The salen complexes ML (M(II) = Cu and Co) and the corresponding tetrahydrosalen complexes M[H-4]L (M(II) = Cu) were investigated by ESR spectroscopy, by magnetic susceptibility, and by quantum chemical study (L(2-) and [H-4]L(2-) are anions of the following : (H(2)L(1) = N,N’-bis(3-tert-butyl-5-methylsalicylidene)-2,3-diamino-2,3-dimethylbutane; H-2[H-4]L(1), N,N’-bis(2-hydtoxy-3-tert-butyl-5-methylbenzyl) 2,3-dimethylbutane; H(2)L(2) = N,N’-bis(3-tert-butyl-5-chlorosalicylidene) H-2 [H-4]L(2) = N,N’-bis(2-hydroxy-3-tert-butyl-5-chlorobenzyl)-2, 3-diamino-2, 3-dimethylbutane). The ESR spectra of Cu(II) complexes in frozen (100 K) toluene solution exhibit a well-resolved perpendicular part. In addition to hyperfine structure, superhyperfine fines are also seen. The superhyperfine structure in the perpendicular region for both CuL(1) and CuL(2) complexes could be well accounted for by the interaction of two equivalent protons along with the two nitrogen nuclei. The protons here belong to the carbon atoms adjacent to the nitrogen nuclei. In the presence of pyridine (5% v/v) there is a considerable shift in both g(parallel to) and g perpendicular to values. The higher g(parallel to) values compared with those of the parent complexes are consistent with the square pyramidal geometry implying axial (py) coordination. An almost negligible effect of an electron withdrawing substituent (X(5) = Cl) On spin Hamiltonian parameters was observed. Due to the aggregation of molecules, no resolved spectrum could be obtained from frozen toluene solutions of both CoL(1) and CoL(2) complexes. The addition of an axial pyridine leads to a better resolution of the spectra. In the presence of dioxygen and pyridine (5% v/v) the frozen (100 K) toluene solution of both CoL(1) and CoL(2) exhibits rhombic symmetry with well-resolved hyperfine structures in all three directions. The shape of the spectra and spin Hamiltonian parameters indicate the interaction of the square pyramidal cobalt core with dioxygen. The interaction of the complexes CoL(py) with molecular oxygen leads to a spin-spin pairing process which results in a partial ligand to metal charge transfer and a large spin density on the oxygen moiety. CoL complexes are low-spin d(7) systems with, mu(eff) = 2.49 mu(B) for CoL(1). The copper complexes Cut and Cu[H-4]L are magnetically normal (mu(eff) 1.81 mu(B) for CuL(2)). The calculated spin densities show that the unpaired electron is localized on the molecular orbital of b(2) symmetry which is almost the d(xy), orbital of the central atom. Only negligible spin density appears at the pyridine nitrogen atom, which is in agreement with the ESR measurements. The [CoL(1)] system exhibits its unpaired electron at the molecular orbital of al symmetry which is the net d(z)2 metal orbital. The INDO/2 method yields the description of the dioxygen adduct which matches well with the generally accepted MO model. The QR-INDO/1 failed in the prediction of the spin pairing process. It prefers either Co(up arrow...O-2(up down arrow) or Co(down arrow)) ...O-2(up arrow up arrow) types of interaction. This may be due to an improper balance of the resonance and exchange contributions to the magnetic coupling.