Rhenium complexes stability

Finally, insertion of alkynes into dihydrogen mononuclear cationic rhenium complexes, stabilized by the triphos (= l,l,l-tris(diphenylphosphinomethyl)ethane) ligand, leads to vinylidene derivatives which react with water or ethanol eliminating silanols or methane, Fig. 18. Solvents were acetone or THF no chromatography was used. 

The diamagnetic ylide complexes 34 have been obtained from the reaction of electron-deficient complexes [MoH(SR)3(PMePh2)] and alkynes (HC=CTol for the scheme), via the formal insertion of the latter into the Mo-P bond. The structural data show that 34 corresponds to two different resonance-stabilized ylides forms 34a (a-vinyl form) and 34b (carbene ylide form) (Scheme 17) [73]. Concerning the group 7 recent examples of cis ylide rhenium complexes 36 cis-Me-Re-Me) have been reported from the reaction of the corresponding trans cationic alkyne derivatives 35 with PR" via a nucleophilic attack of this phosphine at the alkyne carbon. 

By far, the largest number of structurally characterized rhenium complexes contain the metal in the oxidation state +5 . This can be attributed to the high stability of the rhenium(V) oxo, nitrido, and imido cores with a great variety of ligand systems, but is doubtlessly also related to the fact that rhenium complexes are frequently used as nonradioactive model compounds for the development of technetium radiopharmaceuticals. The dominance of 0 , and NR ligands can be... 

Rhenium complexes containing the metal in the oxidation state +III are comparatively numerous. This may be ascribed to the fact that the d configuration of the rhenium center can readily be stabilized by ligands with pronounced donor and rr-acceptor properties. Most of the rhenium(III) compounds are stable against hydrolysis rendering them suitable for nuclear medical applications. 

The ability of polypyridyl ligands to accept electron density from electron-rich rhenium centers and, thus, to contribute to the stabilization of rhenium complexes with the metal in low oxidation states has already been discussed for rhenium(II) compounds. Only small modifications to the polypyridyl ligand or the metal center can create dramatic differences in the properties of the resulting complexes. Generally, the starting materials which have been introduced as precursors for rhenium(II) polypyridyl complexes in Section 5.3.2.6.2, are also appropriate for the synthesis of related rhenium(l) compounds. 

Explain in terms of redox chemistry how (he formation of ehloru complexes stabilizes rhenium(III)... 

Most photosensitizers, however, are reasonably photostable compounds, and their optical properties have been studied in depth. In particular, there has been much interest in ruthenium-based photosensitizers such as [Ru(bpy)3]2+ and [Ru(phen)3]2+, due to their stability and absorption of visible light. Detailed information on their optical properties, including ground and excited state information in relation to photosensitization, has been reviewed by Creutz et al. [16]. Similarly, the photochemistry and photophysics of rhenium complexes, as discussed here, have been reviewed in detail by Kirgan et al. [7]. 

Bruce and co-workers demonstrated mesomorphism in benzylideneaniline complexes bound to octahedral Mn(l) and Re(l), providing that the imine ligand was sufficiently anisotropic these complexes were the first simple octahedral complexes to show N and lamellar phases. They were formed by the or/.4o-metallation reaction of the imine with [MMe(GO)s] (M = Mn, Re). The parent ligands showed smectic and nematic phases at temperatures up to 300 °C, whereas on complexation to Mn(l), only a nematic phase was seen for 11 (11 M = Mn, = 5, 7) and 12 (12 M = Mn X = Y = H n = m = 8) which cleared below 190 °C with decomposition. The related Re(l) complexes yielded materials with very similar transition temperatures and enhanced thermal stability, so that decomposition was not observed at the clearing point.Therefore, in the following part of this study, only rhenium complexes were studied. 

Gladysz has investigated the solution behaviour and configurational stability of these chiral rhenium complexes further, he extensively studied the chiral recognition process that results from the enantioface binding of alkenes, aldehydes and ketones on these rhenium chiral Lewis acids 3.7 and/or 3.8. There is a huge body of papers and reviews on this chemistry, and the reader may consult the referenced review. ... 

Seifert S, Syhre R, Gupta A, Spies H, Johannsen B (1999) Stability studies on 3+1 mixed-ligand technetium and rhenium complexes. In Nicolini M, Mazzi U (eds) Technetium, rhenium and other metals in chemistry and nudear medicine, vol 5. SGE Editoriali, Padova, p 687... 

The chemistry of technetium(II) and rhenium(II) is meagre and mainly confined to arsine and phosphine complexes. The best known of these are [MCl2(diars)2], obtained by reduction with hypophosphite and Sn respectively from the corresponding Tc and Re complexes, and in which the low oxidation state is presumably stabilized by n donation to the ligands. This oxidation state, however, is really best typified by manganese for which it is the most thoroughly studied and, in aqueous solution, by far the most... 

The surprising stability of uni-negative rhenium, the complexes of which are undoubtedly covalent, may be related to the especial strength of the bonds formed by the neutral osmium atom. 

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