(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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