Reactivity of Phosphorus Donors

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.