Group aqua ions

One of the motivations for studying ammonia complexes is that (before deuterium and oxygen isotopes were available) they offered one of the rare opportunities to get an idea about the constitution of aqua ions in solution. However, this does not always proceed as smoothly as in the case of nickel(II). Bjerrum found that Zn(NH3)/2 does not react further with aqueous ammonia, as confirmed by crystal structures containing this tetrahedral complex. However, the zinc(II) aqua ions in crystals are octahedral with N = 6 like magnesium(II) and nearly all 3d group aqua ions. 

The aqua ion is not easily reduced nor oxidized. It is the slowest reacting of the bivalent transition metal ions with e " k = 7.7 X 10 M s ) and the product Mn+q is very reactive. However Mn(CNR)5 (R = a variety of alkyl and aryl groups) is stable and the selfexchange in the Mn(l,ll) hexakis(isocyanide) system has been studied by Mn and H nmr line broadening. The effects of solvent, temperature, pressure and ligand have been thoroughly explored. 

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). 

The writer4,179 analyzed the hydration energy differences derived from Eq. (31) for aqua ions, where E° is known for the process increasing the oxidation number from (z — 1) to z. It turns out that each transition group has a characteristic constant K such that... 

Among the elements in groups IB and 2B, which form bonds of a more covalent character, several investigations have been reported for the copper(II) ion, which has the same distorted octahedral coordination as in the solid state (Table III). The silver ion does not form discrete aqua ions in the solid state, but occurs as Ag(H20)4+ in aqueous perchlorate solutions, with Ag—H20 bond lengths of 2.4 A, expected for a tetrahedrally coordinated Ag+ ion (19, 42). 

The aqua ions M2+ are more acidic than those in the same periods in group 2. Zinc (like Be) is amphoteric, dissolving at high pH to form [Zn(OH)4]2-. The other ions are not amphoteric as they have... 

The control of the Eu redox stability is certainly a key issue for an eventual MRI contrast agent application. With the exception of cryptate complexes of Eu such as Eu (2.2.2) + and Eu (2.2.1) +, the complexation with poly(amino carboxylates) diminishes the redox stability of the Eu state, as compared to the aqua ion (some representative redox potentials are -0.63 V (Eu(H20) ) -0.21 V (Eu(2.2.2)2+) -0.82 V (EuODDA) -1.00 V (EuTETA -) -1.18 V (EuDO-TA ) -1.35 V (EuDTPA )) [111, 112]. Macrocyclic ligands that match in size with the larger Eu ion have a stabilizing effect of the reduced state, whereas carboxylate coordinating groups seem to be unfavorable in this respect. 

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