Density functional and post-Hartree-Fock studies on effective exchange interaction ofd-?-d conjugated systems involvingm-phenylene-type bridge

Density functional and post–Hartree–Fock calculations have been carried out for the metal–π conjugated complexes involving pyrimidine as an m-phenylene-type unit. Total energies for the lowest and highest spin states are used for investigation of d–π–d magnetic interactions via pyrimidine of Mn(II)2–pyrimidine, Cu(II)2–pyrimidine, and Mn(II)Cu(II)–pyrimidine. The natural orbital analysis of the unrestricted Hartree–Fock (UHF) and density functional theory (DFT) solutions of these complexes has been performed to elucidate relative contributions of spin polarization and spin delocalization (superexchange) interactions for determination of the sign of effective exchange integrals. Implications of the calculated results have been discussed in relation to the molecular magnetism of Mn(II) and Cu(II) complexes from the viewpoint of charge and spin density distributions, shapes of the natural orbitals, and their occupation numbers.