The magnetic coupling mechanism in HHeH and OFe2Cl62- systems has been investigated using the density functional theory with the broken symmetry approach. The approximately linear relationship between the coupling constant and the square of the energy difference of the singly occupied molecular orbitals (MOs) can be expected for the slight change of structural parameters. By analyzing the magnetic interaction one conclusion has been drawn that the sigma -like interaction between the magnetic centers and bridging atom plays an important role in the magnetic coupling.