energy. CFSE refers to the stability which results from allocating a transition metal ion within the crystal field produced via a set of ligands. Complexes having high CFSE are thermodynamically more stable (i.e., they possess high values of Ka, equilibrium constant for metal-ligand linkage). ${\left[ {Co{{\left( {en} \right)}_3}} \right]^{3 + }}$ is more stable than ${\left[ {Co{{\left( {N{H_3}} \right)}_6}} \right]^{3 + }}$since its CFSE is higher due to the $en$ ligand being strong compared to $N{H_3}$ in accordance with spectrochemical series. $en$(having more field strength) causes more splitting than $N{H_3}$, therefore splitting energy ($∆_o$) for ${\left[ {Co{{\left( {en} \right)}_3}} \right]^{3 + }}$ will be more and hence also more CFSE.