Polyoxo compound formation

Key points: Acids containing the OH group condense to form polyoxoanions; polycation formation from simple aqua cations occurs with the loss of H₂O. Oxoanions form polymers as the pH is lowered whereas aqua ions form polymers as the pH is raised. As the pH of a solution is increased, the aqua ions of metals that have basic or amphoteric oxides generally undergo polymerization and precipitation. Because the precipitation occurs quantitatively at a pH characteristic of each metal, one application of this behaviour is the separation of metalions,. With the exception of Be₂ (which is amphoteric), the elements of Groups 1 and 2 have no important solution species beyond the aqua ions M(aq) and M2 (aq). By contrast, the solution chemistry of the elements becomes very rich as the amphoteric region of the peri odic table is approached. The two most common examples are polymers formed by Fe(III) and Al(III), both of which are abundant in the Earth’s crust. In acidic solutions, both form octahedral hexaaqua ions, [Al(OH₂ )₆]₃ and [Fe(OH₂)₆]₃. In solutions of pH > 4, both precipitate as gelatinous hydrous oxides:

The precipitated polymers, which are often of colloidal dimensions (between 1 nm and 1 µm), slowly crystallize to stable mineral forms. The extensive network structure of

aluminium polymers, which are neatly packed in three dimensions, contrasts with the linear polymers of their iron analogues. Polyoxoanion formation from oxoanions occurs by protonation of an O atom and its departure as H₂O:

The importance of polyoxo anions can be judged by the fact that they account for most of the mass of oxygen in the Earth’s crust, as they include almost all silicate minerals. They also include the phosphate polymers (such as ATP (11)) used for energy transfer in living cells. The formation of polyoxoanions is important for early d-block ions, particularly V(V), Mo (VI), W(VI), and (to a lesser extent) Nb(V), Ta(V), and Cr (VI); see Section 19.8. They are formed when base is added to aqueous solutions of the ions or oxides in high oxidation states. Polyoxoanions are also formed by some nonmetals, but their structures are different from those of their d-metal analogues. The common species in solution are rings and chains. The silicates are very important examples of polymeric oxoanions, and we discuss them in detail in Chapter 14. One example of a polysilicate mineral is MgSiO₃, which contains an infinite chain of SiO₃^2- units. In this section we illustrate some features of polyoxoanions using phosphates as examples. The simplest condensation reaction, starting with the orthophosphate ion, PO₄^-3, is

The elimination of water consumes protons and decreases the average charge number of each P atom to –2. If each phosphate group is represented as a tetrahedron with the O atoms located at the corners, the diphosphate ion, P₂O₇^-4 (12), can be drawn as (13). Phos phoricacid can be prepared by hydrolysis of the solid phosphorus(V) oxide, P₄O₁₀. An initial step using a limited amount of water produces a metaphosphate ion with the formula P₄O₁₂^-4 (14). This reaction is only the simplest among many, and the separation of products from the hydrolysis of phosphorus(V) oxide by chromatography reveals the presence of chain species with from one to nine P atoms. Higher polymers are also present and can be removed from the column only by hydrolysis. Figure 4.8 is a schematic representation of a two-dimensional paper chromatogram: the upper spot sequence corresponds to linear polymers and the lower sequence corresponds to rings. Chain polymers of formula Pn with n=10 to 50 can be isolated as mixed amorphous glasses analogous to those formed by silicates (Section 14.15). The polyphosphates are biologically important. At physiological pH (close to 7.4), the P–O–P entity is unstable with respect to hydrolysis. Consequently, its hydrolysis can serve as a mechanism for providing the energy to drive a reaction (the Gibbs energy). Similarly, the formation of the P–O–P bond is a means of storing Gibbs energy. The key to energy

exchange in metabolism is the hydrolysis of adenosine triphosphate, ATP (11a), to adenosine diphosphate, ADP (11b):

Energy flow in metabolism depends on the subtle construction of pathways to make ATP from ADP. The energy is used metabolically by pathways that have evolved to exploit the delivery of a thermodynamic driving force resulting from the hydrolysis of ATP.

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هذه الأطعمة تحمي دماغك من الشيخوخة وتزيد حدة الذكاء

خبراء يحذرون من مخاطر "الدهون المخفية" على القلب

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"ميزة جديدة" من إنستغرام تنافس تيك توك.. طال انتظارها

غوغل في مأزق.. بيانات 2.5 مليار مستخدم بقبضة القراصنة

المزيد

مواضيع ذات صلة

Liquid ammonia

Amphoterism

Acidic and basic oxides

Periodic trends in aqua acid strength

Extrinsic semiconductors

Intrinsic semiconductors

Insulators

The Fermi level

Densities of states

Band formation by orbital overlap

The conductivities of inorganic solids

Nonstoichiometry