A Metallic (EDT-DSDTFVSDS)₂·FeBr₄ Salt: Antiferromagnetic Ordering of <i>d</i> Spins of FeBr₄^- Ions and Anomalous Magnetoresistance Due to Preferential π−<i>d</i> Interaction

The 2:1 salt of a new donor molecule, EDT-DSDTFVSDS with FeBr4- ion, (EDT-DSDTFVSDS)2·FeBr4 showed an essentially metallic behavior despite a small upturn in the electrical resistance below ca. 30 K (electrical conductivities at 290 and 4.2 K are 200 and 170 S cm-1, respectively). The Fe(III) d spins of the FeBr4- ions in this salt were subject to antiferromagnetic ordering at 3.3 K by virtue of a strong π−d interaction (Jπd) which is comparable to that in a molecular metallic conductor, λ-(BETS)2·FeCl4, and of a very weak d−d interaction (Jdd). This strong π−d interaction was evidenced by a large and negative magnetoresistance effect (ca. 20% at 5 T) as well as by the appearance of a large dip in the resistance at the magnetic field (ca. 2.0 T) parallel to the easy axis for the spin-flop transition of the Fe(III) d spins.