Hydration enthalpies transition metal ions

Somewhat better data are available for the enthalpies of hydration of transition metal ions. Although this enthalpy is measured at (or more property, extrapolated to) infinite dilution, only six water molecules enter the coordination sphere of the metal ion lo form an octahedral aqua complex. The enthalpy of hydration is thus closely related to the enthalpy of formation of the hexaaqua complex. If the values of for the +2 and +3 ions of the first transition elements (except Sc2, which is unstable) are plotted as a function of atomic number, curves much like those in Fig. 11.14 are obtained. If one subtracts the predicted CFSE from the experimental enthalpies, the resulting points lie very nearly on a straight line from Ca2 lo Zn2 and from Sc to Fe3 (the +3 oxidation state is unstable in water for Ihe remainder of the first transition series). Many thermodynamic data for coordination compounds follow this pattern of a douUe-hunped curve, which can be accounted for by variations in CFSE with d orbital configuration. 

FIGURE 10-7 Enthalpies of Hydration of Transition Metal Ions. The lower curves show experimental values the upper curves result when contributions from spin-orbit splitting, a relaxation effect from contraction of the metal-ligand distance, and interelectronic repulsion energy are subtracted, (a) 2+ ions, (b) 3-b ions. (Reproduced with permission from D. A. Johnson and P. G. Nelson, Inorg. Chem., 1995, 34, 5666 data) and D. A. Johnson and P. G. Nelson, Inorg. Chem., 1999, 4949 data). 1995, 1999, American Chemical Society.)... 

Inclusion of the absolute value of the standard enthalpy of hydration of the proton, Ahyd// (H +, g) = — 1110 kJ mol 1 (derived in Chapter 2), gives the absolute values for the enthalpies of hydration of the transition metal ions. The estimated values are given in Table 7.5. 

The enthalpies of hydration of a selection of transition metal ions were derived. 

The enthalpy of hydration of +2 metal ions is usually in the range of 1600 to 2500 kJ mol-1. Because the sizes of the metal ions decrease in going to the right in the transition series, there will naturally be a general increase in hydration enthalpy in progressing from Ca2+ to Zn2+. Figure 19.8 shows the hydration enthalpies for the +2 ions of the first transition series when plotted in terms of the number of electrons in the d orbitals. 

FIGURE 10-28 Simulated Hydration Enthalpies of M Transition Metal Ions. 

Table 7.5 Estimated values of the standard enthalpies of hydration of some + 2 and + 3 transition metal ions and that forGa ...

Ca +, Mn +, and Zn + have d, and so CFSE is 0. Other metal ions deviate from the expected line due to extra CFSE, with two maxima for the configuration ([V(H20)6]2+ species) and ([Ni(H20)g] + species). Eigure 3.11 presents the hydration enthalpies of divalent transition metal ions. 

Standard electrode potentials and stability of different oxidation states of transition metal ions in aqueous solutions The stability of a particular oxidation state in solution can be explained in terms of its electrode potential which in turn depends upon enthalpy of sublimation, ionization energy and hydration energy. 

Although there is no space to develop a detailed discussion of the solubilities of compounds of the transition elements, the general insolubility of their + 2 and + 3 hydroxides is important. The rationale underlying their insolubility can be summarized (i) the hydroxide ion is relatively small (152 pm ionic radius) and the ions of the +2 and +3 transition metals assume a similar size if their radii are increased by 60-80 pm, and (ii) the enthalpy of hydration of the hydroxide ion (—519 kJ mol ) is sufficiently negative to represent a reasonable degree of competition with the metal ions for the available water molecules, thus preventing the metal ions from becoming fully hydrated. Such effects combine to allow the lattice enthalpies of the hydroxides to become dominant. 

Enthalpies of hydration for some +2 metal ions of the first transition series. 

A simple ionic bonding model accounts for many properties of transition metal complexes, including variations in the hydration and lattice enthalpies and the ionic radii of the metal ions. The observation of high- and low-spin states for complexes of some metal ions can also be explained. 

Other evidence for CFT comes from the enthalpies of hydration for the divalent metal ions of the first transition series. Again, a linear trend is expected, with the enthalpy of hydration decreasing across the series. The experimental data are shown by the solid blue line in Figure 16.11. The largest deviations from the expected trend occur for those d" electron configurations having the greatest CFSE, namely d andd . If the CFSE is subtracted from the experimental enthalpy of hydration, the expected linear trend is observed, as shown by the solid red line. 

FIGURE 3.11 Hydration enthalpies of transition 3metal ions. (Data from J. Barrett, Inorsanic Chemistry in Aqueous Solution [Cambridge, UK The Royal Society of Chemistry, 2003].)... 

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