Spectroscopic study of (two-dimensional) molecule-based magnets: [MII(TCNE)(NCMe)2][SbF6] (M = Fe, Mn, Ni)

The M-[TCNE] (M = 3d metal; TCNE = tetracyanoethylene) system is one of the most interesting classes of molecule-based magnets, exhibiting a plethora of compositions and structures (inorganic polymer chains, 2D layers, 3D networks, and amorphous solids) with a wide range of magnetic ordering temperatures (up to 400 K). A systematic study of vibrational (both infrared and, for the first time, Raman) properties of the family of new TCNE-based magnets of M-II(TCNE) (NCMe)(2)[SbF6] [M = Mn, Fe, Ni] composition is discussed in conjunction with their magnetic behavior and newly resolved crystal structures. The vibrational properties of the isolated TCNE center dot- anion in the paramagnetic Bu4N [TCNE center dot-] salt and recently characterized 2D layered magnet Fe-II(TCNE)(NCMe)(2)[FeCl4] are also reported for comparison. Additionally, a linear correlation between nu(C=C) (a(g)) frequency of the TCNE ligand and its formal charge Z (the spin density on the pi* orbital), Z = [1571 - nu(C=C) (a(g))]/154.5 [e], is presented. It is shown that monitoring Z by Raman spectroscopy is of great use in providing information that allows understanding the peculiarity of the superexchange interaction in M-[TCNE] magnets and establishing the structure-magnetic properties correlations in this class of magnetic material. (C) 2011 American Institute of Physics. [doi:10.1063/1.3599047]