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Rhenium, the last of the nonradioactive elements, was not discovered until 1925 (1). It is one of the three rarest elements in the earth's crust (0.0007 parts per million) and is isolated from the flue ash of molybdenum smelters. The organometallic chemistry of rhenium was little explored until recently, in part because common starting materials for rhenium chemistry are moderately expensive [$5 per gram for Re sub 2 O sub 7 and $8 per gram for Re sub 2 (CO) sub 10 ] and in part because rhenium has found only limited use in catalysis (Re-Pt mixtures are used in the catalytic reforming of petroleum, and Re sub 2 O sub 7 --SnMe sub 4 -Al sub 2 O sub 3 has been used as a heterogeneous catalyst for the metathesis of functionalized olefins).
In the past 10 years, there has been an explosive growth in the organometallic chemistry of rhenium as research groups from around the world have synthesized and isolated an astounding array of stable organorhenium compounds. There are three characteristics of rhenium that make its chemistry distinctive. First, the high strength of bonds made by rhenium to other elements stabilizes ordinarily reactive species and makes possible the isolation of normally unstable structural types. Second, the huge range of oxidation states available to rhenium has made it possible to isolate organometallic derivatives ranging in oxidation state from -III in (Character omitted) to VII in C sub 5 Me sub 5 ReO sub 3 (Me, methyl). Third, the reluctance of rhenium to form coordinatively unsaturated complexes enhances the stability of rhenium intermediates and gives rise to unusual reaction mechanisms. These characteristics are shared by other transition metals, particularly those in the third transition series, but they are particularly pronounced for rhenium.
In this article, recent advances in organorhenium chemistry are highlighted that showcase some of the current trends in organometallic chemistry, including the concept of concerted organometallic reactions that avoid high-energy coordinatively unsaturated metal complexes, the preparation of novel pi-propargyl complexes, the observation of agostic metal alkyls in which C-H bonds interact with a metal center, the use of chiral-at-metal complexes, the chemistry of compounds with multiple bonds between metals, the isolation of high-valent organometallic complexes, and the role of metals in the catalysis...