Magnetic properties of organic-based Ni[TCNE](MeCN)$_{2}$][BF$_{4}$] magnet.*

A new organic-based magnet of Ni[TCNE][BF$_{4}$]( MeCN)$_{2-\delta}$ (\textbf{1}) composition ($\delta $ = 0.15; TCNE = tetracyanoethylene) was synthesized via reaction of NBu$_{4}$(TCNE) and Ni(NCMe)$_{6}$(BF$_{4})_{2}$ in CH$_{2}$Cl$_{2}$. Zero field cooled and field cooled magnetizations, $M(T)_{ZFC}$ and $M(T)_{FC}$, at 0.5 mT rise sharply below 70 K indicative of an onset of a magnetic transition. $M(T)_{ZFC}$ reaches maximum at 25 K followed by a rapid decrease suggesting antiferromagnetic (AF) interaction. In contrast, $M(T)_{FC}$ rises upon further cooling signifying a strong irreversibility in accord with sharp increase of a remanant magnetization below 30 K and hysteretic behavior of $M(H)$. The $M(H)$ at 2 K increases rapidly with field and approaches saturation above $\sim $ 0.5 T. At 9 T $M(H)$ reaches 2.24 $\mu _{B}$ that is significantly higher than 1.30 $\mu _{B}$ expected for AF coupled Ni(II) S = 1 and [TCNE]$^{-}$ (S = 1/2) suggesting a ferromagnetic (FM) interaction. The unpaired Ni$^{II}$ spins and those on the [TCNE]$^{-}$ reside in orthogonal orbitals resulting in FM coupling. Assuming that similarly to Fe[TCNE][FeCl$_{4}$](MeCN)$_{2}$ \textbf{1} consists of Ni$^{II}-\mu _{4}$-[TCNE]$^{-}$ layers we believe that the decrease of $M(T)_{ZFC}$ below 25 K is due to AF coupling \textit{between the layers} while the interaction \textit{within the layer} is FM in contrast to the AF one reported for Fe, V, and Mn analogues.