Osmium five-coordinate

The reluctance of the carbyne carbon to react with nucleophiles is revealed by the reaction with LiEt3BH (see Scheme 6). Here the most electrophilic site is not the carbyne carbon but the ipara position of the aryl ring in the carbyne substituent Both ruthenium and osmium five coordinate, cationic, carbyne complexes undergo this reaction. The structure of a representative example, the osmium compound derived from the p-tolyl carbyne complex, has been determined by X-ray crystallography [16]. The unusual vinylidene complex reacts with HCl to produce a substituted benzyl derivative. The reaction may proceed through the intermediate a-vinyl complex depicted in Scheme 6 although there is also the possibility that the vinylidene compound is in equilibrium with the carbene tautomer as shown below. 

Scheme 8.20 Preparation of the five-coordinate osmium carbynes 75a,b and their reactions with chalcogens, HCI and chlorine to afford the six-coordinate carbene osmium(II) complexes 76a,b, 77a,b, and 78a,b (a R = Me, b R = Ph E = S, Se, Te)...

The five coordinate osmium(II) complex OsHCl(CO)(PCy3)2 was synthesized by Moers and coworkers and was foimd to take up SO2 as a sixth ligand. The SO stretching frequencies (1284 and 1109 cm" ) indicated >/ -planar SO2 binding, which was confirmed... 

Osmium tetroxide forms very stable, five-coordinate adducts with several nitrogen donors, e.g., quinuclidine or hexamethylenetetramine1-5. 

Diamagnetic, brown, monomeric diolatodioxobis(amine)osmium(VI) esters of adenosine 411, or five-coordinated monodiolato-amine osmium(VI) esters 5 with m-dioxo coordination12... 

If the nitrogen donor is a secondary 1,2-diamine, the initially formed hexacoordinated osmi-um(VI) ester of type 4 loses a molecule of water upon heating forming a five-coordinated square-based pyramidal osmium(VI) complex of type 647... 

A five-coordinate osmium hydride complex oxidatively adds HCl ... 

Fig. 14 Widely investigated half-sandwich iron and five-coordinate osmium boryl systems (showing definition of the torsion angle 0 for the former)...

Scheme 11 The synthesis of five-coordinate osmium(II) boryl complexes 8.104-8.108...

Substitution of water molecules by Cl or Br has been reported for the chal-cogenide clusters [Mo3Y7(H20)6j (Y = S, Se). Finally, dirhodium cyclo-pentadienyl derivatives with -OH bridges have been reported. An unusual five-coordinated hydrido-hydroxo mononuclear derivative of osmium is obtained upon reaction of HOsCl(CO)(PPr 3)2 with KOH in methanol. ... 

The reduction (22) of Fe(DMPE)2Cl2 was carried out in THF using two equivalents of sodium naphthalenide. The expected product, Fe (DMPE)2, was not obtained for reasons which will soon become apparent. Instead, two other products were obtained. The first was Fe(DMPE)2H(CioH7), analogous to previously reported ruthenium and osmium (15,16) compounds, except that it exists in solution as an equilibrium mixture of cis and trans isomers. The second product was a zero-valent complex, but instead of being four coordinate it is five coordinate, having the structure shown in 2 and displaying an AB4 P( H)... 

Early reports on interactions between redox enzymes and ruthenium or osmium compounds prior to the biosensor burst are hidden in a bulk of chemical and biochemical literature. This does not apply to the ruthenium biochemistry of cytochromes where complexes [Ru(NH3)5L] " , [Ru(bpy)2L2], and structurally related ruthenium compounds, which have been widely used in studies of intramolecular (long-range) electron transfer in proteins (124,156-158) and biomimetic models for the photosynthetic reaction centers (159). Applications of these compounds in biosensors are rather limited. The complex [Ru(NHg)6] has the correct redox potential but its reactivity toward oxidoreductases is low reflecting a low self-exchange rate constant (see Tables I and VII). The redox potentials of complexes [Ru(bpy)3] " and [Ru(phen)3] are way too much anodic (1.25 V vs. NHE) ruling out applications in MET. The complex [Ru(bpy)3] is such a powerful oxidant that it oxidizes HRP into Compounds II and I (160). The electron-transfer from the resting state of HRP at pH <10 when the hemin iron(III) is five-coordinate generates a 7i-cation radical intermediate with the rate constant 2.5 x 10 s" (pH 10.3)... 

The pattern of chemical reactions observed for these compounds clearly sets them apart from "Fischer-type" carbyne complexes of GroupVI e.g., W(hCR)X(CO)4. Whereas the "Fischer-type" complexes typically react with nucleophiles at the carbyne carbon all of the reactions observed for the five coordinate mthenium and osmium complexes, including the cationic examples, are electrophilic additions to the MsC bond. The following sections deal individually with, protonation, addition of halides of the coinage metals, addition of chlorine and chalcogens, and finally an attempted nucleophilic addition where the nucleophile is directed to a remote site on the aryl ring of the carbyne substituent. 

The metal-carbon triple bond chemistry of ruthenium and osmium described in this article bears a close resemblance to the metal-carbon double bond chemistry of these elements as exemplified by the methylene complexes [26]. In both systems two structural classes are found, five coordinate (trigonal bipyramidal, formally zero oxidation state) and six coordinate (octahedral, formally +2 oxidation state). In both systems the five coordinate compounds exhibit multiple metal-carbon bonds which are rather non-polar and typically undergo addition reactions with electrophilic reagents. On the other hand the six coordinate compounds, both M=C and M=C, begin to show electrophilic character at the carbon centres especially in cationic complexes. Further development of the carbyne chemistry of ruthenium and osmium will depend upon the discovery of new synthetic methods allowing the preparation of a broader range of compounds with widely differing carbyne substituents. 

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