Ruthenium: Organometallic Chemistry

Abstract Unlike iron, whose organometallic chemistry dates from the end of the nineteenth century, that of its second‐row transition‐metal congener ruthenium had to wait for the discovery of the sandwich compound ferrocene in 1951. The impetus from that breakthrough stimulated all areas of organometallic research, and ruthenocene was reported the very next year. More so than in its coordination chemistry, it has become apparent that the organometallic chemistry of ruthenium has many features in common both with iron and with its heavier analog osmium. Thus, many complex types and reactions that are discussed below have counterparts with either iron or osmium or both. As is the case in all transition‐metal elements, there is a tendency for substitution or addition and elimination reactions to be slower when proceeding down a group in the periodic table. The middle elements, in particular ruthenium and rhodium, profit from this trend; for example, hydrido‐metal and alkyl‐metal bonds are generally more stable and consequently more abundant in ruthenium species than in analogous structures of iron. Conversely, rigid structures have been found in osmium complexes in cases where the ruthenium analogs are either unstable or highly fluxional. Accordingly, iron complexes are generally recognized as stoichiometric agents for organic synthesis, while ruthenium complexes were used as catalysts, and of course, many organic reactions characteristic to ruthenium catalysts were developed and reported so far. On the other hand, osmium complexes are generally stable to be used for the elucidation and confirmation of the intermediates of ruthenium‐catalyzed reactions. However, the chemistry of ruthenium was found to be far behind, compared with those of the same second‐row transition metals, rhodium and palladium, in the Groups 8–10. The most important reason is that the appropriate zerovalent mononuclear ruthenium complex, Ru0, which could generate 16‐electron species in solution was not readily available; the attempts to synthesize zerovalent ruthenium triphenylphosphine complexes were often unsuccessful because of the orthometalation reaction to form RuII complexes, which were usually inactive for catalytic reactions except for hydrogenation and isomerization of CC unsaturated bonds. With recent progress in inorganic and organometallic chemistry, organic synthesis catalyzed by ruthenium complexes has attracted much attention, and a large number of useful and catalytically active ruthenium complexes have been prepared. Consequently, in this updated “Ruthenium: Organometallic Chemistry”, the slightly old topics were omitted, while new topics about application of ruthenium complexes as homogeneous catalysts for both organic synthesis and metathesis reactions, as well as recent application of ruthenium complexes as anticancer agents, were added together with the corresponding new references.