CHARGE-DENSITY, COVALENCE, AND THE GROUND-STATE OF THE [FE(CN)6]3-ION IN CS2K[FE(CN)6]

Charge-density X-ray diffraction experiments have been performed on Cs2K[Fe(CN)6] at 295 and 85 K. The Fe(CN)6 fragment is almost octahedral in geometry, but the Cs-K environment is quite non-cubic. The Fe-CN bonding can be described by an initial t2g5e(g)0 Fe configuration which loses 1.4(2) e by pi- backbonding from t2g orbitals onto the cyanide ligand and gains 1.0(2) e by sigma-bonding from cyanide into e(g) orbitals. The observed Fe 3d-t2g distribution is anisotropic and changes little between 85 and 295 K, indicating that the cubic T2g ground term of the ferricyanide ion is split by more than 200 cm-1. The ground state corresponds to a spin-hole wavefunction with large \d(xz) >, smaller \d(yz) >, and negligible \d(xy) > components. An analysis of e.s.r. data supports this interpretation. Polarized neutron diffraction results support the pi-backbonding, sigma-bonding model and emphasize the importance of both covalence and spin polarization. The ground state at 4.2 K may differ from that at 85 K, being a roughly equal mixture of \d(xz) > and \d(xy) >, but more data are required to be certain.