Tungsten complexes redox reactions

For a number of years, phenolic substances were dosed by colorimetric techniques, based on redox reactions usually known as Folin Ciocalteau methods, even if a number of adjustments were developed to fit different matrix characteristics. The Folin Cioalteau reagent is a mixture of phosphomolybdic and phosphotingstic acids, with molybdenum in the 6+ oxidation state and, when the reaction takes place, it is reduced to form a complex called molybdenum blue and tungsten blue. In this complex, the mean oxidation state is between 5 and 6 and the formed complex is blue so it can be read spectrophotometrically at 750 nm. 

The one non-redox reaction catalyzed by a tungsten enzyme involves the hydration of acetylene to acetaldehyde [5], This same reaction is catalyzed by WO(mnt)22- but not by W02(mnt)22- [186], The five-coordinate W(IV) complex should have an open site that may be available to coordinate and activate the acetylene substrate [194],... 

A comprehensive review of spectrophotometric methods for the determination of ascorbic acid (1) was presented. Most of the methods are based on the reducing action of ascorbic acid, making use of an Fe(III)-Fe(II) redox system, and to a lesser extent Cu(II)-Cu(I), V(V)-V(IV) and phosphomolybdate/phosphotungstate-molybdenum/tungsten blue redox systems. A kinetic spectrophotometric method for the determination of L-ascorbic acid and thiols (RSH) was developed, whereby the absorbance of the Fe(II)-phen complex formed during the reaction of 1 or RSH with Fe(III)-phen was continuously measured at 510 nm by a double beam spectrophotometer equipped with a flow cell. The linearity range for 1 was 4-40 p,M and for RSH 8-80 xM. The method was validated for pharmaceutical dosage forms . 

These complexes are excellent models for theoretical studies. The octacyano complexes of molybdenum and tungsten are stable and inert toward substitution reactions and therefore very suitable for theoretical studies of redox reactions and application of the Marcus theory. The photoreactivity of these systems is also proving to be important. The 0X0- and nitridocyano complexes of Mo(IV), W(IV), Tc(V), Re(V), and Os(VI) are very good candidates for kinetic studies of substitution reactions with both mono- and bidentate ligands and are of interest especially in view of the large variations in the observed reactivity. 

Spec A = 665 nm of the titanium-methylene blue-ascorbic acid redox reaction Spec A= 420 nm of the yellow Tiron derivative (pH = 5.2 to 5.6) Spectrophotometric determination of tungsten with thiocyanate after both tungsten(VI) and molybdenum(VI) are extracted into chloroform as benzoin a-oxime complexes... 

Metal alkylidyne complexes undergo a variety of oxidation and reduction reactions as well as redox-induced transformations of the alkylidyne ligands. A method for the direct transformation of Fischer-type carbyne complexes into Schrock-type alkylidyne complexes was developed in our laboratory. Bromine oxidation of the /ra/7, -carbyne bromo tetracarbonyl complexes 49 of molybdenum and tungsten in the presence of dimethox-yethane affords the dme-stabilized alkylidyne tribromo metal complexes 50 [Eq. (42)] (81). For alkyl-substituted complexes (R = Me, CH2CMe3)... 

Competition experiments again feature prominently in another discussion of the possible role of transient five-coordinate [Co(NH3)5] in induced and in spontaneous aquation of pentaaminecobalt(III) derivatives. " The operation or nonoperation of the D mechanism at various cobalt(III) centers and at penta-cyanoferrate(II) still requires a few experiments providing unambiguous results. Its operation at molybdenum(O)- and tungsten(0)-penta or tetracarbonyl complexes seems more firmly based. The question of its operation at pentacyanoferrates(III) does not seem to have caused much concern. The only recent paper which mentions kinetics of such a reaction, replacement of 2-methyl imidazolate in [Fe "(CN)5(2-Meimid)] ", reports that the limiting first-order rate constant is 2.3 x 10 s at 298 K, but is more preoccupied with redox catalysis by traces of iron(II) than with simple substitution. 

Oxidation of formic acid by Co aq is believed to involve substitution at Co aq as first step. However it did not prove possible to estimate formation rate constants in this study they were in fact assumed to be much larger than rate constants for the subsequent redox steps. Reaction of cw-[Co(en)2(OH2)2] with tungstate takes place much more rapidly than other formation reactions of aquo-cobalt(III) complexes, and indeed falls within the stopped-flow range. These reactions cannot therefore involve cobalt-oxygen bond breaking, but must rather be substitution at tungsten(VI). ... 

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