FACTORS AFFECTING STABILITY OF METAL
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M.Sc., 3rd Semester
Department of Chemistry
Factors affecting stability of metal complex
a) Metal related factor
b) Ligand related factor
Coordination chemistry is the study of a class of compounds formed by metals for ex, when an excess of aqueous potassium cyanide is added to aqueous ferrous sulphate a yellow solution is formed
The product [Fe(CN)6]4- is called a metal complex ion. It can be isolated as its potassium salt K4 [Fe(CN)6] stability of a metal complexes vary over a very wide range of values.
No single factors is expected to account for there relative stabilities of coordination compounds they are affected by many physical and chemical factors. Some of these factors are discussed below.
Physical Factor - Thermodynamic stability
Chemical Factor - Metal related factor
Ligand Related factor
PHYSICAL FACTORS LIKE
If the terms stability is used without any modification, then it refers to the thermodynamic stability
Types of stability constant
1. stepwise stability constant ‘ K’
2. overall stability constant ‘ß ‘
CHEMICAL FACTORS LIKE
). Metal related factor;
In complex metal ion is electron pair acceptor therefore
Lewis acid. Higher the Lewis acidity, higher is acceptor
tendency and stabler is the complex.
Size, charge and electron configuration of metal ion is the stability.
a] Size- smaller the size higher is Lewis acidity stabler complex.
Ex [Fe(H2O)6]2+ - less stable
[Co(H2O)6]2+ - more stable
Because Co2+ is smaller than Fe2+
b] Charge- Higher the charge higher is Lewis acidity stabler the complex
Ex [Fe(H2O)6]2+ - less stable
[Fe(H2O)6]3+ - more stable
Because Fe3+ ion
c) Electronic Configuration Of Metal Ion
Metal ion with pseudo inert gas configuration forms stabler
complex then those having inert gas configuration. Pseudo inert
gas configuration-metal ion with (n-1) d10ns2np6
ex K+and Cu+
where as charge size nearly the same. But Cu+ forms
Stabler complexes. Because the cations of non-transition elements
have there inert gas configurations. (n-1)s2p6 in the outermost shell.
Where as the cations of transition elements have pseudo-inert gas
configurations (n-1) d10ns2np6 in the outermost shell. Where
d-orbitals are poor shielding and higher effective nuclear charge so
ligand attracts more. Hence Cu+ is stabler complex.
Ligand Related Factor
). Nature of ligating atoms:
Ligating atoms like N , O and F forms stable bonds with small
metal ion like Li+, Co3+,Ti4+(HSAB Concept)
Ligating atom like S and P form stable complexes with larger
metal ions, pb2+,Hg2+ and Cu+(HSAB concept)
b) Basicity of ligands:
Electron donor tendency higher the electorn donar tendency
stabler is the complex basicity of organic ligand can be increased by
Electron donor subsitutents.(+ I effect)
Ex; - NH3, CH3-NH2
Where CH3-NH2 is more basic better electron donor form stable
c) Chelating Ability Of Ligand.
Chelating ligands form stabler compleres thus
en (H2 N- CH2 –CH2-NH2) form stable complex the NH3
Ex: [Cu (NH4)]2+ stabler then [Cu(en)2]2+ therefore en is a chelating agent.
Chelate-complex with ring structure.
Chelate effect is thermodynamic effect can be explained using entropy
factor. During chelate formation. There is an increase in entropy
I,e S is +ve. G is decreases.
i.e thermodynamic stability increases.
[Co(H2O)6]3+ + 3en [Co(en)3]3+ + 6H2O
No of 1 + 3 1 + 6
4 7 particles
There is a net increases in the number of particles larger the number
Of particles Higher is entropy therfore more the number of chelate
ring higher is the stability.
d) Steric Factor
One more factor of significance in chelating is the steric factor This is associated with the presence of a bulky group. Either attached to the donor atom or near enough to it to cause mutual repulsion between the ligands which results in the weakening of the metal to ligand bonds.
This is illustrated with following examples.
Oxine forms stable complexes with metal ions like Mg2+, Al3+ etc.
2- methyl oxine does not forms stable complexes.
4- methyl oxine forms stabler complexs than oxine because due to steric hindrance here N and O are donar atom in oxine
In 2- methyl oxine the –CH3 hindrance the complex formation it is adjacent to N donar atom.
In 4- methyl oxine the – CH3 group opposes + I effect. In 4- methyl oxine the N is more basic due to + I effect of CH3.
[Ni(2-methyl orime)2] < [Ni (oxime)2] < [Ni( 4-methyloixme)2]
Because steric hindrance and basicity of ligand
Form this following factor we can able
to identify the more and less stable metal
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