Theoretical studies on ferromagnetic behavior of [Cr(C5(CH3)5)2]+[TCNE]- and [Mn(C5(CH3)5)2]+[TCNQ]-

By using broken-symmetry hybrid-DFT (UB3LYP and UB2LYP) calculation, the effective exchange integrals (J values) of [Cr(C-5(CH3)(5))(2)](+)[TCNE](-)[Cr(C-5(CH3)(5))(2)](+) and [Mn(C-5(CH3)(5))(2)](+)[TCNQ](-)[Mn(C-5(CH3)(5))(2)](+) were determined theoretically. Those calculated models were reduced to 3-spin-sites models from X-ray crystallographic data of charge transfer 3D crystal. The calculated results showed that effective exchange integrals were positive and the signs of spin densities on the cyclopentadienyl rings were negative. These results supported the so-called McConnell I mechanism for ferromagnetism proposed by Kollmar et al. and our previous calculations. Natural orbital analysis made it clear that the orbital overlap between SOMO on metals and SOMO on TCNE or TCNQ cations was nearly zero. These results indicated that orbital orthogonality was an important key factor for explaining the ferromagnetism of those systems. (c) 2006 Elsevier Ltd. All rights reserved.