OH2, Water

OFeN4C37H36, Iron, carbonyltetrakis(2-iso-cyano-l,3-dimethylbenzene)-, 26 57 OH2, Water... 

OC4H8, Furan, tetrahydrohafnium, niobium, scandium, titanium, vanadium, and zirconium complexes, 21 135-139 OH2, Water ... 

Where R is the gas constant, T the temperature (K), Fthe Faraday constant and H2 is the relative partial pressure (strictly, the fugacity) of hydrogen in solution, which for continued evolution becomes the total external pressure against which hydrogen bubbles must prevail to escape (usually 1 atm). The activity of water a jo is not usually taken into account in elementary treatments, since it is assumed that <7h2 0 = U nd for dilute solutions this causes little error. In some concentrated plating baths Oh2 0 I O nd neither is it in baths which use mixtures of water and miscible organic liquids (e.g. dimethyl formamide). However, by far the most important term is the hydrogen ion activity this may be separated so that equation 12.1 becomes... 

Since a small trace of water suffices to produce a large effect, the equilibrium of (43) evidently lies far in favor of the right-hand side (see also Sec. 115), indicating that a water molecule dissolved in ethyl alcohol provides a vacant level for an additional proton that lies lower than the level occupied by the protons in the OH2 group of the (C2H3OH2)+ ion. A proton captured in this lower level of (HaO)+ will have to wait until it receives the necessary energy before it can move back to an alcohol molecule. In the meantime the (H30)+ ion can merely contribute to the electrical conductivity by drifting slowly in the field only when the proton has returned to an alcohol molecule can the rapid proton jumps be resumed. 

The structure of the aqua complex (Figure 1.51), which is an active intermediate in some catalytic systems, shows the Ru-OH2 distance to be some 0.1 A longer than in the ruthenium(III) hexaqua ion, indicating a possible reason for its lability the water molecule also lies in a fairly exposed position, away from the bulk of the EDTA group. 

Water [579] is present in the structure of true crystalline hydrates [580] either as ligands co-ordinated with the cation (e.g. [Cu(OH2)4]2+ in CuS04 5 H20) or accommodated outside this co-ordination sphere within voids left in anion packing, further stabilized by hydrogen bonding (e.g. the remaining water molecule in CuS04 5 H20). 

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. 

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. 

By contrast, the acidity of the metal salts used in these cements has a less clear origin. All of the salts dissolve quite readily in water and give rise to free ions, of which the metal ions are acids in the Lewis sense. These ions form donor-acceptor complexes with a variety of other molecules, including water, so that the species which exists in aqueous solution is a well-characterized hexaquo ion, either Mg(OH2)g or Zn(OH2)g. However, zinc chloride at least has a ternary rather than binary relationship with water and quite readily forms mixtures of Zn0-HCl-H20 (Sorrell, 1977). Hence it is quite probable that in aqueous solution the metal salts involved in forming oxysalt cements dissolve to generate a certain amount of mineral acid, which means that these aqueous solutions function as acids in the Bronsted-Lowry sense. 

PtMe2(OR)(N-N)(OH2)] OH (47) (Eq. 6.16) [30, 31]. These complexes, likely resulting from an oxidative ROH addition, were characterized by elemental analysis, IR and NMR spectroscopy, conductivity measurements and conversion to derivatives containing weakly coordinating bulky anions. These reachons are of interest because they represent the first examples of oxidation of plahnum(ll) complexes with alcohols and provide the first stable alkoxoplahnum(lV) complexes. The alkoxo-platinum(lV) bond is inert against solvolysis by alcohols, water and even dilute perchloric acid. 

X-ray diffraction studies on several forms of the enzyme have demonstrated that the active site is composed of a pseudo-tetrahedral zinc center coordinated to three histidine imidazole groups and either a water molecule [(His)3Zn-OH2]2+ (His = histidine), or a hydroxide anion [(His)3Zn-OH] +, depending upon pH (156,157). On the basis of mechanistic studies, a number of details of the catalytic cycle for carbonic anhydrase have been elucidated, as summarized in Scheme 22... 

The stability of an unsubstituted carbene is quite low in water. Highly correlated ab initio MO calculations have been used to study the energetics and mechanism governing the reaction between the radical CH2 and H20 in the gas phase and in solution, and it was found that methylene reacts in a barrierless fashion to produce the ylide-like intermediate methyleneoxonium, H2C-OH2, which in turn undergoes a 1,2-hydrogen shift to produce CH3OH.128 The presence of substituents appears to stabilize carbenes toward water.129... 

Pyromellitate, the tetracarboxylate of benzene, forms red crystals of formula [Co2(C6H2 (C00)4)4]-18H20 from aqueous silica gel, and feature infinite chain-like polyanions with [Co(OH2)4[C6H2(COO)4)]2]ra2" stoichiometry where Co is in an octahedral environment of four waters and two traras-disposed carboxylates.436... 

Although there is a tendency to associate coordinated water with Werner-type complexes, where it is extensively established, organometallic aqua ions are known.945 The simple [(Cp )Co(OH2)3]2+ has been established, and is prepared via Equation (8). The lower pATa is 5.9, similar to values in aminecobalt(III) compounds, and reversible deprotonation and dimerization has been identified as part of the reactions of the aqua ion.946... 

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