PHOSPHINE E(B) AND C-B VALUES

This article presents a scale of sigma donor strengths for phosphines, The phosphine parameters, E(B) and C-B, can be used in combination wi th previously reported parameters for oxygen, nitrogen, and sulfur don ors to measure sigma-basicity. The advantages of combining donors from more than one family to analyze reactivity are demonstrated, When sub stituents 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 concl usions that can be drawn from substituent constant correlations. QALE analyses also treat only single families and are subject to the same l imitations. The dual parameter E(B), C-B basicity scale is less relian t 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 suppor t the idea that the pi-acceptor properties of the phosphines decrease regularly as their sigma basicity increases. It is shown that the phos phine sigma donor, E(B) and CB(p)arameters inappropriately can correla te reactivity toward accepters that pi-back-bond when a data set invol ves only phosphines. In order to obtain reliable acceptor parameters a nd to detect a pi-back-bond component in donor-acceptor interactions, donors from families other than phosphines have to be studied with the pi-donor.