Experimental electron density distributions of [Cu(NO3)(2)(H2O)(2)(pm)(2)] (1) and [MCl2(pm)(2)] (2:M=Co, 3:M=Fe) (pm=pyrimidine) showing antiferromagnetic interactions were determined by the X-ray diffraction method using the multipole expansion refinement. In the crystal structures, the pyrimidine ligand bridges metal atoms to form one-dimensional chains in 1, and three-dimensional networks in 2 and 3. The electron populations in the d-orbitals calculated from the multipole population parameters revealed that the unpaired electrons are located in d(X)(2)-y(2) orbital for each complex. These results show that the strong interactions between the bridging ligand and the unpaired electrons cause the antiferromagnetic interactions mediated by the bridging ligands.