Paramagnetic Hexacyanometalates. The Diversity of Spin Distribution Studied by 13C and 15N MAS NMR Spectroscopy

With the aim of probing the spin density distribution in the open-shell cyanometallates Cs2K[M(CN)6] (M = Cr, Mn, Fe), K3[M(CN)6] (M = Mn, Fe), K4[M(CN)6] (M = Cr, Mn), and K4[V(CN)7] have been studied by solid-state 13C and 15N NMR spectroscopy. The signals appear in strongly shifted and broad ranges (13C, −2100 to −8900 ppm; 15N, −1900 to 2400 ppm) except K4[V(CN)7], which is NMR-silent. Analysis of the isotropic signal shifts yields negative spin density in all carbon 2s orbitals (up to 12.2% at the six ligands of [Mn(CN)6]3–) and positive spin density in all nitrogen 2s orbitals (up to 1.1% at the six ligands of [Mn(CN)6]4– and [Fe(CN)6]3–). This is in accord with the induction of alternating spin at the CN ligands by successive polarization of σ bonds triggered by the spin center M n+. The signal shift anisotropies are related to spin in the carbon and nitrogen 2pπ and 2pσ orbitals. In the case of Cs2K[Cr(CN)6] and K4[Cr(CN)6] much positive spin is found in the nitrogen 2pπ orbitals, which corresponds to direct M → N spin transfer. On Cs2K[M(CN)6] (M = Mn, Fe), the 2pπ spin density at nitrogen is negative. The results are in accord with and extend the data of polarized neutron diffraction and EPR spectroscopy. Owing to high signal resolution, small deviations of the [M(CN)6] n− ions from octahedral symmetry and disorder of crystal layers have been detected. This corresponds to the crystal symmetry and to Jahn–Teller distortion. The disorder entails a scatter of spin densities. In the case of K4[Mn(CN)6] it reaches 19% for the C 2s orbitals and 80% for the N 2s orbitals with regard to the respective smallest spin population.