Advances in metal chemistry of quinonoid compounds: new types of interactions between metals and aromatics.

This Account presents an overview of current research activities that focus on novel types of interactions between cationic transition metal complexes and arene systems and on unprecedented quinonoid complexes which result from such interactions. When a negatively charged phenoxy group is present in a position para to the metal in a high oxidation state, intramolecular charge transfer occurs, giving the corresponding metallaquinones or quinone methide complexes. In addition, two types of interactions involving low-valent metal compounds have been observed: methylene arenium complexes which result from positive charge transfer to the aromatic ring and sigma-bonded C-H and C-C agostic complexes of cationic metals. These sigma-complexes are proposed as intermediates in metal-based bond activation processes.