183W NMR

Structures known to form with either metal also form with the entire intermediate range of mixed-metal combinations 180). Both molybdenum and tungsten form the heptametalate [M7O24]6 in aqueous solution. The existence of mixed heptametalates has been shown by the application of 170 NMR 182) and 183W NMR spectroscopy 180). In fact, molybdenum can replace any or all of the tungsten atoms in [W7O24]6 so that at least 19 mixed-metal structures can be observed 180). Approximate relative stability constants have been determined for the intermediate species in the series... 

The fi isomer of [(B W Cy]6 ( tungstate X ) is difficult to isolate in pure form, but can be obtained indirectly from reduced tp. There seems little doubt, based on comparisons with heteropolyanions, and from JH and 183W NMR, that the anion structure corresponds to that of /S-[SiWi2O40]4-.52 The composition and structure of tp are still unknown. Although W6 and W24 formulas have been suggested, recent ultracentrifugation measurements indicate a dodecamer. Reduction of tp by two equivalents/12W causes transformation to a new species tp, which, upon reoxidation, slowly is converted to tp. potassium salt of oxidized tp has been isolated. 

ESR and l70 NMR spectra of le -reduced SiW O o demonstrate that the unpaired electron is weakly trapped on a W atom at low temperatures but undergoes rapid hopping (intramolecular electron transfer) at room temperature (Section II). Anions generated by 2e (and 4e -) reduction are ESR-silent, but 170 and 183W NMR spectra show that the additional electrons are fully delocalized (on the NMR timescale) at room temperature and generate ring currents analogous to those produced by the 7i-electrons of benzene. In contrast, in the case of le -reduced PMoW iO, the electron is localized on a more reducible Mo atom at room temperature (251). 

The first systematic studies of polyoxometalate—rare-earth complexes were published in 1971 (Peacock and Weakley, 1971a, 1971b). Two classes of polytungstate complexes were described and these can now be seen to have common structural features. The complexes of Y, La, Ce(III and IV), Pr, Nd, Sm, Ho, Er, and Yb in which R W = 1 10 were readily formed by reaction of WO - with the appropriate rare-earth salt, but are stable only within the pH range 5.5-8.5. More recent 183W-NMR studies (Inoue et al., 2003) indicate that in solution the anions containing the heaviest R (Tm, Yb and Lu) are partially decomposed. 

Since the solid-state anion structures, shown in Figure 7 both have overall C2 symmetry the 183W-NMR spectra should show 11 (or 17) lines if the same structures are retained in solution. A multinuclear NMR (P, W, O) study of... 

The complexes listed in Table 8 yield solutions containing the monomeric anions. Such solutions exhibit 183W-NMR spectra consistent with the expected symmetry (Ci or Cs) of a monomer and with chemical shifts distinct from those of the 1 2 complexes. 

Nine derivatives of 1, [RSb9W2i086]16 (R = La-Gd, Dy, Yb), have been isolated and characterized by elemental analysis, magnetic susceptibility, and infrared spectra. The four-line 183W-NMR spectrum of the lanthanum derivative is consistent with the D3h symmetry of the parent structure, but the emission spectrum of the europium derivative suggests a lower symmetry in the solid state (Liu et al., 1992). No structural determinations of these derivatives have yet been reported. 

W NMR Data for a Phosphenium Group in Cationic Phosphenium Complexes... 

All nine possible anions/isomers of ds-[V2MoxW6 xOi9]4 have been detected in aqueous solution by 183W-NMR spectroscopy.61... 

---