5.1. Metal Ions in Solution 5.1.1 Hydration and Hydrolysis of Metal Ions In the aqueous processing of metals, we are concerned with two major questions: (i) What happens to metal ions in solution? and (ii) What happens to a solid placed in an aqueous environment? Let us for a moment focus our attention on the water molecules which constitute the solvent phase of the aqueous solution. Two hydrogen atoms and an oxygen atom come together to form a water molecule by sharing their outer electrons: oo oO o o o + H + H = o oH o o oO o H (5.1) Each pair of shared electrons constitutes a covalent bond. It can be seen from this simplified picture that in a water molecule, the oxygen atom is surrounded by 8 outer electrons, four of which are involved in covalent bonding and 4 of which are unshared. We can add to this picture, the fact that oxygen has a higher affinity than hydrogen for electrons. Thus in a water molecule, the electron cloud is denser around the oxygen nucleus than around the hydrogen nuclei. This effect is acknowledged by ascribing a dipolar structure to the water molecule: H+ (-) O H+ Thus like a magnet, the water molecule has a positive and a negative pole. Now let us consider a metal ion immersed in this sea of dipoles. Let us take the simplest case, i.e., the hypothetical situation of a metal ion Mz+ immersed in water with no other ions being present. Through its charge, this ion exerts some influence on the surrounding water molecules. The metal ion is positively charged and by analogy with the well known behavior of magnets, we would expect it to attract the negative pole of a water molecule. This is called an electrostatic interaction: H+ H Mz+ + OH Mz+ O + (5.2) H Depending on the nature of the metal ion, several more water molecules may be attracted towards it in a similar manner. Thus we say that metal ions are hydrated in aqueous solution. We would expect that this interaction will be most intense for the water molecules which are nearest to the central metal ion. These nearest neighbor water molecules constitute the first or inner hydration sphere of the hydrated ion. The hydration number is the number of such nearest neighbor water molecules, and is generally taken to be equal to 6. Thus in aqueous solution, we can represent the dissolved metals ions as M(H2O)6z+, e.g., Ni(H2O)62+, Al(H2O)63+, etc. In addition to the attractive Mz+-O- interaction noted above, we must recognize that a repulsive Mz+-H+ interaction also exists. In fact, we may go so far as to say that the hydration of a metal ion really involves two simultaneous processes: (a) attraction of the negative pole of the water molecule to the positively charged metal ion, and (b) repulsion of the positive pole of the water molecule by the positive charge on the metal ion. These two processes, in competition, can cause a splitting of the bound water molecule: H+ - M2+ + O H+ M2+ H+ - H+ M2+ O - O H+ H+ Mz+ (OH)- + H+ (5.3) The term hydrolysis describes processes such as Equation 5.3 in which the O-H bonds of water are broken. The resulting OH--containing product is termed a hydroxo metal complex. It is conventional practice to express the formation of a hydroxo complex as indicated in Equation 5.4. Mz+ + H2O = MOH(z-1)+ + H+ However, in view of our previous treatment of hydration, we can also write: (5.4) M(H2O)6z+ + H2O = M(H2O)5(OH)+ + H3O+ (5.5) The hydrolysis process may not necessarily stop with the splitting of one water molecule; it may involve the other water molecules in the inner hydration sphere. Thus we speak of successive or stepwise hydrolysis: M(H2O)62+ = M(H2O)5(OH)+ + H+ M(H2O)5(OH)+ = M(H2O)4(OH)2o + H+ M(H2O)4(OH)2o = M(H2O)3(OH)3- + H+ M(H2O)3(OH)3- = M(H2O)2(OH)42- + H+ etc. (5.6) 5.1.2 Ionization of Water and Hydration of the Proton A similar electrostatic process involving only water molecules, leads to the ionization of water and the hydration of the proton. The ionization of water may be viewed in terms of the transfer of a proton from a water molecule to a second water molecule, yielding a hydrated hydrogen ion and a hydroxyl ion: H O+H H - +H O O - H O +H - + H - +H +H H2O + H2O = H3O+ + OH- O O + H + H + H (5.7) The species H3O+ is called the oxonium ion and it may be further hydrated to give the trihydrated oxonium ion, H3O(H2O)3+: H H O H H O O H H H H O H In the above structure the dashed lines are used to emphasize the fact that the O-H bonds between the surrounding water molecules and the oxonium protons are weaker than those within the oxonium ion. The weaker bonds represent hydrogen bonds. It is common practice to express the ionization of water without explicitly indicating the hydration of the proton: H2O = H+ + OH- (5.8) However there are situations where it is necessary to invoke the hydrated nature of the proton. 