Electron-transfer reactions often occur directly from donor to acceptor, and their rates tend to be linear with reactant concentration, or at most second order when metal atoms act as promoters. Yuasa et al. now report an example of third-order kinetics in which Sc3+ ions are needed to self-assemble a competent electron-acceptor complex. No reaction was observed between an Ir(2-phenylpyridine)3 donor to a p-benzoquinone (denoted “Q”) acceptor, which would have had to drive the electron more than 1 volt uphill. However, addition of Sc3+ ions formed a structure in which three Sc3+ ions are sandwiched between two Q molecules, as determined by electron spin resonance. The one-electron reduction that forms the Q −-3 Sc3+-Q complex is third order in Sc3+ and second order in Q. — PDS
J. Am. Chem. Soc. 10.1021/ja037098c (2003).