Water aquo complexes

The FI2O molecules of these aquo-complexes constitute the iimer solvation shell of the ions, which are, in turn, surrounded by an external solvation shell of more or less uncoordinated water molecules fomiing part of the water continuum, as described in section A2.4.2 above. Owing to the difference in the solvation energies,... 

Consider now the aquo-complexes above, and let v be the distance of the centre of mass of the water molecules constituting the iimer solvation shell from the central ion. The binding mteraction of these molecules leads to vibrations... 

We have already seen that in the aquo-complex which is usually formed when a simple transition metal salt dissolves in water, the... 

In an aquo-complex, loss of protons from the coordinated water molecules can occur, as with hydrated non-transition metal ions (p. 45). To prevent proton loss by aquo complexes, therefore, acid must usually be added. It is for these conditions that redox potentials in Chapter 4 are usually quoted. Thus, in acid solutions, we have... 

When a copper(II) salt dissolves in water, the complex aquo-ion [Cu(H2p)6P is formed this has a distorted octahedral (tetragonal) structure, with four near water molecules in a square plane around the copper and two far water molecules, one above and one below this plane. Addition of excess ammonia replaces only the four planar water molecules, to give the deep blue complex [Cu(NH3)4(H20)2] (often written as [Cu(NHj)4] for simplicity). TTo obtain [Cu(NH3)6], water must be absent, and an anhydrous copper(II) salt must be treated with liquid ammonia. 

In the presence of appropriate ligands, the values may be affected sufficiently to make Cu(l) stable but since the likely aquo-complex which Cu(I) would form is [Cu(H20)2], with only two water ligands, the (hypothetical) hydration energy of Cu is therefore much less than that of the higher charged, more strongly aquated [Cu(H20)e]. ... 

Chemical Variety. The term species refers to the actual form in which a molecule or ion is present in solution. Eor example, a metal ion may occur in natural waters, as a free metal ion, ie, an aquo complex Me(H20), an inorganic or organic complex, and it may be present in dissolved or... 

Water of crystallization, aquo complexes and solid hydrates... 

Many ionic compounds contain what used to be referred to as water of crystallization . For example, magnesium chloride can exist as a fully hydrated salt which was formerly written MgCla.bHjO, but is more appropriately written Mg(OH2)eCl2, since the water molecules occupy coordination sites around the magnesium ions. This is typical. In most compounds that contain water of crystallization, the water molecules are bound to the cation in an aquo complex in the manner originally proposed by Alfred Werner (1866-1919) in 1893 (Kauffman, 1981). Such an arrangement has been confirmed in numerous cases by X-ray diffraction techniques. 

CO3 species was formed and the X-ray structure solved. It is thought that the carbonate species forms on reaction with water, which was problematic in the selected strategy, as water was produced in the formation of the dialkyl carbonates. Other problems included compound solubility and the stability of the monoalkyl carbonate complex. Van Eldik and co-workers also carried out a detailed kinetic study of the hydration of carbon dioxide and the dehydration of bicarbonate both in the presence and absence of the zinc complex of 1,5,9-triazacyclododecane (12[ane]N3). The zinc hydroxo form is shown to catalyze the hydration reaction and only the aquo complex catalyzes the dehydration of bicarbonate. Kinetic data including second order rate constants were discussed in reference to other model systems and the enzyme carbonic anhy-drase.459 The zinc complex of the tetraamine 1,4,7,10-tetraazacyclododecane (cyclen) was also studied as a catalyst for these reactions in aqueous solution and comparison of activity suggests formation of a bidentate bicarbonate intermediate inhibits the catalytic activity. Van Eldik concludes that a unidentate bicarbonate intermediate is most likely to the active species in the enzyme carbonic anhydrase.460... 

Laughlin et al. [122] analysed chloroform extracts of tributyltin dissolved in seawater using nuclear magnetic resonance spectroscopy. It was shown that an equilibrium mixture occurs which contains tributyltin chloride, tributyl tin hydroxide, the aquo complex, and a tributyltin carbonate species. Fluorometry has been used to determine triphenyltin compounds in seawater [123]. Triph-enyltin compounds in water at concentrations of 0.004-2 pmg/1 are readily extracted into toluene and can be determined by spectrofluorometric measurements of the triphenyltin-3-hydroxyflavone complex. 

