Hexaaqua ion

Rhodium complexes with oxygen ligands, not nearly as numerous as those with amine and phosphine complexes, do, however, exist. A variety of compounds are known, iucluding [Rh(ox)3] [18307-26-1], [Rh(acac)3] [14284-92-5], the hexaaqua ion [Rh(OH2)3] [16920-31 -3], and Schiff base complexes. Soluble rhodium sulfate, Rh2(804 )3-a H2 0, exists iu a yellow form [15274-75-6], which probably coutaius [Rh(H20)3], and a red form [15274-78-9], which contains coordinated sulfate (125). The stmcture of the soluble nitrate [Rh(N03)3 2H20 [10139-58-9] is also complex (126). Another... 

Consider some vanadium ions in aqueous solution. Pale violet solutions of vanadium(ii) salts contain the [V(H20)6] ion. The vanadium(ii) center is only weakly polarizing, and the hexaaqua ion is the dominant solution species. Aqueous vanadium(ii) solutions are observed to be unstable with respect to reduction of water by the metal center. In contrast, vanadium(ni) is more highly polarizing and an equilibrium between the hexaaqua and pentaaquahydroxy ion is set up. The of 2.9 means that the [V(OH2)6] ion (Eq. 9.17) only exists in strongly acidic solution or in stabilizing crystal lattices. 

The implications of these mechanistic studies for our understanding of environmental iron sequestration by siderophores is as follows. The hydroxyl containing aqua ferric ions will tend to form ferri-siderophore complexes more rapidly than the hexaaqua ion and ferrous ion will be sequestered more rapidly than the ferric ion. However, once in a siderophore binding site the ferrous ion will be air oxidized to the ferric ion, due to the negative redox potentials (see Section III.D). This also means that Fe dissolution from rocks will be influenced by mineral composition (other donors in the first coordination shell) as well as surface reductases in contact with the rock, and of course surface area (4,13). 

Fig. 8. Energies calculated with a polarizable continuum model, differences of the sums of all metal-oxygen bond lengths, AS(M-O), and energy profiles for water exchange on rhodium(III) and ruthenium(II) hexaaqua ions.

Interestingly, the reorientational time is about 2-3 times larger than expected for a hexaaqua ion. Indeed, the second sphere water molecules... 

The hexaaqua ion, [Cr(OH2)6]3+, is also the final product in the reduction process (7, 12). The complete process using Fe(II) as a reducing agent is shown in the reaction sequence (8)-(ll), where a Fenton-type (15) mechanism is proposed (7,16), with [(H20)5CrIV0]2+ behaving as the chromium equivalent of the -OH radical (16). 

We now consider a hexaaqua ion as an example (Fig. 3) at which the angular overlap model can be applied quite naturally, but on which a parametrization based on the electrostatic model — though formally equivalent (24) — would necessarily be somewhat artificial. 

Since the barrier for such rotations may be expected to be quite low, we may here seek an explanation for the fact (2 a) that the spectra of the hexaaqua ions have a more pronounced temperature dependence than have similar ions with other ligands. 

Most simple hydrated Fe111 salts of oxyadds such as nitrate and perchlorate are presumed to contain the hexaaqua ion [Fe(H20)6]3+ though few crystallographic studies are available.1 104 The... 

In all its complexes both neutral and ionic Ti111 is normally octahedral. In dilute acids the main species is [Ti(H20)6]3+, which hydrolyzes (pAT = 1) to give [Ti(0H)(H20)5]2+. The hexaaqua ion occurs in alums such as CsTi(S04)2-12H20 and in simple salts such as [Ti(H20)6](/ -MeC6H4S03)3 3H20.54... 

The divalent state is the common and most stable oxidation state. In neutral or acid aqueous solution there is the very pale pink hexaaqua ion [Mn(H20)6]2, Which is resistant to oxidation as shown by the potentials... 

The propensity of aluminum salts to form the hexaaqua ion, [A1(H20)6] +, upon exposme to water requires that most aluminum oxyacid salts be prepared by nonaqueous routes. Some of these salts are of particular technological value. 

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