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

¹H NMR spectra of pyridine and α-, β-, and γ-picoline coordinated to the paramagnetic heteropolyanion [H_mSiW₉O₃₇Cu₃]^(10-m)- (SiW₉Cu₃) are reported. NMR lines are assigned to [H_mSiW₉O₃₇Cu₃(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 protons exceed 70% of those in analogous nickel complexes. The nuclear longitudinal relaxation times are 0.006, 0.06, and 0.1 s for α-, β-, and γ-H in pyridine coordinated to SiW₉Cu₃. The electronic correlation time calculated from the T₁ values is in the order of 10^-11 or 10^-12 s, which is two or three orders of magnitude smaller than those of ordinary copper complexes. It is suggested that spin frustration causes rapid electronic relaxation in these complexes in which the three Cu²⁺ ions form an equilateral triangle.