Calorimetric measurements of the interaction of the free-radical base 2, 2, 6, 6-tetramethylpiperidine-N-oxyl with the Lewis acids boron trifluoride and bis(hexafluoroacetylacetonato)copper(II) are reported. Using data previously reported, 2 the C and E numbers of the base were determined to be 6.21 and 0.915. The C/E ratio is much larger than other typical diamagnetic oxygen donor molecules, e.g., ethyl acetate, dimethyl sulfoxide, and pyridine N-oxide, etc. The 1:1 adduct of Cu(hfac)2 with the free-radical base was isolated and its magnetic properties were investigated. It was found that the adduct exhibited large intramolecular antiferromagnetic coupling. Various mechanisms of spin-spin interaction are proposed to explain its magnetic behavior. By comparing the enthalpy predicted for the formation of this adduct from the E and C correlation with that measured experimentally, the contribution to the stability of this adduct from spin-spin interaction can be estimated. The implication of these results to the bonding in systems involving coordinated O2 are discussed.