TRIMETALLO DERIVATIVES OF LACUNARY 9-TUNGSTOSILICATE HETEROPOLYANIONS.2. ISOTROPIC NMR SHIFTS IN PYRIDINE-TYPE LIGANDS COORDINATED TO THE PARAMAGNETIC 9-TUNGSTO-3-CUPRIO(II)SILICATE ANION

H-1 NMR spectra of pyridine and alpha-, beta-, and gamma-picoline coor dinated to the paramagnetic heteropolyanion [HmSiW9O37Cu3]((10-m)-) (S iW9Cu3) are reported. NMR lines are assigned to [HmSiW9O37Cu3(ptl)(n)] ((10-m)-) (n = 1, 2 or 3; m = 0, 1 or 2; ptl = pyridine-type ligands) on the basis of their pH and [SiW9Cu3]/[ptl] dependence. The isotropic NMR shifts come mainly from the contact shifts, which for some proton s exceed 70% of those in analogous nickel complexes. The nuclear longi tudinal relaxation times are 0.006, 0.06, and 0.1 s for alpha-, beta-, and gamma-H in pyridine coordinated to SiW9Cu3. The electronic correl ation time calculated from the T-1 values is in the order of 10(-11) o r 10(-12) s, which is two or three orders of magnitude smaller than th ose of ordinary copper complexes. It is suggested that spin frustratio n causes rapid electronic relaxation in these complexes in which the t hree Cu2+ ions form an equilateral triangle.