Reactivities and selectivities of free and metal-coordinated carbocations

Abstract

Most quantitative comparisons of carbocations, e.g. hydride affinities, heats of ionization, pKR+ values, or solvolysis rate constants have been restricted to certain groups of compounds and do not allow the direct comparison of structurally different entities, for example ordinary carbenium ions, heteroaromatic cations, or cationic metal π-complexes. Such a comparison can be based on the kinetics of the reactions of these electrophiles with non-charged nucleophiles (e.g. π-systems, n-nucleophiles, or hydride donors). It is shown that the use of a floating scale of reference nucleophiles allows the development of a comprehensive electrophilicity scale (E-scale) which can be used for calculating absolute rate constants for the reactions of carbocations with a large variety of nucleophiles. The relationship of this electrophilicity scale with other "stability scales" is discussed.