Credit: © 2008 ACS

Open-shell transition metal ions — those without a full d-shell of electrons — with only two coordinating ligands have interesting reaction chemistry and properties. Although some examples of two-coordinate iron complexes are known, most increase the amount of coordination on the metal centre through interactions to remote parts of the ligands. This means the complexes bend away from the 180° expected.

Now, William Reiff from Northeastern University in Boston and Gregory Girolami from the University of Illinois in Urbana, together with co-workers from the USA and Korea, have made a two-coordinate iron complex that is nearly linear. The compound has two tertiary-butyl amide ligands and the N–Fe–N angle is 179.5°, with no significant contact between the iron atom and the carbon atoms at the periphery of the ligands.

The resulting ground state is nearly degenerate, and gives rise to a large magnetic moment of 5.55 Bohr magnetons — higher than the 4.90 predicted for high-spin Fe(II) species. This is also responsible for an exceptionally large internal hyperfine field, which is larger than all known iron compounds, apart from another linear two-coordinate complex. These results show that the absence of Jahn–Teller distortions in linear complexes can produce unusual magnetic effects.