The high-spin-low-spin (HS-LS) transition in iron(II) complexes was studied by the recently developed quantum chemical effective Hamiltonian method. This method uses a trial wave function which is an antisymmetrized product of the fully correlated function of d-electrons and of the Slater determinant of the ligand MOs instead of the conventional Hartree-Fock single determinant trial wave function built of the molecular orbitals spread over an entire complex. This approach allowed us to explicitly take into account the d-electron correlations, the weak covalence of the metal-ligand bonds, and the electronic structure of the ligands. The cooperativity effects in the HS-LS transition occurring in the crystals are briefly discussed and the contribution from the Coulomb forces to the intermolecular interaction responsible for the cooperativity is estimated.