[iamback]

Then why in the above reversible reaction, Iodide reacts fastest with C+ and F- slowest ?

First of all it is not a reversible reaction. I admit C-I bond is the weakest. You are saying that if C-I is weakest(rather it is), why it is not favoring the the formation C-F bond which is more stronger ? Now, nucleophilicity does not depend on how stronger the bond is gonna be, or how strong the bond it would make. It depends on polarisability.

C-F bond is stronger, does that mean F^- would be a good nucleophile. It is stronger as " Bond strength (energy) can be directly related to the bond length/bond distance." C-F is the strongest single bond in organic chemistry—and relatively short—due to its partial ionic character.

Iodide is relatively good nucleophile in protic solvents. This is because its large size allows it to polarize electron density in such a way that it will still be negatively charged enough to attack an electrophile after picking up a proton from the protic solvent. In aprotic solvent, it's actually a worse nucleophile than all the other halogen anions.

According to thermodynamics, the more stable the product the faster it should be formed, isn't it?

You yourself said that 'nucleophilicity is a kinetic property'.

[iamback]

Also refer this for your answer : http://chemistry.stackexchange.com/questions/4152/how-can-a-group-be-both-a-good-nucleophile-and-a-good-leaving-group

[Borek]

According to thermodynamics, the more stable the product the faster it should be formed, isn't it?

iamback already marked this part of your post. This is definitely wrong - kinetics and thermodynamics are separate phenomena and there is no simple relationship between both.

[souro10]

According to thermodynamics, the more stable the product the faster it should be formed, isn't it?

iamback already marked this part of your post. This is definitely wrong - kinetics and thermodynamics are separate phenomena and there is no simple relationship between both.

Yes, it is indeed wrong. I was arguing from the other side. 

I was expecting a proper argument about nucleophilicity being a kinetic property.
The reasons that were being pointed out so far, weren't satisfying to me.
When someone argues nucleophilicity with pkB values, these arguments naturally arise. 

Then why in the above reversible reaction, Iodide reacts fastest with C+ and F- slowest ?

First of all it is not a reversible reaction. I admit C-I bond is the weakest. You are saying that if C-I is weakest(rather it is), why it is not favoring the the formation C-F bond which is more stronger ? Now, nucleophilicity does not depend on how stronger the bond is gonna be, or how strong the bond it would make. It depends on polarisability.

This is the simple answer that one is looking for. Nucleophilicity does not depend on how stronger a bond is going to be, it depends on how fast the nucleophile donates it's electrons.

Now, pkB definitely depends on how stable the conjugate acid is. pKb is actually the equilibrium constant of the reversible reaction. In that case, is it plausible to provide pKb data to a question on nucleophilicity?

Hence, I argued.

[orgopete]

Iodide is relatively good nucleophile in protic solvents. This is because its large size allows it to polarize electron density in such a way that it will still be negatively charged enough to attack an electrophile after picking up a proton from the protic solvent. In aprotic solvent, it's actually a worse nucleophile than all the other halogen anions.

Could you post data to support this?

As I recall, fluoride does indeed jump past iodide in aprotic solvents by increasing its rate, but iodide was relatively independent of a solvent effect. That was some time ago and my memory could be incorrect.

[Babcock_Hall]

I just checked Table 4.9 (pp. 333-334) in Lowry and Richardson's textbook (Mechanism and Theory in Organic Chemistry).  The values of n_CH3I for NH₂OH and NH₂NH₂ are 6.60 and 6.61, respectively.  The solvent is methanol in these reactions.  The value for NH₃ is 5.50  This is a logarithmic scale, BTW.

[orgopete]

I love it when someone looks up actual data for their argument (snack given).

[ARITRA]

Nucleophilicity of NH2NH2 is greater than NH2OH, however considering the fact that the conjugate acid of NH2OH is much more stable than that of NH2NH2, NH2NH2 should be a better base as well. Across a period electronegativity increase that is tendency to donate the lone pair electrons decrease and if electrons cannot be donated fast enough, nucleophilicity which is a KINETIC criterion decreases across a period. Also, for a similar reasons, the basicity also decrease across a period. SO we can say across a period, basicities parallel nucleophilicities. Hence, the result.

[SimantiniSP]

The nucleophilicity order is NH2-NH2>NH2-OH>NH3
This can be explained on the basis of two factors:
1)ALPHA EFFECT:
In both NH2-NH2 that is hydrazine and NH2-OH that is hydroxylamine, nucleophilic attack by lone pair of electrons is enhanced by the lone pair of electrons on the adjacent atom. The phenomenon is called "alpha effect". This effect is more pronounced in case of hydrazine compared to hydroxylamine, because lone pair of electrons on nitrogen is more polarizable than the lone pair of electrons on oxygen atom.
2)MOLECULAR ORBITAL THEORY
in NH3, no lone pair repulsion occurs hence NUCLEOPHILICITY IS MINIMUM
in NH2-OH, oxygen with lone pair is more stable hence charge repulsion is less
in NH2-NH2, charge repulsion is more between the two nitrogen atoms, so charge repulsion being more, hence NUCLEOPHILICITY IS MAXIMUM
any suggestions are welcome and appreciated