Hydrated transition metal ions

Ligand Substitution in MA by the Porphyrin. Even in the most labile hydrated transition-metal ion, Cu2+, ligand substitution generally occurs more slowly than the preceding diffusion step (37). It is useful to look at reaction (8) in relation to the much faster reactions (9) and (10), which have been carefully studied by Diebler (38). 

We have discussed several different types of equilibria so far, including acid-base equilibria and solubility equilibria. We now turn to equilibria of another type, which primarily involve transition metal ions in solntion. Transition metal ions tmd to be good electron acceptors (good Lewis acids). In aqneons solntions, water molecules can act as electron donors (Lewis bases) to hydrate transition metal ions. For example, silver ions are hydrated by water in solntion to form Ag(H20)2 (flq ). Chanists often write aq) as a shorthand notation for the hydrated silver ion, but the bare ion does not really exist by itself in solntion. 

Bronsted acidity can also be introduced by hydrolysis of hydrated transition metal ions, and ammonium exchange with subsequent removal of ammonia under heating. 

In an aquo-complex, loss of protons from the coordinated water molecules can occur, as with hydrated non-transition metal ions (p. 45). To prevent proton loss by aquo complexes, therefore, acid must usually be added. It is for these conditions that redox potentials in Chapter 4 are usually quoted. Thus, in acid solutions, we have... 

Persulfate (41) reacts with transition metal ions (e.g. Ag, Fe21, Ti31) according to Scheme 3.42. Various other reduetants have been described. These include halide ions, thiols (e.g. 2-mercaptoethanol, thioglycolic acid, cysteine, thiourea), bisulfite, thiosulfate, amines (triethanolamine, tetramethylethylenediamine, hydrazine hydrate), ascorbic acid, and solvated electrons (e.g. in radiolysis). The mechanisms and the initiating species produced have not been fully elucidated for... 

Figure 8-8. Hydration energies for divalent transition metal ions.

One of the most promising techniques for studying transition metal ions involves the use of zeolite single crystals. Such crystals offer a unique opportunity to carry out single crystal measurements on a large surface area material. Suitable crystals of the natural large pore zeolites are available, and fairly small crystals of the synthetic zeolites can be obtained. The spectra in the faujasite-type crystals will not be simple because of the magnetically inequivalent sites however, the lines should be sharp and symmetric. Work on Mn2+ in hydrated chabazite has indicated that there is only one symmetry axis in that material 173), and a current study in the author s laboratory on Cu2+ in partially dehydrated chabazite tends to confirm this observation. 

Cerium(IV) oxidations of organic substrates are often catalysed by transition metal ions. The oxidation of formaldehyde to formic acid by cerium(IV) has been shown to be catalysed by iridium(III). The observed kinetics can be explained in terms of an outer-sphere association of the oxidant, substrate, and catalyst in a pre-equilibrium, followed by electron transfer, to generate Ce "(S)Ir", where S is the hydrated form of formaldehyde H2C(OH)2- This is followed by electron transfer from S to Ir(IV) and loss of H+ to generate the H2C(0H)0 radical, which is then oxidized by Ce(IV) in a fast step to the products. Ir(III) catalyses the A -bromobenzamide oxidation of mandelic acid and A -bromosuccinimide oxidation of cycloheptanol in acidic solutions. ... 

In aqueous solutions, in which the most probable ligand is the water molecule, most of the lower oxid ation states (i.e. + 2, + 3 and some of the + 4 states) of transition metal ions are best regarded as hexaaqua complex ions, e.g. [Feu(H20)6]2 +. In these ions the six coordinated water molecules are those that constitute the first hydration sphere, and it is normally accepted that such ions would have a secondary hydration sphere of water molecules that would be electrostatically attracted to the positive central ion. The following discussion includes only the aqua cations that do not, at pH = 0, undergo hydrolysis. For example, the iron(III) ion is considered quite correctly as [Fe(H20)6]3 +, but at pH values higher than 1.8 the ion participates in several hydrolysis reactions, which lead to the formation of polymers and the eventual precipitation of the iron(III) as an insoluble compound as the pH value increases, e.g. ... 

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. 

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. 

The temperature-dependent irreversibility demonstrates that the ion-exchange behavior of NaX towards bivalent cations depends strongly upon the thermal history of the sample. The rather pronounced differences in behavior of transition-metal ions, also observed in synthetic zeolite 4 A (9) is in very sharp contrast with the nearly identical, either hydrated or crystallographic, dimensions of these ions (10). Obviously, this observation raises important questions as to the value of the current interpretation (nearly) exclusively in terms of physical dimensions of ions and pore width. In contrast, the similarity of behavior in mont-morillonite is remarkably close the AG0 value for the replacement of Na by either Ni, Co, Cu, or Zn is —175 cal ( ll)/equivalent, irrespective of the nature of the cation (11). Therefore, the understanding of their difference in behavior in zeolites must take other effects into consideration. 

Zinc can be removed from carbonic anhydrase on dialysis against a chelating agent at pH about 5 (37, 38). The apoenzyme is inactive but the gross conformation of the protein is maintained (37, 39, 40). The metal-chelating site can accomodate any of the divalent transitional metal ions from Mn2+ to Zn2+ as well as Cd2+ and Hg2+ (38,41). Most of these metallocarbonic anhydrases have low activities or are inactive, however. Only Zn2+ and Co2+ are efficient activators. As shown in Table 3, this narrow metal-ion specificity is observed for the CO 2 hydration as well as for the esterase reactions. 

The hydrated electron is the most powerful reductant (E7 = -2.9 V) IP has a somewhat higher reduction potential (E7 = -2.4 V for a compilation of reduction potentials, see Wardman 1989). Often, both H and eaq are capable of reducing transition metal ions to their lower oxidation states [e.g., reactions (4) and (5)]. 

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