Exchange coupling behavior of cyano-bridged binuclear Fe(II)-Ni(II) complexes: a density functional theory combined with broken-symmetry approach

Molecular magnetism in a series of cyano-bridged Fe(III)-Ni(II) complexes has been investigated using hybrid density functional theory (DFT) B3LYP combined with broken symmetry (BS) approach. Two structural factors of the r distance (Ni-N-brid) and the theta angle (Ni-N-brid-C-brid) on J are analyzed by us in detail. The calculated results show that the J values increase with the increase of the r distance when the theta angles are smaller than 150 degrees. When theta is 150 degrees, the J values are almost the same for the different r distances. However, when theta is larger than 150 degrees, the J values will decrease with the increase of the r distance. However, for all of the r distances, the J values will decrease with the increase of the theta angle except for when the theta angles are in the range 120-130 degrees or 170-180 degrees. Then we use above trends to interpret the experimental and calculated J values of the complete structures for a series of cyano-bridged Fe(III)-Ni(II) complexes. In addition, we find that Fe(m) mainly displays its polarization effect and although the largest part of the spin density is located at the Ni(II), there is an important delocalization of the unpaired electron, mostly to the six donor atoms, through the calculated spin density distributions.