Charge density, covalence, and the ground state of the [Fe(CN)6]3– ion in Cs2K[Fe(CN)6]

Abstract

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 t_2g⁵e_g⁰ Fe configuration which loses 1.4(2) e by π backbonding from t_2g orbitals onto the cyanide ligand and gains 1.0(2) e by σ bonding from cyanide into e_g orbitals. The observed Fe 3d–t_2g distribution is anisotropic and changes little between 85 and 295 K, indicating that the cubic T_2g 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 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 |d_xz〉 and |d_xy〉, but more data are required to be certain.