Aqua-ions complex formation

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). 

Enormous acceleration of substitution at ruthenium(II) can be obtained by appropriate choice of ligands. Following the demonstration of remarkably rapid water exchange and complex formation (7d or D in mechanism) at the organometallic aqua-ion [Ru(ri5-C5Me5)(H20)3]2+ (150), comparably rapid substitution has been demonstrated at... 

Plutonium(IV), hydrolysis of, 19 698 Plutonium-231, 19 670 Plutonium-238, 19 668, 669, 675 special precautions for, 19 703 Plutonium-239, 19 669 Plutonium aqua ions, thermodynamic values for, 19 693t Plutonium carbide, 4 649t stoichiometry, 4 651 Plutonium carbide (2 3), 4 649t Plutonium carbides, 19 690-691 Plutonium cations, 19 692 Plutonium chalcogenides, 19 691 Plutonium complexes bonding in, 19 694—695 formation constants for, 19 697t... 

In the Re(V) and W(IV) aqua oxo complexes, comparison of both the complex formation of the [MO(OH2)(CN)4], by NCS ions and the water exchange (k iq) shows a relative increase in reactivity of approximately 3 orders of magnitude (Table II), which is in direct agreement with the previously (1, 2, 50) concluded dissociative mechanism. The increase in Lewis acidity of the Re(V) center compared to that of W(IV) is expected to result in a much less reactive system in a dissociative activated mode. 

The aqua ion Au(H20)4+ has not been characterized either in solution or in the solid state. Most of the substitution studies have involved the halide complexes AuXj and Au(NH3) (Ref. 319). A number of earUer generalizations have been confirmed. Rates are very sensitive to the nature of both entering and leaving ligands and bond formation and breaking are nearly synchronous. The double-humped energy profiles witnessed with Pd(II) and Pt(II) are not invoked the five-coordinate species resulting from an associative mechanism is the transition state ... 

It has been tacitally assumed in this discussion that the second-order formation rate constants measure the simple water substitution process. Although this must apply when unidentate ligands replace coordinated water, a composite process could describe the replacement by multidentate ligands. However, consideration of rate constants for successive formation and dissociation processes suggests that the overall rate of complex formation with flexible bidentate (and probably multidentate) ligands such as diamines, dipyridyl, glycine is probably determined by the rate of expulsion of the first water molecule from the metal aqua ion (56, 80, cf. 3 and 84). 

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... 

If the solution has high fluoride concentration and low pFl values, anhydrous fluorides can be crystallized. The risk of substituting fluoride ligands by OFl ions increases when the pFl approaches the neutral regime the formation of mixed aqua-ftuoro complexes is favored at lower fluoride concentrations and low temperature. 

Aqua ions are known but not very stable. Substitution of Pt in aqueous solution is sometimes zero-order in the added ligand, L, or can have both L-dependent and L-independent contributions to the rate, probably because intermediate formation of an unstable aqua complex is the rate-determining step for the L-independent pathway. A large number of O-donors, particularly anionic ones, give stable complexes, for example, carbonate, acetate, oxalate, acetylacetonate, and alkoxide. Tetrameric platinum(II) acetate is formed by formic acid reduction of Pt solutions in acetic acid. It does not appear to be a very useful synthetic precursor for Pt chemistry. The acetylacetonate [Pt(acac)2] is monomeric and square planar. 

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