Electronic Structure, Heisenberg Coupling Constants, and Metal−Metal Bond in Dimeric Iron(II) Organometallics with the Metal Centers at Variable Distances: A Density Functional Approach

Density functional calculations were performed to investigate the electronic structure, the magnetic coupling, and the bonding in Fe2(NCH2)2(NCH)2(C6H5)2,Fe2(C6H5)4, and Fe2(HNCH)4 model systems. The corresponding real compounds are characterized by strongly reduced magnetic moments, while metal-metal distances vary in a wide range from uncommonly short to slightly long. The results show the absence of a direct Fe(II)-Fe(II) bond for the first two compounds: both the electronic structure studies and the Heisenberg coupling analysis give indications in this direction. The reduced magnetic moment results essentially from strong antiferromagnetic interactions via bridging ligands. Heisenberg coupling constants were calculated for each complex, which show an antiferromagnetic coupling of four unpaired electrons on each center. In the third compound there is a relevant direct Fe-Fe interaction, although the short Fe-Fe distance results mainly from a strong metal-bridging ligand interaction.