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The superexchange interaction between transition-metal ions via oxygen dianion was investigated by the ab initio molecular orbital (MO) method. It is found that the magnitude of the effective exchange integral (J ab) for the CuOCu unit is far larger than those of the NiONi, CrOCr and FeOFe units. Implications of this result are discussed in relation to the high T c superconductivity for Ba-La-Cu-O and R-Ba-Cu-O (R=Y, etc.).

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A generalized molecular orbital (GMO) theory for magnetically interacting organic compounds has been presented. The theory is applied to the calculations of the effective exchange integrals (Jab) for the cyclophane-type carbene dimers whose para and ortho-isomers have the high-spin ground state. The calculated Jab-values are consistent with observations.

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