Rhenium: Organometallic Chemistry
Authors: Romão Carlos C. 
Published: March 2006

The organometallic chemistry of Rhenium spans 11 oxidation states. Since many ligands stabilize several oxidation states, this chemistry is described by ligand type. Re‐Re multiple bonded organometallic complexes, Polynuclear organometallic complexes, and Bioorganometallic Chemistry, uncoverable in this fashion, are treated separately.

The CO ligand dominates the chemistry of the lower oxidation states (Re−IIIto ReII) most of which is derived from Re2(CO)10and the consequences of the rupture of its Re‐Re bond. [Re(CO)5]•, [Re(CO)5]−and Re(CO)5X are the most important products of Re‐Re homolysis, reduction and oxidation, respectively, and originate a vast chemistry. [ReXx(CO)yLz] is the largest family of organorhenium complexes. The luminescentfac‐[ReX(CO)3L2] molecules assume high practical relevance in photoresponsive materials and sensors.

Polyhydrides [ReHxLy] lose H2upon thermal, photochemical or oxidative activation promoting alkane dehydrogenation. The abundant [Re(CO)xHy] clusters are important building blocs for catalysts and materials. [ReH(CO)4]nare analogues of cycloalkanes.

Mononuclear homoleptic alkyls and aryls of Re in high oxidation states are limited to ReMe6and [Re(2‐MeC6H4)4]. Instead, carbenes and carbynes, as the remarkable [Re(≡CCMe3)(=CHCMe3)(CH2CMe3)2], are common. ReVIIoxo and imido complexes also originate carbene/carbyne complexes active in olefin metathesis catalysis. ReO3Me (MTO) is a uniquely versatile catalyst in a wide variety of oxidation and O transfer catalysis. ReO2Me derivatives are active catalysts namely in aldehyde olefination.

η2‐, η3‐, η4‐ (dienes), η6‐ (arenes) and η7‐ (cycloheptatrienyl) ligands play a limited role in the organorhenium chemistry. Exceptional is, however, the activation of η2‐arenes by ReIcomplexes. The dearomatized benzene ring in TpRe(CO)(MeIm)(η2‐C6H6) reacts as a diene. Unusual alkyne complexes appear in ReIIIand ReVoxides, e.g. ReR(O)(η2‐RCCR) (R = H, alkyl). [Cp'Re(CO)2(η3‐propargyl)]+complexes reveal a fascinating series of structural transformations and rearrangements.

The cyclopentadienyl ligand (η5‐C5R5 = Cp’) shapes the large families of complexes of the fragments Cp’2Re (rhenocene) Cp'Re(CO)2, [Cp'Re(NO)(PPh3)]+and Cp'ReOxXy. Hydrocarbyl transformations at these fragments and the understanding of the stereochemical control in enantioselective transformations of ligands bound to the chiral‐at‐metal fragment [Cp'Re(NO)(PPh3)]+are of outstanding relevance. Isoelectronic neutral or negative 6 e donors may enhance these transformations and support new ones.

[Cp*Re(CO)2]2and [Re(≡CCMe3)(OCMe3)2]2are rare examples of unsupported dimers of 16e fragments. Re isocyanide and nitrosyl complexes follow most of the structural and chemical patterns found for the carbonyl ligand. However, recent findings revealed that the participation of the O atom of the NO ligand in H bonding may lead to unexpected reactivity.

Polynuclear Re complexes span a bewildering variety of synthetic methods and structural motifs. The ultimate rationale for their study is cooperative catalysis, as modelled in alkene hydrogenation with a Re/Pt binuclear complex and applied in petroleum refining with PtxReyclusters.

Bioorganometallic chemistry is rapidly evolving into practical medical applications in diagnostics and therapy. Stable Re(CO)3fragments decorated with biologically active molecules are able to target Re compounds to specific tissues/organs. New synthetic methods have been developed for the synthesis of the required molecules in aqueous solution, avoiding the use of CO gas and classical high pressure synthesis.