Hydration shell transition metal ions

In the case of ions for which the ion-water binding is very strong (the transition-metal ions particularly), the hydration number may be greater than the coordination number, because more than one shell of waters moves with the ion and the hydration number will encompass all the water molecules that move with it, while the coordination number refers to the ions in just the first shell. 

The Cr ion has a special place in the history of solvation because it was the first ion for which the lifetime of the water in its hydration shell was measured. This work was done by Hunt and Taube in 1957. The exchange of water between the hydration shell and the surroundings was slow, so the change in the concentration of the isotope could be measured. Tbe lifetime found is 1.6 x lO s (about 6 months). Tho e is evidence of outer-sphere water, so the value for the total hydration water that travels with the ion is much higher than the value of about 6 found for the first shell in transition-metal ions. 

The figure of six, rather than four, is used because of the experimental evidence that transition-metal ions undergo six coordination in the first shell. Correspondingly, the hydration numbers are 10-15. 

Hydration Shell Properties of Transition Metal Ions°... 

The dynamics of aqueous divalent post-transition-metal ions have also been studied by Rode s group (Lim et al. 2009 Bhattacharjee et al. 2009 Azam et al. 2010 Rode and Lim 2010). Their peculiar behaviour is ascribed to the lone pair electrons, so that in the proximal hydration hemisphere they are only slightly water stmcture makers, but in the distal hydration hemisphere they are slightly structure breakers. Even for the first hydration shell the RMRTs are much smaller than for the transition metal cations, and for the second shell the RMRTs are 120 % for Ge +, 107 % for Sn +, and 140 % for Pb +. The non-monotonically varying values with the atomic number should be noted. 

The structural parameters of the second hydration shell of the divalent transition metal ions in the first row of the periodic table from Mn " " to Zn " ", summarized in Ohtaki and Radnai (1993), does not show clear conclusions for the structural change of the ions with the atomic number. 

These results provide an overview of the structure of aquaions and show that a broad classification into labile and stable species can be made. Those of the former category can be represented by a relatively weak and variable hydration shell and include the alkali ions (other than Li+), Ag+, Ca, and ND4+. On the other hand, cations of the transition metals, the rare earths, and small, highly charged ions such as Be +, Mg, and AP, which have well-defined hydration shells, form stable aquaions. Cations such as Cu + and Li+ are intermediate, having exchange times in the range lO -lO sec. 

A spectroscopic study of Eu -Br showed that the increase in the Br concentration causes a larger enhancement in the intensity and band area of the -> transitions than for the Fj -> transitions. These modifications in the spectra were attributed to changes in the structure and nature of the inner solvation sphere of Eu in the excited state as compared to that of the ground state (Marcantonatos et al. 1984). The differences in intensity between absorption and emission bands would, therefore, reflect formation of inner-sphere complexes by Br in the excited state while outer-sphere complexation would dominate the ground state. It was proposed (Marcantonatos et al. 1981, 1982) that excitation of Eu " ion to the state would result in an expansion of the 4f and a shrinkage of the 5p orbitals with an overall decrease in the metal ion radius. The consequent contraction of the iimer shell would be expected to produce more compact and less easily disrupted outer hydration spheres for both ( Dj) Eu(H20)g and ( Di)Eu(H20)g with a possible increase in kobs-... 

Generally, one may state that the PES of the reaction of electrophilic substitution in the region of a transition state is much simplified as compared with the nucleophilic substitution. Therefore, the effects of solvation and small amounts of catalysts, particularly those coordinating the metal centers in XXIV, may considerably exceed the magnitude of the above-examined structural effects. One may appropriately point to the calculations on the hydrated methonium ion performed by the CNDO/2 method with a special parametrization in super-molecular approximation, and with the surrounding of 5 and 10 water molecules taken into account. They led to the conclusion that the pyramidal C4V structure, rather than the structures or Dj, is in solution energetically the most favored [83]. The structure of the first hydrated shell is shown by the formula XXVI ... 

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