Polyoxometallates

Key points: In their highest oxidation states, metals in Groups 5 and 6 readily form polyoxometallates and heteropoly oxometallates; pH is crucial in determining which compounds form. Apolyoxometallate is an oxoanion containing more than one metal atom. The H₂O ligand created by protonation of an oxido ligand at low pH can be eliminated from the central metal atom and thus lead to the condensation of mononuclear oxometallates. A familiar example is the reaction of a basic chromate solution, which is yellow, with excess acid to form the oxido-bridged dichromate ion, which is orange:

In highly acidic solution, oxido-bridged Cr (VI) species with longer chains are formed. The tendency for Cr (VI) to form polyoxo species is limited by the fact that the O tetrahedra link only through vertices: edge and face bridging would result in too close an approach of the metal atoms. By contrast, it is found that five- and six-coordinate metal oxido complexes, which are common with the larger 4d- and 5d-series metal atoms, can share oxido ligands between either vertices or edges. These structural possibilities lead to a richer variety of polyoxometallates than found with the 3d series metals. Chromium’s neighbours in Groups 5 and 6 form six-coordinate polyoxo complexes (Fig. 19.12). In Group 5, the polyoxometallates are most numerous for vanadium, which forms many V(V) complexes and a few V(IV) or mixed oxidation state V(IV) V(V) poly oxido complexes. Polyoxometallate formation is most pronounced in Groups 5 and 6 for V(V), Mo (VI), and W(VI).

It is often convenient to represent the structures of the polyoxometallate ions by poly hedra, with the metal atom understood to be in the centre and O atoms at the vertices. For example, the sharing of O atom vertices in the dichromate ion, Cr₂O₇₂, may be depicted in either the traditional way (7) or in the polyhedral representation (8). Similarly, the important M6 O19 structure of [Nb₆O₁₉ ]^8- , [Ta₆O₁₉ ]^8-, [Mo₆O₁₉ ]^2- , and [W₆O₁₉ ]^2- is depicted by the conventional or polyhedral structures shown in Fig. 19.13. The structures for this series of polyoxometallates contain terminal O atoms (those projecting out from a single metal atom) and two types of bridging O atoms: two-metal bridges, MOM, and one hypercoordinated O atom in the centre of the structure that is common to all six metal atoms. The structure consists of six MO₆ octahedra, each sharing an edge with four neighbours. The overall symmetry of the M₆O₁₉ array is O_h. Another example of a polyoxometallate is [W₁₂O₄₀ (OH)₂]^10- (9). As shown by this formula, the polyoxoanion is partially protonated. Polyoxometallate anions can be prepared by carefully adjusting pH and concentrations, for example polyoxomolybdates and polyoxotungstates are formed by acidification of solutions of the simple molybdate or tungstate:

Mixed metal polyoxometallates are also common, as in MoV₉O₂₈^5- , and there is a large class of heteropoly oxometallates, such as the molybdates and tungstates, that also in corporate P, As, and other heteroatoms. For example, [PMo12 O40 ]3 contains a PO4 3 tetrahedron that shares O atoms with surrounding octahedral MoO6 groups (10). Many different heteroatoms can be incorporated into this structure, and the general formula tion is [X(+N)Mo₁₂O₄₀ ]^(8-N) where X(+N) represents the oxidation state of the hetero atom X, which may be As(V), Si(IV), Ge(IV), or Ti(IV). An even broader range of heteroatoms is observed with the analogous tungsten heteropolyoxoanions. Heteropoly- oxomolybdates and tungstates can undergo one-electron reduction with no change in structure but with the formation of a deep blue colour. The colour seems to arise from the excitation of the added electron from Mo(V) or W(V) to an adjacent Mo(VI) or W(VI) site.

Figure 19.13 (a) Conventional and (c) polyhedral representation of the six edge-shared octahedra as found in [M₆O₁₉ ]^2-. The intermediate structure (b) shows the process of constructing the polyhedral representation in progress.

اخر الاخبار

اخبار العتبة العباسية المقدسة

جامعة العميد تنظم ورشة تدريبية عن برنامج منصة التعليم العالي

قسم الشؤون الفكرية ينظم برنامجًا دينيًّا لكشافة الكفيل في النجف الأشرف

العتبة العباسية المقدسة تقدم دعوة لجامعتي الكوفة والطوسي للمشاركة في مشروعها القرآني الخاص بطلبة الجامعات

قسم الشؤون الفكرية يصدر مجموعة جديدة من الإصدارات المعرفية والثقافية

المزيد

الآخبار الصحية

كيف يمكن لحفنة من الجوز أن تؤثر على نومك وتركيزك؟

دراسة: كوب ميلك شيك واحد قد يكون "قنبلة دماغية"

هل يعد "الباراسيتامول" خيارا آمنا للنساء الحوامل؟

دراسة تكشف الرابط بين العلاقات العاطفية والرضا عن الحياة

المزيد

الآخبار التكنلوجية

كارثة صامتة.. "الهواء الملوث" يسرق نور عيون الأطفال

دراسة: رصد أول دليل على وجود "إشعاع هوكينغ"

رسالة بلغة لا تعرفها؟.. "واتساب" يترجمها لك فورا

"إنستغرام".. ميزة جديدة لكشف "الكذب في العمر"

المزيد

مواضيع ذات صلة

Thermochemical correlations

Magnetic measurements

Octahedral complexes

Sulfide complexes

Disulfides

Mononuclear oxido complexes

Metal oxides

Structural trends

Oxidation states down a group

Intermediate oxidation states in the 4d and 5d series

Intermediate oxidation states in the 3d series

High oxidation states