Second hydration sphere

The second hydration sphere may be considered as "outer-sphere complexing , a phenomenon which is extremely important in all reactions inaqueoiis solution. Coordination of a third hydration sphere will... 

Aqueous solutions of nickel(II) salts in the absence of strong coordinating species are usually green because of the [Ni(H20)ft]2+ cation.1447 The number of coordinated water molecules in the inner-shell complex is now well ascertained in the temperature range -30 to 30 °C by means of electronic and both 170 and LHNMR spectra.1448-1451 The most recent value of the coordination number is 5.85 0.2.145 Neutron diffraction studies of NiCl2 in D20 solution led to estimates of the Ni—O bond distance within the [Ni(D20)6]2+ cation in the range 195-220 pm.1452 A second hydration sphere of about 15 water molecules has also been proposed. 

Ni(H20)6] + (73) is the main cation formed when Ni is dissolved in aqueous solution in the absence of other coordinating ligands a second hydration sphere of some 15 water molecules has been proposed. In addition, most simple salts written as NiX2-xH20 actually contain the hexaaqua ion [Ni(H20)6] +. Under alkaline conditions, nF-hydroxy... 

Mean hydrogen bond energies AE for two hydration shells each water molecule of the first hydration shell is coordinated to three water molecules of the second hydration sphere. 

Woods, M., Zhang, S., Ebron, V.H., and Sherry, A.D. (2003) pH-sensitive modulation of the second hydration sphere in lanthanide(III) tetraamide-DOTA complexes a novel approach to smart MR contrast media. Chemistry - A European Journal, 9, 4634 640. 

It can also be mentioned that the LAXS data (142) can be interpreted as if the Tl( OH2)e ion is surrounded by a second hydration sphere ( 0H2), like many other metal ions in water (203). The Tl-0 distance in the studied 1M solution is about 4.1 A, which would indicate tetrahedral bonding of OH2 [a lone pair of OH2 is directed towards the metal ion, while the other lone pair is accessible to accept a hydrogen bond (205)]. However, interpretation of longer/weaker interactions in the radial distribution curves obtained from LAXS measurements is always associated with considerable uncertainty. In the present case, the Tl-0 distance could also be interpreted as originating from the second coordination sphere consisting of loosely associated perchlorate ions. 

Proton chemical shifts in aqueous solutions of Al i nitrate, perchlorate, and sulphate have been measured over a range of temperatures, and the effects of hydrolysis and addition of acid have been studied in detail. These arise from changes in the interactions between the bulk water and three different environments, viz. [Al(OH2)e] + itself, the second hydration sphere of this cation, and the broken-water structure around the anion. ... 

Hydration spheres created by H metal center surrounded by water molecules. The dashed line marks the border between H the first and second hydration sphere. 

Structure breaker. A similar problem is encountered with the study of the hydration of Au+ ions (Armunanto et al. 2004), where absolute Af/ r values for t of 2 and 0.5 ps are reported (unusually shorter for the first than the second hydration sphere), but not the corresponding value for bulk water. The relative MRT values for the first and second shells were reversed in a more recent study (Lichtenberger et al. 2011). Still, also Au+ was considered to be a structure-breaking ion. The hydration dynamics of T1+ (Vchirawongkwin et al. 2007) share with those of Au" the phenomenon that the RMRT of the first hydration sphere (76 %) is shorter than that of the second sphere (88 %) for t of 0.5 ps, but the list of RMRT values given in this paper for Rb+, Cs+, Ag+, and Au+ are at variance with their being classified as water structure breakers in the papers quoted above. This point has not been explained so far. 

Aqueous quadrivalent ions have also been studied with respect to the residence time of water molecules in their second hydration spheres. The most recent publication (Messner et al. 2011) reports the RMRT values 320 % for Zr +, 910 % for Hf +, and 480 % forU + (also 1060 % forAl +). The non-monotonical variation of the MRT with the atomic number of the ion is to be noted. Finally, for the uranyl ions, the RMRT values 250 % for UO2" " (Frick et al. 2010) and 320 % for U02 (Frick et al. 2009) were reported. The MRT values for second shell hydration of tri-and tetravalent cations were recently summarized by Hofer et al. (2011). 

The range of the coefficient of P2c is due to that of the number of water molecules in the second hydration sphere of the cations. The opposite signs of Pd and P2c and their partial compensation then produce the observed signs of 5i, in agreement with the water structure-making and -breaking properties of the cations. For the anions the correlation is ... 

The second solvation sphere is more difficult to characterize because of the rapid exchange with the water molecules from within the solution. Studies undertaken according to the aforementioned techniques and via simulations imply that, for the majority of cations, the immediate environment of the solvated ions consists of 12 water molecules. The average 0-0 distance between the oxygen atoms of the first and second hydration spheres, which is about 2.7 A, is shorter than the average intermolecular distance between water molecules in pure water (about 2.9 A) [11,12]. The number of layers of water molecules attracted and oriented by a cation increases with its polarizing strength. 

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