The synthesis and the characterization of several mono- and dinuclear middle transition metal derivatives of 1,10-phenanthroline-5,6-dione, 1, are presented. The reaction of 1 with CrCl 2(THF) 2 gives CrCl 2( O, O′-C 12H 6N 2O 2)(THF) 2, 2, while the halides of iron(II), cobalt(II) and nickel(II) afford adducts of general formula MX 2( N, N′-C 12H 6N 2O 2), M = Fe, 4, Co, 5, X = Cl; M = Ni, 6, X = Br. DFT calculations on CrCl 2(L)(THF) 2 with L = O, O′-C 12H 6N 2O 2 or O, O′-C 14H 8O 2 allowed a direct comparison of the coordination properties of 9,10-phenanthrenequinone and 1,10-phenanthroline-5,6-dione to be made. Dinuclear compounds of general formula CrCl 2(THF) 2( O, O′-C 12H 6N 2O 2- N, N′)MX n L m , M = Zr, 7, X = Cl, n = 4, m = 0; M = Cr, 8, X = Cl, n = 2, L = THF, m = 2; M = Fe, 9, Co, 10, X = Cl, n = 2, m = 0; M = Ni, 11, X = Br, n = 2, m = 0, are prepared from 2 and the corresponding metal halide, while VCp 2( O, O′-C 12H 6N 2O 2- N, N′)FeCl 2, 12, is synthesized by reacting 4 with VCp 2. The electronic properties of the different complexes are investigated by magnetic moment measurements and EPR spectroscopy.