Bond crystalline hydrates

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). 

The crystal structures of several members of the zwitterionic complex family were determined. Like in the anionic spirosilicates, the structures were found to vary continuously between TBP and SP, with Si—O bond lengths essentially equal to those in anionic analogs. More than one crystalline modification was found for some of the zwitterions, and even those (even for the same complex ) were substantially different in molecular geometry for 60 the percent deviation from TBP - SP was calculated to be 34.9% in the monoclinic crystal, 70.0% in the orthorombic, 86.2% in the crystalline hydrate and 96.3% in another crystal form of the monohydrate46 47. These results were interpreted in terms of variation in hydrogen bonds in the different crystal modifications. In the hydrates... 

These stability effects are apparent in the equilibrium constants for hydration of ketones and aldehydes. Ketones have values of Keq of about 10-4 to 10-2. For most aldehydes, the equilibrium constant for hydration is close to 1. Formaldehyde, with no alkyl groups bonded to the carbonyl carbon, has a hydration equilibrium constant of about 40. Strongly electron-withdrawing substituents on the alkyl group of a ketone or aldehyde also destabilize the carbonyl group and favor the hydrate. Chloral (trichloroacetaldehyde) has an electron-withdrawing trichloromethyl group that favors the hydrate. Chloral forms a stable, crystalline hydrate that became famous in the movies as knockout drops or a Mickey Finn. 

Malonate and the dianions of the other aliphatic dicarboxylic acids H02 C(CH2 ) CO,H (n = 1-8) have been isolated as crystalline hydrates, and some of these have been dehydrated.293 X-Ray structural analyses have been reported for the malonate dihydrate,294 which has trans H2Os and bridging malonates of type (110), and for the succinate tetrahydrate and adipate dihydrate 295 the succinate reputedly has a trans- fAn02 (H2 0)4 ] polyhedron with bridging dicarboxylate, presumably as in (109). The monoanion of maleic acid (111) is only unidentate in crystalline [Mn(H-maleate)2-(H20)4] 296 but the equivalent compound of phthalic acid is a bis-chelated octahedral [Mn(0,0)2 (H2 0)2] species.297 Clearly other interactions in the solids, as well as the bonding interactions with Mn", define structure in the carboxylate compounds. 

Crystalline hydrates of metal ions and of organic substances, especially those with N—H and O—H bonds, are numerous. For metal ions, the oxygen is always bound to the metal and the lone pairs on it can be directed toward the metal and involved in bonding but can, however, also form H bonds. There is hence flexibility, allowing stabilization in lattices of many different types of hydrated structure. 

The acid is tribasic at 25°C, pK — 2.15, pK2 = 7.1, pA 12.4. The pure acid and its crystalline hydrates have tetrahedral P04 groups connected by hydrogen bonds. These persist in the concentrated solutions and are responsible for the syrupy nature. For solutions of concentration < 50%, the phosphate anions are hydrogen bonded to the liquid water rather than to other phosphate anions. 

The fact that many crystalline hydrates melt, such as Ca(W03)2-4H20, while more complex binary hydrated systems have a tendency to form glasses is an indication of the critical role of the water molecule and the hydrogen bond. 

Various models are reported in the literature for correcting the bond lengths and angles of H2O molecules in solid hydrates for thermal motion and anharmonicity. However, as recently shown , both positive and negative terms exist, which partially compensate for each other. Therefore, data obtained by neutron diffraction do not show systematic errors larger than 3 pm and 2°, respectively, and, hence, the differences between the average water molecule in crystalline hydrates and that in the gas phase discussed above should be real. ... 

Although each of these ion-dipole bonds (Sec. 1.21) is weak, in the aggregate they supply a great deal of energy. (Wc should recall that the ion dipole bonds in hydrated sodium and chloride ions provide the energy for the breaking down of the sodium chloride crystalline lattice, a process which in the absence of water requires a temperature of 801. ) Just as a hydrogen ion is pulled out oj the molecule by a hydroxide ion, so a halide ion is pulled out by solvent molecules. 

In the crystalline hydrates of some acids a proton is transferred to the water molecule forming the ion, which can form three hydrogen bonds. 

Many metal carboxylates are prepared in the form of hydrates and water is lost in a lower temperature range than that required for the onset of anion decomposition [86]. The removal of water from the structure must be accompanied by substantial bond redistribution within the anhydrous phase so formed, and this can sometimes lead to structural reorganization within the solid [142]. Nickel salts, prepared in the form of crystalline hydrates, however, show a low degree of lattice order after water removal [116,118,121]. The crystal structures of few dehydrated metal carboxylates are known in any detail, an omission that must be remembered in formulating reaction mechanisms. 

The removal of ligands (including H2O) from coordination compounds may be accompanied by recrystallization and either a change in coordination number, structure or ligands present through formation of new bonds with the anions, anation, etc. Constituent water in crystalline hydrates may be as coordinated ligands and/or structural water. A scheme for the classification of dehydration reactions is described in Table 7.3. 

When the solvate happens to be water, these are called hydrates, wherein water is entrapped through hydrogen bonding inside the crystal, and strengthens the crystal structure, thereby invariably reducing the dissolution rate (Table 4). The water molecules can reside in the crystal either as isolate lattice, where they are not in contact with each other as lattice channel water, where they fill space and metal coordinated water in salts of weak acids, where the metal ion coordinates with the water molecule. Metal ion coordinates may also fill channels, such as in the case of nedocromil sodium trihydrate. Crystalline hydrates have been classified by structural aspects into three classes isolated lattice sites, lattice channels, and metal-ion coordinated water. There are three classes, which are discernible by the commonly available analytical techniques. 

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