I agree with you that trimethoxyphosphine is expected to be a stronger nucleophile than trimethylphosphine. Both compounds have pyramidal structures in which phosphorous, the central atom, has a lone pair of electrons ready to attack a given electrophile & each has 3 electron-releasing methyl groups. It is true that the (O) in Me-O-P gives more electronegativity which means a polar O-P bond but the (O) has 2 lone pair of electrons which will enhance electron-releasing to the (P) atom rendering it to become more nucleophilic.

This is my expectation but I stand for correcting it if there is another convincing explanation.

The alpha effect is hardly understood. Also in real life systems you have to consider a numer of effects which may be competing.

In trimethyl phosphite 2 effects lower the nucleophilicity.

1. Polarization: Electron density is drawn from the phosphorous which makes it less nucleophilic. Also the polarized bonds will have a higher p-character at the phosphorous resulting in a higher s-character for the lone pair. (both effects are interwined)

2. Resonance: The lone pair will interact with sigma* O-C bonds. This interaction will stabilize the lp more than a n_P -> sigma*_CH interaction in Me3P

It is in general questionable if the alpha effect is present for pairs of second row hetero atoms with first row heteroatoms

If you want to learn more about stereoelectronic effects I recommend you the great book from Igor Alabugin

Chapter Introduction

... except hypochorite in which the electronegativity difference is small

The nucleophilicity depends also from the partner electrophile the phosphine is reacting with, considering the HSAB theory.

Three methyl groups attached to a P atom makes the nucleophile a soft one, reacting faster with a soft electrophile (by orbital control). In the case of trimethoxyphosfine the nucleophile is harder, reacting better wit an hard electrophile (by charge control).

Then, trimethylphosfine will react faster with methyl iodide than with methyl fluoride, but the opposite is expected with trimethoxyphosfine, that will react faster with methyl fluoride than with methyl iodide.

I think that trimethyl phosphine is the stronger nucleophile.

is stronger nucleophilic than trimethyl phosphine (Me3P) due to electron donating mesomeric effect of methoxy group phosphorus atom becomes more electron donor i.e. hence more nucleophilic.

Dear Mohammad Javaherian many thanks for the interesting technical question. It's interesting to see that there are already some rather differing answers. I agree with Sebastian Jung in that the "alpha effect" does not play any important role here. I don't see a mesomeric effect of the methyl groups either because the phosphorus atom has a free electron pair and not a positive charge. Instead, the electron-withdrawing oxygens in trimethylphosphite will lead to the effect that the free electron pair at phosphorus has more s-character, making P(OMe)3 a weaker nucleophile. In contrast, the free electron pair at the P atom in trimethylphosphine has more p-character so that PMe3 is the stronger nucleophile and a better donor ligand toward transition metals. For some very useful overview on the different properties of various ligands please have a look at the following article:

http://www.f.u-tokyo.ac.jp/~kanai/seminar/om/Hartwig2.pdf

Me3P have more Nucleophilicity than (MeO)3P because of the methoxy group in this case withdrawl group through inductive effect..as you know methoxy group has two effects inductive and mesomeric in this case Inductive effect more effective than mesomeric effect therefore Me3P more nucleophile..