Magnetic ordering of new molecule-based magnet: [Fe(TCNE)(NCMe)$_{2}$][FeCl$_{4}$].

The magnetic properties of [Fe(TCNE)(NCMe)$_{2}$][FeCl$_{4}$] (TCNE = tetracyanoethylene), a molecule-based magnet synthesized via reaction of FeCl$_{2}$(NCMe)$_{2}$ with TCNE in CH$_{2}$Cl$_{2}$. $M(T)$ is discussed. Both \textit{$\chi $'}($T)$ and \textit{$\chi $}''($T)$, ac susceptibilities are almost frequency independent, and exhibit a sharp peak at $\sim $90 K in accord with $T_{c}$. The zero field cooled (ZFC) and field cooled (FC) magnetic susceptibilities, \textit{$\chi $}$_{ZFC}(T)_{ }$and \textit{$\chi $}$_{FC}(T)$, at 5 Oe rise sharply below 95 K indicative of a magnetic transition. \textit{$\chi $}$_{ZFC}(T)$ reaches maximum at 88 K followed by a rapid decrease suggesting an antiferromagnetic(AFM) ground state attributed to AFM coupling between ferrimagnetically ordered [Fe[TCNE](NCMe)$_{2}$]$^{+}$ layers. In contrast, \textit{$\chi $}$_{FC}(T)$ rises upon further cooling suggesting a strong irreversibility and indicating the presence of a remanant magnetization below 90 K, which increases upon cooling. [Fe(TCNE)(NCMe)$_{2}$][FeCl$_{4}$] is the initial member of a new class of magnets. It is the first metal-TCNE magnet with direct bonding between metal ion and [TCNE]$^{- }$whose structure has been determined, and it possesses a novel planar $\mu _{4}$-[TCNE]$^{-}$ spin coupling unit.