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FACTOR'S AFFECTING STABILITY OF COMPLEXES:

 (1) Charge on central metal cation:
In general stability of Complexes is directly proportional to the magnitude of charge on central metal atom. Thus Complexes of Fe +3 are more stable than Fe+2
(2) Size of central metal cation:
As size of the central atom ion decreases the stability of the Complex increases. This is applicable when Oxidation state of central metal ion is same in all the cases. For example Zn+2 < Cu+2 < Ni+2 < Co+2 < Fe+2 < Mn+2
(3) Nature of the ligands:
The size and charge of ligands is also an important factor of in deciding the stability of complexes.
(i) If the Ligand is smaller, it can approach the central metal ion more closely forming a stable bond.
(ii) High charge ligand will form a strong bond. Thus high charge and small size of ligands leads to Formation of stable Complexes.
(iii) Charge density of the ligand= charge/size. More the charge density more is the stability of the Complex. For example fluoride (F- ) will form more stable Complexes and Iodide (I-) forms least stable Complexes.
(4) Chelating effects:
Formation of five and six membered chelate from polydentate ligands enhance the stability of Complexes in comparison to monodentate ligands. This is called chelate effect of chelation. In general, the more number of chelate rings in the Complex, the more will be stability of the Complex.
(5) Formation constant of Complexes:
Stronger is the metal- ligand bond, less is the dissociation of Complex ion in the Solution and hence greater is the stability of complex. Thus the larger the numerical value of formation constant, the thermodynamically more stable is the complex.

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