The epr spectra have been observed for a series of molecular oxygen adducts prepared from Co(acacen) [acacen = (CH3-C(0~)=CHC(CH3)=NCH2-)2]. These reversibly formed adducts, which in general contain an additional base, are unambiguously shown to be monomeric in both fluid and frozen solution. They are further shown to be best characterized as superoxo (02") adducts of Co(III). Suggestion is also made as to the probable geometry of these complexes.Cobalt complexes carrying molecular 02 have been the object of extensive investigations.1-7 Until recently the oxygen adducts studied have generally been dimeric, containing two atoms of cobalt for one molecule of oxygen. However, several examples of monomeric 1:1 complexes of molecular oxygen with cobalt(II) have now been chemically characterized.7-9* We have examined the epr absorption of a series of adducts of Co(acacen),10 A, in order to confirm their postulated monomeric nature and to elucidate their electronic CH3 z y CH3 _)c-0 t/yD-cf Hcf ß Xh-X=NZ! XN=c( CH3 ch,-ch, CHs A B-A-YCrystalline oxygen adducts of A with pyridines, e.g., py-A-02, in the sixth coordination position exhibit magnetic moments8 ranging from 1.49 to 1.89 BM, indicating the presence of a single unpaired electron. Our epr measurements in both liquid and frozen solution clearly demonstrate the monomeric nature of these adducts by the presence of eight resolved hyperfine splittings from a single cobalt 59Co nucleus. Furthermore, a careful consideration of the spin Hamiltonian parameters for the compounds studied indicates that the unpaired electron resides primarily on the oxygen molecule, and that in partitioning charges the adducts are best thought of as superoxide adducts of cobalt(III). We have also observed that it is possible to form a solution-stable oxygen adduct of A with water as the base in the sixth coordination position, H20-A-02, and there is evidence to suggest a pentacoordinate oxygen adduct in the absence of a base, A-02, although these two adducts have not been isolated as crystalline solids.structures. These adducts are designated as B-A-Y (or ABY) where B = py, CN-py, Me-py, DMF, H2G, or open, and Y = 02 or open. Abbreviations are explained in Table I.The parent compound A contains a low-spin cobalt(II) ion, as indicated by a magnetic moment of 2.16 BM. Although resolvable epr spectra of A were not usually observed at 77 °K, or above,1 11 1:1 adducts of A with a pyridine base, e.g., py-A, exhibit epr spectra in frozen solution which are quite comparable to those shown by adducts of cobalt(II) phthalocyanine12 or by vitamin Bi2r.