Paramagnetic Prussian Blue Analogues CsMII[MIII(CN)6]. The Quest for Spin on Cesium Ions by Use of 133Cs MAS NMR Spectroscopy

The Cs-133 magic-angle spinning NMR. spectra of the paramagnetic compounds CsMII[M-III(CN)(6)], M-II = Ni, Co, Fe, Mn; = Co, Fe, yield unusually large and temperature-dependent signal shifts (up to -950 ppm relative to CsCl at 298 K). Comparison with the spectra of the diamagnetic analogues CsM[Co(CN)(6)], M = Zn, Cd, shows that the shifts are largely due to the unpaired electrons. This is ascribed to through-bond transfer of spin to the Cs+ ions, while the through-space effect of the magnetic moments on the signal shifts is shown to be virtually negligible. The mechanism inducing negative spin at Cs+ is discussed. The magnitude of the spin density (average: vertical bar 5.8 X 10(-3)vertical bar (a.u.) (-3)) suggests that Cs+ is involved in magnetic exchange interactions of corresponding Prussian blue derivatives.