Five-coordinate ruthenium intermediate

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 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. 

The addition of mercuric halides to five-coordinate intermediate stage where HgX+ has formed a bond with the central metal to afford a cationic complex. Reactions of the ruthenium(O) and osmium(O) complexes (XIV) with mercuric halides illustrate this type of behavior (37). Such reactions do not always go to completion. For... 

Five-coordinate d complexes add halogens under very mild conditions, usually with loss of a neutral ligand to form cis dihalo compounds. Comparison with the analogous ruthenium(O) and osmium(O) compounds (XIV) reveals the stepwise nature of halogenation. In the case of the osmium complex a cationic intermediate (XLIV), which has been isolated and characterized, is formed (39). Warming this cationic complex in tetrahydrofuran results in the loss of a mole of CO and the formation of the cis dihalide (XLV). The intermediate ruthenium complex is apparently too... 

Substitution is, of course, much slower in [Ru(pc)L2] than in [Fe(pc)L2]. A D mechanism operates in both series of complexes the five-coordinate intermediates show very little discrimination. Variable-temperature proton nmr studies of axial ligand exchange in 1-methylimidazole- and 4-r-butylpyridine-benzyl isonitrile-ruthenium-tetraphenylporphyrin complexes show that tetraphenylporphyrin has a much smaller cis effect here than in analogous iron(II) systems. Again tt-bonding effects are important in determining kinetic parameters. ... 

There have been several reviews of mechanisms of photosubstitution in rhodium(III) complexes. Bond indexes for ground and excited states have been discussed in relation to D2h species. " The observation of stereospecificity has been discussed in relation to lifetimes for triplet singlet deactivation and geometric rearrangements. Direct evidence has been presented to support the intermediacy of, and role of rearrangement in, five-coordinate intermediates in ligand field irradiation experiments. Rhodium(III) has been discussed in relation to cobalt(III) and iridium(III), and to ruthenium(II) and ruthenium(III) as well. ... 

The mechanism of axial ligand substitution with a series of ruthenium(II) phthalocyanine adducts is dissociative D), the five-coordinate intermediate possessing little or no ability to discriminate between nucleophiles. The cis effect of tetraphenylporphyrin is considerably less in Ru(III) complexes than in their Fe(II) analogs. An estimate of the relative labilizing effects of the ligands L—L and Cl in the complexes [Pd(L—LiCb] (where L—L = l,2-bis(diphenylphosphino)ethane, o-phenylenebis(dimethylarsine), en, phen, bipy, or l,2-bis(phenylthio)-ethane) have been made through a study of the reaction of these complexes with en. 

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