This article presents a scale of σ donor strengths for phosphines. The phosphine parameters, E B and C B, can be used in combination with previously reported parameters for oxygen, nitrogen, and sulfur donors to measure σ-basicity. The advantages of combining donors from more than one family to analyze reactivity are demonstrated. When substituents are changed in a family of donors, the change in the C B/E B ratio is small and this is shown to lead to limitations in the conclusions that can be drawn from substituent constant correlations. QALE analyses also treat only single families and are subject to the same limitations. The dual parameter E B, C B basicity scale is less reliant upon steric effects than QALE analyses to rationalize trends in the reactivity of phosphines. This leads to an alternative interpretation of a large body of phosphine chemistry. A model is presented to support the idea that the π-acceptor properties of the phosphines decrease regularly as their σ basicity increases. It is shown that the phosphine σ donor, E B and C B parameters inappropriately can correlate reactivity toward acceptors that π-back-bond when a data set involves only phosphines. In order to obtain reliable acceptor parameters and to detect a π-back-bond component in donor−acceptor interactions, donors from families other than phosphines have to be studied with the π-donor.

A greatly expanded database has led to a revised set of E B and C B basicity parameters for the analysis of the reactivity of phosphorus donors with the ECW model. Reactivity and spectral shift analyses are reported for 79 acceptors. General patterns for the reactivity of phosphorus donors emerge, for example, π-back-bond acceptor tendencies are shown to decrease as the σ basicity increases. Steric effects are encountered with a few acceptors, and a procedure is offered that uses an extension of ECW to quantitatively confirm steric repulsions. Very different interpretations of reactivity result from ECW compared to those from analyses that use one-parameter electronic basicities, χ, and cone angles, θ. When donor orders reverse with acceptor softness (covalency), one-parameter electronic basicity scales cannot accommodate the reversals. It will be shown that the cone angle is used inappropriately to compensate for changing softness in the χ and θ correlations. Attempting to compensate for softness using θ leads to an exaggeration of the steric contributions and to unprecedented interpretations of reactivity trends when the parameters for different acceptors are compared. The meaning of the correlations is questioned. Donors for which θ does not correlate to E B and C B parallel those that require extra parameters, e.g., E ar, to obtain good fits with the QALE procedures. It is also shown that the extra QALE terms provide parameters that enable one to correlate systems which demonstrate nonlinear free-energy behavior, clouding even further the meaning of QALE correlations.

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