Assessing the stereoelectronic properties of pyrrolyl phosphines and related ligands. The quantitative analysis of ligand effects (QALE)

By application of the QALE model (quantitative analysis of ligand effects) to the standard reduction potentials (E degrees values) and the standard enthalpies of reduction (Delta H degrees) of the eta-Cp(CO)(PZ(3))Fe(COMe)(+/0) couple (PZ(3)= a phosphorus(III) ligand), and nu(CO) for eta-Cp(CO)(PZ(3))Fe(COMe)(0), we have determined that a minimum of four parameters are necessary to describe the stereoelectronic properties of the set of ligands PPhi(Pyr)(3-i) (Pyr = pyrrolyl) and P(NC4H8)(3) (NC4H8=pyrrolidinyl). These parameters are chi(d), theta, E-ar and the pi acidity parameter, pi(p). The values of these parameters were determined by linear regression analysis of a set of QALE equations. The coefficients of these equations were based on the analyses of data for PR3, PPhiR3-i, P(p-XC6H4)(3), P(OR)(3), and P(O-p-XC6H4)(3). The parameters for P(Pyr)(3) are chi(d)=31.9 +/- 0.7, theta=145 +/- 3, E-ar=3.3 +/- 0.2 and pi(p)=1.9 +/- 0.2; and for P(NC4H8)(3) the parameters are chi(d)=-1.2 +/- 1.4, theta=145 +/- 5, E-ar=-0.6 +/- 0.4 and pi(p)=0.9 +/- 0.3. P(Pyr)(3) is a poor sigma donor that possesses an E-ar parameter comparable to P(p-XC6H4)(3) and a pi acidity that is about two thirds that of P(OR)(3) and half that of P(O-p-XC6H4)(3). On the other hand, P(NC4H8)(3), which is one of the strongest sigma donor phosphorus(III) ligands, is a weak pi acid with a value for E-ar that is statistically indistinguishable from zero. PPhi(Pyr)(3-i) and P(NC4H8)(3) appear to be isosteric to P(p-XC6H4)(3).