Coordinated Heteropolyanions

Niobium and tantalum compounds form adducts with virtually all types of neutral ant anionic donors. The coordination chemistry of the higher halides is widely developed, and thei activity as Friedel-Crafts catalysts is another manifestation of their Lewis acidity. The stron acceptor capacity of the high valent metal compounds tends to favor the formation of dimers and sometimes of higher condensation products, which competes with coordination with othe donor molecules. Numerous simple anionic or heteropolyanionic species, but little cationi chemistry, and no simple metal salts, are known. 

Why should the early transition metals form so many polyoxoanions The answer lies in the size of the M5/6+ cations and their -acceptor properties.1,5 The effective ionic radii of V5+ (0.68 A), Mo6+ (0.77 A) and W6+ (0.74 A) are consistent with the observation that these cations adopt four-, five- and six-fold coordination by oxide ion. With very few exceptions V, Mo and W atoms in heteropolyanions are six-coordinate. On the other hand Cr6+ (0.58 A) hap a maximum coordination number of four in oxides and oxoanions. Few isopoly- and heteropoly-chromates are known and they are all based on groups of corner-shared Cr04 tetrahedra [Cr207]2-, [Cr3O10]2-, [Cr4Oi3]2-, [03SOCrO3]2-, [02I0Cr03]-,... 

Heteropolyanions and isopolyanions are polymeric oxoanions (polyoxometalates) (2, 3, 5, 6). The structure of a heteropolyanion or polyoxoanion molecule itself is called a primary structure (5, 6, 77). There are various kinds of polyoxoanion structure (Section II.A. 1). In solution, heteropoly anions are present in the unit of the primary structure, being coordinated with solvent molecules and/ or protonated. Most heteropolyanions tend to hydrolyze readily at high pH (Section 1I.C). Protonation and hydrolysis of the primary structure may be major structural concerns in solution catalysis. Heteropoly compounds in the solid state are ionic crystals (sometimes amorphous) consisting of large polyanions, cations, water of crystallization, and other molecules. This three-dimensional arrangement is called the secondary structure. For understanding catalysis by solid heteropoly compounds, it is important to distinguish between the primary structure and the secondary structure (5, 6, 17). Recently, it has been realized that, in addition... 

The known addenda and heteroatoms incorporated in heteropolyanions are summarized in Table II (20). The structures in this table of polyanions with Se(IV), Te(lV), Sb(IIl), Bi(III), Ti(IV), and Zr(IV) still need to be con-finned, since tetradendral coordination of these ions with oxide ions is seldom observed (2). 

The heteropolyanion stabilizes protonated intermediates by coordination in solution and the pseudoliquid phase as well as on the surface, thus lowering the activation energy and accelerating reactions. Several protonated intermediates including the protonated ethanol dimer and monomer [18], the protonated pyridine dimer [12, 19], and protonated methanol [20] have been detected in the pseudoliquid phase directly by use of X-ray diffraction (XRD), IR or solid-state NMR. In solid-state H NMR, the chemical shift for the protonated ethanol dimer, (C2HsOH)2H+ is 9.5 ppm down-field from tetramethylsilane, which lies in the range of supcracids reported by Olah et al. [18]. This fact also supports the strong acidity of heteropolyacids. 

Metals can be coordinated with a heteropolyanion in three different ways to form metal-coordinated polyanions, which show unique catalytic activities for various reactions. The first method is the simple combina-... 

Terminal alkenes can be hydrogenated selectively in the presence of PdCI2 [63] or RhCl(PPhj)3 [64] and heteropoly compounds. The catalytic system is also highly active for the production of urethane or isocyanate compounds by the reductive carbonylation of nitrobenzene. It is considered that polyoxometalate coordinating with Pd2+ in the reduced form is the active species, since easily reducible heteropolyanions are more active [63]. 

Other Heteropolyanions with Tetrahedrally-coordinated Heteroatoms 656... 

Pope, M. T. Isopolyanions and Heteropolyanions. In Comprehensive Coordination Chemistry Wilkinson, G. Gillard, R. D., McCleverty, J. A., Eds. Pergamon Press Oxford, 1987 Chapter 38. 

From the physico-chemical and thermodynamic analysis of the molten systems KF-K2M0O4-B2O3 and KF-K2Mo04-Si02, it can be concluded that the formation of heteropolymolybdates containing boron, [BM06O24] , and silicon, [SiMo 12040]" , as a central atom is most probably responsible for an easy molybdenum deposition. Besides, the entry of fluorine atoms into the coordination sphere of molybdenum in the heteropolyanions lowers the symmetry and thus, also the electrochemical stability of such electro-active species. 

Face-sharing between coordination polyhedra in a heteropolyanion was first detected in CeMoi2Oii2 (23). The MoOg octahedra form six facesharing pairs (Fig. 6) corners are shared between octahedra in different pairs, and in addition each octahedron shares a face with the central... 

Table 6. References to X+ Z+ Mii04oHv<14- -J - >- anions in which tetrahedrally-coordinated X+n has the d° or d10 configuration. For the general reaction of Z+z (including Ti(IV), V(V), and Mo(VI)) with unsaturated heteropolyanions...

The presence of an actual water molecule coordinated to Z, when the anion contains two hydrogen atoms, was first demonstrated (55) for the SiConWn and SiComWn anions. The water in the first was shown to be replaceable by pyridine, and in the second by pyridine and ammonia. Crystalline salts of the product anions were isolated. The phenomenon seems to be a general one for XZWn anions containing Co (II) (55, 213), Co(III) (55, 203, 213), Ni(II) (55, 213), and Cr(III) (222). As predicted (55), it has also been observed for the X2C0W17 anions (176). These anions and their substitution products therefore constitute a class of hybrid complexes, intermediate between normal heteropolyanions with metal heteroatoms and conventional coordination complexes, and the significance of this has been discussed elsewhere (55). 

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