Aqua-ions acidity

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. 

This supramolecular approach has been used as a way to facilitate the crystallization of cuboidal aqua ions and to capture single intermediates present in complex solution mixtures. For example, [M3Q4Clx(H20)9.x] (Q = S, Se) supramolecular adducts have been isolated for x =1-5 from aqueous HCl solutions by varying the acid concentrations where the aggregates contain in each case only one of all possible isomers [46]. In addition to the hydrogen bonds, other kinds of interactions, namely C1---H20, C1---C1 and Q---Q contacts, are also involved in the network propagation. This supramolecular approach has also been efficiently employed for the crystallization of a large number of aqua complexes within the [M3M Q4] cubane-type family, where M is a transition or post-transition metal [47]. 

In the case of the rhenium aqua-ion [Re(OH2)3(CO)3]+ (33b) the question has been posed whether complex-anion can be considered to be a Bronsted acid. Titrations with hydroxide in water yielded a pKa value of 7.55 which is exceptionally low for a +1 cation. After the deprotonation of one coordinated water molecule, polymer formation over (/r-OH) bridges was initiated and the two compounds [Re3(/T3-OH)(/T-OH)3(CO)9r (35) and [Re2(/i-OH)3(CO)6] were (36) isolated and structurally characterized (Scheme 6). 

Hydrolysis of ferric solutions is readily induced by addition of base. Initially, at rather acid pH, the purple ferric aqua-ion Fe(H20)63+, upon addition of base undergoes the first deprotonation step (equation 5), which is followed by reversible dimerization (equation 6), giving a yellow solution of mono- and dinuclear species ... 

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... 

This equation shows that not only a high metal-ion concentration, but also a high pH, often favors the formation of higher polynuclear species, since y generally increases more rapidly than x. For many aqua metal ions, however, the precipitation of insoluble hydroxides sets an upper pH limit, so that in practice it is possible to study the oligomerization reactions only within a narrow pH region defined by the magnitude of the first acid dissociation constant of the monomeric aqua ion and the pH at which insoluble hydroxide formation occurs. 

Condensation to monohydroxo-bridged complexes is often described by Eq. (28), for which the equilibrium constants Kd are related to those defined by Eq. (27) by Kd = Q2i/Qh, where is the first acid dissociation constant of the mononuclear aqua ion. 

Decarboxylation of carbonate complexes is usually effected by acid hydrolysis with the formation of a C02 free oxide or hydroxide complex.128 All such reactions involve a protonated (bicarbonate) intermediate but there are some useful deferences which, in many instances, may be reconciled with the three main structural types of carbonate complexes. Both unidentate and chelate carbonates readily yield C02 on acidification, while there is a greater resistance to C02 loss when the carbonate is a bridging ligand. Unidentate carbonate complexes decarboxylate with the initial formation of a bicarbonate intermediate and subsequent loss of C02 without rupture of the M—O bond, viz. structure (3). By contrast, in chelate carbonate complexes, cleavage of the M—O bond occurs (with ring opening) with the formation of a bicarbonate aqua ion before the loss of C02, viz. equation (5).29... 

However, the reaction requires only a general acid catalyst rather than the specific acid catalyst H+, and the corresponding reactions of the soft thioether may be better mediated by softer Lewis acids such as Cu+, Ag+, Hg2+, Pd2+, Pt2+ or Au3+. In many cases the aqua-ted metal ion is the most convenient Lewis acid, but in the case of some metals, particularly the second and third row transition metal ions, the aqua ions are not isolable and other complexes (particularly those with chloride ligands) are equally effective. The role of these softer metal ions as Lewis acids is two-fold. Firstly, the sulfur is co-ordinated to the metal, which increases the polarisation of the C-S bond and enhances the electrophilic character of the carbon, and, secondly, the thiol (or thiolate) leaving group is stabilised by co-ordination (Fig. 4-39). 

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