how could you estimate the relative ordering of the electron -donor of the power PR3 Ligands?,when provided with sets of different ligands of the PR3 type and a large supply of (CO)5W(thf) with which to make a series of complexes of the form(CO)5(PR3)(by substitution of thf with PR3).
The ability to donate the electron pair depends on many factors. The donor orbital should have the relatively high energy, be close in size to the free orbital of acceptor and correspond its symmetry, be well-polarized (for soft acid-base interaction, which is actual for phosphine ligands) and contain enough amount of donor atoms able to distribute the arising charge. Therefore we can make such conclusions: 1) R-radicals should be large enough and contain electron-donor groups, such as alkyl ones. 2) Aryls are preferable compared with alkyls as they are better polarizing substituents. 3) Electron-donor groups in aryl core will enhance the donor ability of the phosphine ligands due to mesomeric effect.