This paper reports on the synthesis, X-ray structure, magnetic properties, and DFT calculations of [{HC(CMeNAr)(2)}Mn](2) (Ar = 2,6-iPr(2)C(6)H(3)) (2), the first complex with three-coordinate manganese(l). Reduction of the iodide [{HC(CMeNAr)(2)}Mn(mu-l)](2) (1) with Na/K in toluene afforded 2 as dark-red crystals. The molecule of 2 contains a Mn-2(2+) core with a Mn-Mn bond. The magnetic investigations show a rare example of a high-spin manganese(l) complex with an antiferromagnetic interaction between the two Mn(l) centers. The DFT calculations indicate a strong s-s interaction of the two Mn(l) ions with the open shell configuration (3d(5)4s(1)). This suggests that the magnetic behavior of 2 could be correctly described as the coupling between two S-1 = S-2 = (5)/(2) spin centers. The Mn-Mn bond energy is estimated at 44 kcal mol(-1) by first principle calculations with the B3LYP functional. The further oxidative reaction of 2 with KMnO4 or 02 resulted in the formation of manganese(III) oxide [{HC(CMeNAr)(2)}Mn(mu-O)](2) (3). Compound 3 shows an antiferromagnetic coupling between the two oxo-bridged manganese(III) centers by magnetic measurements.