Aqua Ions of Molybdenum

Molybdenum is at present unique in having aqua ions in five oxidation states. Whereas the complexities of Mo(VI) aqueous solution chemistry have been understood in general terms for some time, it is only in the last 15 years that the aqua ions of the lower oxidation states II through V have been identified, and their structures clearly established. Metal aqua ions are notoriously difficult to crystallize for X-ray diffraction studies, and structures of derivative complexes 

The tetra- p-acetato-dimolybdenum(II) can be prepared from hexacarbonyl molybdenum by a method already described.2 The yellow crystals obtained are used for the second stage without further purification. 

Conversion to the tetrapotassium octachlorodimolybdate(II) dihydrate3 is achieved as follows. Concentrated hydrochloric acid (130 mL), in a 250-mL conical flask, is cooled to 0° in ice, and saturated with hydrogen chloride gas for seven minutes. Potassium chloride (3.6 g) is added with stirring, and the mixture warmed to room temperature. The tetra-p-acetato-dimolybdate(II) (2.6 g) 

The diamagnetic molybdenum(ll) aqua ion is characterized by an absorption maximum at 510 mm (e = 370 M 1 cm ) (8 - 8 transition), with a minimum at 420 mm (e = 120 M cm 1) for the dimer. Pure samples do not have a peak at 370 nm. The oxidation state of the molybdenum may be verified as 11 by addition of a 100-fold excess of Fe(lll), to convert Mo(II) to Mo(Vl) (20 minutes required), and titration of the generated Fe(Il) with standard Ce(lV), using ferroin as the indicator. 

A structure containing two quadruply bonded molybdenum(ll) atoms surrounded by eight water molecules in an eclipsed configuration can be assigned, in view of the similarity in absorption spectrum of the aqua ion, the 

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The Mo(IV) aqua ion was first reported by Souchay et al. as a monomeric species in 1966 (93), and the Mo3044+ core structure has been confirmed by single-crystal X-ray structure analyses, as described in this review, 95Mo NMR (94), EXAFS structure analysis (95), and 180-labeling experiments (96), after the appearance of many contradictory reports (97). Information on the reduction products of the Mo(IV) aqua ion is also available (98), Richens and Sykes have summarized the preparation of the different aqua ions of molybdenum in oxidation states II to V (99). 

TABLE I Summary of Single Oxidation State Aqua Ions of Molybdenum... 

Aqueous Chemistry. Molybdenum has weU-characterized aqueous chemistry in the five oxidation states, VI, V, IV, III, and II. A listing of aqua ions is given in Table 2. Except for the Mo(VI) species all of the aqua ions are only soluble or stable in acidic media (17). The range of aqueous ions known for molybdenum is far broader than that of other elements. 

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... 

The sulfur-bridged molybdenum aqua ions have been prepared by the following methods (1) reduction of [Mo202S2(cys)2] with Na[BH4], °- (2) reduction of [Mo203S(aq)] with Na[BH4], ... 

Noteworthy also is the large difference in the metal-aqua bond distance in the molybdenum(IV) and rhenium(V) complexes The rhenium-aqua bond is 0.13 A shorter than the molybdenum-aqua bond, which is more than one would expect from the ionic radii of these metal ions and is probably a result of the higher positive charge on the rhenium atom. The relative strong rhenium-aqua bond resulted in a relative strong acidic water ligand compared with that of the molybde-... 

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