Stability of co-ordination
compounds
The stability of
compounds means in a most general sense, the compounds exist under suitable
conditions may be stored for a long period of time. However when the formation
of complexes in solution is studied, two types of stabilities, thermodynamic stability and kinetic
stability are considered. In
the language of thermodynamics, the equilibrium constants of a reaction are the
measure of the heat released in the reaction and entropy change during
reaction. The greater amount of heat evolved in the reaction, the most stable
are the reaction products. Secondly, greater the increase in entropy during the
reaction, greater is the stability of products. The kinetic stability of
complexes refers to the sped with which transformation leading to the
attainment of equilibrium will occur. Here we are mainly concerned with the
thermodynamic stability of the complex compound.
Thermodynamic stability vs.
kinetic stability and their correlations:
The most stable states of a kinetic reaction are those of the
reactants, in which an input of energy is required to move the reaction from a
state of stability, to that of reacting and converting itself to
products. Kinetics is related to reactivity.
In contrast, the most stable state of a thermodynamically favorable reaction is
the products, because the reaction occurs spontaneously, without the
need for energy to be added. Thermodynamics is related to
stability.
Therefore,
something that is unreactive will desire to stay in the form of reactants,
which will require an input of energy to cause the reaction to go forward,
converting reactants into products. This is illustrated in example #3 below. A
reactive species does not require an input of energy to be converted from
reactants to products, because its most stable and preferred state is that of
the products. Instead, a thermodynamically favorable
reaction requires energy to be converted from products back to
reactants.An energy source moves the reaction forward (kinetics corresponds to
movement). The same is for thermodynamically favorable reactions, except
that the reaction must be stimulated backward from products to reactants.
Cumulative or overall and stepwise constants:
A cumulative or overall
constant, given the symbol β, is the equilibrium constant for the formation of
a complex from reagents. For example, the cumulative constant for the formation
of AH2 is given by
The stepwise constant,
K, for the formation of the same complex from A2- and H+ in
two steps is given by:
For 1st
equation:.........................
For 2nd
equation.........................
Further reading:
http://shodhganga.inflibnet.ac.in/bitstream/10603/3975/11/11_section%20c.pdfhttp://www.hyperquad.co.uk/documents/HQfaq.pdf
http://chemwiki.ucdavis.edu/Physical_Chemistry/Equilibria/Chemical_Equilibria/Principles_of_Chemical_Equilibria/Kinetically_vs_Thermodynamically_Stable
http://classes.uleth.ca/200403/chem3820a/3820%20lecture%20chapter_9_part1_2004.pdf
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