Ferromagnetic dimer interactions in $Cu2Cl4(CH3CN)2$

The magnetic properties of $Cu2Cl4(CH3CN)2$ are reported. Initially reported by Rundle and Willett [J. Chem. Phys. 40, 838, (1964)], the title compound consists of dimeric Cu(II) ions doubly bridged by two chloride ions. Each Cu(II) ion is further bonded to an additional nonbridging chloride ion and a nonbridging acetonitrile molecule. The dimers stack along the crystallographic $a$ axis to form a ladder structure. Static magnetization measurements indicate the two Cu(II) ions within a dimer interact ferromagnetically with $J/kB=40±2 K,$ while a weak ferromagnetic interaction, $J′/kB=0.9±0.1 K,$ also exists between dimers. Ferromagnetic exchange is unusual for this structural motif and is attributed to the reduction of electronic density on the bridging ions due to the electron-accepting nature of the nitrile group in acetonitrile.