When Is a Proton Not a Proton?

In spite of the large number of protonated complexes, their use in catalytic cycles has been little explored. The existing examples suggest that catalytic processes initiated by the protonation of a low-valent metal hold promise for the development of selective, efficient, and atom-economical addition reactions under very mild conditions, if the protonation occurs with weak acids. The acidity required depends upon the basicity of the metal, and the latter can be increased by the use of more electron-rich ligands (see figure).