The Mn(hfac)₂ complex of the paramagnetic 4-(benzoxazol-2′-yl)-1,2,3,5-dithiadiazolyl ligand is reported (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato-). The Mn(II) and radical ligand spins are coupled antiferromagnetically (AF) in the coordination complex. Short sulfur–oxygen contacts between molecules provide an efficient pathway for AF coupling between the radical ligand of one molecule and the Mn(II) of a neighbouring molecule, resulting in a large total spin ground state (S_T = 4) for a pair of molecules.