5.1.3 Polymerization and Precipitation The hydrolysis products encountered above may react further with each other to give polymeric species: M(H2O)5(OH)2+ + M(H2O)5(OH)2+ = [(H2O)4M-(OH)2-M(H2O)4]4+ + 2H2O (5.9) For example, Fe(H2O)5OH2+ can undergo the following dimerization reaction: Fe(H2O)5OH2+ + Fe(H2O)5OH2+ = OH Fe(H2O)4]4+ + 2H2O [(H2O)4 Fe OH (5.10) In other words, the two OH groups form bridges between the two nuclei, i.e., the two Fe3+ ions. These polynuclear species can in turn become hydrolyzed: OH OH Fe(H2O)4]4+ = [(OH)(H2O)3Fe [(H2O)4Fe OH Fe(H2O)3(OH)]2+ + 2H+ (5.11) OH The presence of this new hydrolyzed species can then lead to more polymerization: OH OH Fe(H2O)3(OH)]2+ + [(OH)(H2O)3Fe [(OH)(H2O)3Fe OH OH OH = [(OH) (H2O)3Fe Fe(H2O)3(OH)]2+ OH Fe(H2O)2 OH OH Fe(H2O)3(OH)]4+ + 2H2O (5.12) (H2O)2Fe OH OH Hydrolysis-polymerization reactions can proceed until finally the resulting polymers become too large to remain soluble, i.e., they achieve colloidal dimensions. When this stage is reached, we say we have precipitated a solid. In this hypothetical system consisting of only one type of metal ion in water (i.e., in the absence of anions or other metals ions), each polymerization reaction involves the elimination of water and therefore ultimately we would expect metal hydroxides to be formed. EXAMPLE 5.1 Hydrolysis reactions For each of the polynuclear hydroxo species below, write down the corresponding chemical equation that describes its formation via interaction of Mz+ and H2O. (a) Solution Be6(OH)84+ (b) Zr4(OH)88+ (c) Mg4(OH)44+ (d) Co2OH3+ (a) 6Be2+ + 8H2O = Be6(OH)84+ + 8H+ (b) 4Zr4+ + 8H2O = Zr4(OH)88+ = 8H+ (c) 4Mg2+ + 4H2O = Mg4(OH)44+ + 4H+ (d) 2Co2+ + H2O = Co2OH3+ + H+ 5.1.4 Structures of Polycations and Polyoxoanions The species of interest here are those in which metal ions are linked by hydroxyl (MOH-M) and/or oxo (M-O-M) bridges. In the case of complexes based on M(II), M(III), and M(IV) atoms, the hydroxyl bridge is used almost exclusively. Table 5.1 presents a summary of the structural information on these species. (see Baes and Messmer, p. 420). It can be seen that symmetrical structures are preferred. In these structures, up to six cations are organized into groups of two, three, four, or six and the coordination number of the hydroxyl ion may be 2 or 3. The structure of the square-planar M4(OH)8(H2O)16 complex is illustrated in Figure 5.1a (B&M, p. 157). Adjacent metal cations are linked by a double bridge of hydroxyls. Each metal ion is eight-coordinate - through the oxygens of four hydroxyls and four water molecules. The M6(OH)84+ and M6(OH)126+ structures are illustrated respectively in Figures 5.1b and 5.1c. (See Baes & Mesmer, p.378) In addition to the species listed in Table 5.1, Al(III) forms the complex Al13O4(OH)247+. This species (Figure 5.1d; see Baes and Mesmer, p.118) consists of a central AlO45- tetrahedron surrounded by twelve AlO6 octahedra; the octahedra are linked by shared edges. In the case of the M(V) and M(VI) atoms, O2- bridges are prominent. The elements which form these polyoxoanions come predominantly from the left-side of the d-block: V(V), Nb(V), Ta(V), Cr(VI), Mo(VI), and W(VI). Table 5.2 shows that the polymerization ranges from dimers to 19-mers. The typical structure consists of MO6 octahedra linked at the corners or edges, as illustrated in Figure 5.2 (Huheey). Table 5.1 Polynuclear hydroxo complexes (Baes and Mesmer, p. 420) Species Cation (source) M2OH3+ M4(OH)88+ Be2+, Mn2+, Co2+, Ni2+, Zn2+, Cd2+, Hg2+, Pb2+ Cu2+, Sn2+, UO22+, NpO22+, PuO22+, VO2+, Al3+ Sc3+, Ln3+, Ti3+, Cr3+ Th4+ Be2+, Hg2+ Sn2+, Pb2+ Al3+, Cr3+, Fe3+, In3+ UO22+, NpO22+, PuO22+ Sc3+, Y3+, Ln3+ Mg2+, Co2+, Ni2+, Cd2+ Pb2+ Zr4+, Th4+ M6(OH)84+ Be2+, Pb2+ M6(OH)126+ Bi3+ M2(OH2(2z-2)+ M3(OH)33+ M3(OH)4(3z-4)+ M3(OH)5(3z-5)+ M4(OH)44+ Probable structure = Mz+, o = OH- M4 square with eight OHions, one centered over and under each edge. M6 octahedron with eight OH- ions centered on faces. M6 octahedron with 12 OHions centered along edges. Table 5.2 Polyoxoanions M2 V(V) V2O74- M3 M4 V3O93- V4O124- M6 Nb6O198- Ta(V) Ta6O198- M12 M19 Cr2O72Mo7O246- Mo(VI) W(VI) M10 V10O286- Nb(V) Cr(VI) M7 W6O192- Mo19O594W12O4110- See B&M under the various metal ions, Huheey, pp.755-764, SAL, pp.164-165) (a) (c) (b) (d) Figure 5.1 Structures of polycations: (a) M4(OH)8(H2O)168+ , (b) M6(OH)84+ (c) M6(OH)126+ , (d) the Al13O4(OH)247+ polycation. Figure 5.2 Evolution of the structures of polyoxoanions. (Huheey, p. 757)
