V Aqua Dimer

This ion has been prepared by three different methods. These involve the reduction of sodium molybdate(VI) in 10 A/ hydrochloric acid with hydrazine hydrochloride at 80°, the aquation of dipyridinium pentachlorooxomolyb-date(V) in 0.05 M HC104, and the reduction of sodium molybdate(VI) with ammonium hexachloromolybdate(III). ° The first of these is described here. 

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Reactions with hexachloroiridate(IV) (fast), 02 (t /2 1 min), and aqua V02+ (t,/2 10 min in a typical experiment) yield the molybdenum(V) aqua dimer as the only identifiable product. At much slower rates, 02 and hexachloroiridate(lV) also oxidize Mo(V) to Mo(VI). 

To the ice-cooled Mo(V) aqua dimer [MOjOjS faq)] (0.035M per dimer, 200 mL in 1MHC1) in a beaker, is added Na[BH ] (5.5 g, NatBH4]/ Mo 10) gradually with stirring. Concentrated HCl (10 mL) is added slowly and the solution turns from orange-red to brown. 

Molybdenum has an extensive aqueous solution chemistry for oxidation states II through VI. It is unique in having aqua or aqua/oxo ions for all five states in acidic solution (pH < 2). These are well defined in all but the Mg 1 case, the study of which is complicated by the existence of rapid equilibria involving protonated/deprotonated monomer/dimer (and higher) forms. The VI state is without question the most stable and in contrast to Crvi is only the mildest of oxidants. Compounds which have contributed to the development of the aqueous solution chemistry, including the aqua ions themselves, are considered under Section 36.1.2. It is only since 1971 that the aqua forms of oxidation state II-V ions have been identified, and... 

Shimizu et al. used simple rhodium-aqua ions (Rh3+) immobilized onto polymer-modified electrodes to perform the electrochemical reduction of NAD+ [114]. Rh3+ was loaded onto polymeric anion doped-polypyrrole membranes coated on the surface. Electrochemical reduction of NAD+ with immobilized Rh3+ was performed at —0.85 V, where Rh3+ was reduced to Rh+. NADH was produced without detectable formation of NAD-dimers. 

Picolinic acid (pyridine 2-carboxylic acid) complexes of chromium(III) have been the subject of a number of studies. Complexation by picolinic acid in water/ethanol (30% v/v) follows an ion-pairing, Eigen-Wilkins type mechanism.Activation parameters suggest an associative character for the reaction of the aqua complex. Chelated complexes of chromiuni(ni) and picolinic acid are the products of the rapid, inner-sphere reduction of [Co (pico)(NH3)5p with chromium(II). The reaction of the related 4-carboxylic acid complex of cobalt(III) with chromium(II) is also rapid in contrast, pyridine-3-carboxylic acid (nicotinic acid) complexes undergo slower reactions. A -hydroxy-bridged dimeric complex [Cr2(pico)4(OH)2] has also been prepared. A study of magnetic properties in the temperature range 16-300 K leads to J - —6 cm and g = 2, typical for such complexes. 

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