Transition metal-phtahalocyanine(Pc) compound, TPP[Fe(Pc)(CN)_2]_2, which is one of molecular conductors of charge transfer type with 3/4-filled conduction band consisting of LUMO of Pc molecules, shows characteristic features in transport and magnetic properties resulting from localized magnetic moments S=1/2 associated with Fe^+3 atoms. We construct an effective tight-binding model of this system and study the mechanism of exchange interaction, J, between d and π electrons based on both

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... order perturbation of transfer integrals between d and π orbitals and numerical diagonalization. It is found that there is no hybridization between d-orbitals and LUMO of π-orbitals and then super-exchange interaction in the Anderson model does not exist. Instead, processes associated with Hund's rule both on d and π orbitals, which may be called "the double Hund's exchange mechanism", turn out to play important roles and the sign of resultant J can be either ferromagnetic or antiferromagnetic depending on model parameters because of competition among various processes. By taking account of magnetic anisotropy due to spin-orbit interactions and comparing with experimental results, it is indicated that J is antiferromagnetic and of the order of 100K.