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 t2g5eg0 Fe configuration which loses 1.4(2) e by π backbonding from t2g orbitals onto the cyanide ligand and gains 1.0(2) e by σ bonding from cyanide into eg 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 |dxz〉, smaller |dyz〉, and negligible |dxy〉 components. An analysis of e.s.r. data supports this interpretation. Polarized neutron diffraction results support the π-backbonding, σ-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 |dxz〉 and |dxy〉, but more data are required to be certain.