Adsorption of Ag on the surface of PdO is also an interesting option offered by colloidal oxide synthesis. Silver is a well-known promoter for the improvement of catalytic properties, primarily selectivity, in various reactions such as hydrogenation of polyunsaturated compounds." The more stable oxidation state of silver is -F1 Aquo soluble precursors are silver nitrate (halide precursors are aU insoluble), and some organics such as acetate or oxalate with limited solubility may also be used." Ag" " is a d ° ion and can easily form linear AgL2 type complexes according to crystal field theory. Nevertheless, even for a concentrated solution of AgNOs, Ag+ does not form aquo complexes." Although a solvation sphere surrounds the cation, no metal-water chemical bonds have been observed. 

The trichloroammine can have either a cw-chloride replaced by water to give the cw-aquo complex, or it can have the trans-chloride replaced, and then each of these two isomers in turn can hydrolyze further. They will each lead to a common cw-diaquo species, and the cw-dichloroaquoammineplatinum(II) will also lead to a possible /raws-diaquo isomer. 

A 1 1 aquo complex was prepared with the chiral crown 1,3 1, 3 4,6 4, 6 -tetra-0-methylene-2,2 5,5 -bis-0-oxydiethylene-di-D-mannitol (9). In the crystal the host molecule has C2 symmetry, and the hydrogen bonded water guest molecule lies on the twofold axis. The oxygen atom of the water sits above the crown and is hydrogen bonded (H -O = 1.96 A) to two ether linkages adjacent to the six-membered rings. 

For interfacial systems, potential functions should ideally be transferrable from the gas-phase to the condensed phase. Aqueous-mineral interfaces are not in the gas phase (although they may be close, see (7)), but both the water molecules and the atoms/ions in the substrate are in contact with an environment that is very different from their bulk environment. The easiest different environment to test, especially when comparing with electronic structure calculations, is a vacuum, so there is likely to be a great deal of information available on either the surface of the solid or the gas-phase polynuclear ion or the gas-phase aquo complex (i.e., Fe(H20)63+, C03(H20)62-). The gas-phase transfer-ability requirements on potential functions are challenging, but it is difficult to imagine constructing effective potential functions for such systems without using gas-phase systems in the construction process. This means that any water molecules used on these complexes must also transfer from the gas phase to the condensed phase. A fundamental aspect of this transferability is polarization. 

In aqueous media the trivalent rare earth ions are strongly hydrated, and the formation of an aquo complex [M(OHa) ]3+ (where n is larger than six, perhaps eight or nine) takes place. There is also a distinct lowering of pH on dissolving the salts of rare earths in water. The extent of lowering of pH depends essentially on the concentration of the salt and the nature of the particular rare earth ion. The heavier rare earth ions which possess small ionic radii show a greater tendency to hydrolyse. Certain anions like the halides, sulphates and nitrates tend to form ion-pairs in aqueous solution. There is, however, spectroscopic evidence [258] that the formation of an ion-pair readily takes place in an alcoholic medium also. 

The effect of OH and O 2 on the reactivity of complexes deserves special mention. It was found that some hydroxy complexes—e.g., Al(OH)4 and Zn(OH)4 2, are extremely inert to attack by e aq (11). Since no orbitals are involved in these cases, it must be concluded that OH is an extremely poor bridge for electrons. Hydroxy complexes which still contain water in their inner sphere, like cuprate (II), chromate (III), or plumbate(II) do not differ as much from their corresponding aquo complexes. The inhibitory effect of O-2 on the reactivity of the central atom is not sufficient to suppress the reactivity of the metal ions in their higher states of oxidation. Thus we find that Mn04 or Cr207 2 react at diffusion-controlled rates (122). 

In acidic aqueous solution the carbonato-compound [Ru(bpy)2C03] yields the diaquo complex which can be isolated72. Davies and Mullins had obtained this aquo complex by reaction of the dichloro-compound with silver ion in aqueous solution50. The interesting point in this latter work is that aquation is very rapid thus, if water is an acceptable solvent the dichloro-compound in hot aqueous solution may be the best reagent. 

Oxygen Donors. The formation and stability constants oi complexes between Pt (Pd, Rh, Ir, Os, Ru) and o-coumaric acid have been determined by pH titration.31 The results indicate that a 1 2 complex is formed with Pt. Acid-base properties of aquo-complexes formed from [Pt(X)2(OH)2(NH3)(MeNH2)] (X = Cl, Br or N02) in aqueous solutions have been examined using potentiometric titration experiments.186 The Ka of co-ordinated water was lower for (X)2 = (H20)2 than for (X)2 = (H20) (OH-). 

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