AbstractA series of radical complexes containing manganese carbonyl, 1,3,6,8‐tetramethyl‐9,10‐phenanthroquinone (PQ1), 2,4,5,7‐tetramethyl‐9,10‐phenanthroquinone (PQ2) and triphenylphosphine are prepared and their ESR spectra are recorded and analyzed with computer simulations. From the analysis of the proton, manganese and phosphine hyperfine splitting constants, the nature of the orbital interaction and also the election transfer character are revealed. PQ1−and PQ2−Mn(CO)4radicals are of more anion radical character than the unsubstituted PQ−Mn(CO)4radical. The fact that the Mn and P hfsc increases and the proton hfsc decreases as the phosphine ligand increases indicated that an increasing interaction exists between the PQ pπ* and the Mn dπ orbitals with increasi