Complexes of manganese(II)-containing aminoxyl radical substituted phosphine oxide ligands are reported. The compounds [(o-nitronyl nitroxide-phenyl)diphenylphosphine oxide]bis(hexafluoroacetylacetonato)manganese(II), 3, and bis([p-nitronyl nitroxide-phenyl) diphenylphosphine oxide] bis(hexafluoroacetylacetonato)manganese(I 4, prepared by addition of the free radical phosphine oxides to Mn(hfac)(2), were structurally characterized. Complex 3 is mononuclear, containing an O,O-chelating ortho-substituted radical phosphine oxide ligand, while in 4 the para-substituted ligands bridge two Mn(hfac)(2) units to yield a binuclear molecular rectangle. The magnetic behavior of both systems is dominated by a strong antiferromagnetic Mn(II)-aminoxyl interaction (J= -213 (3), -218 (4) cm(-1) with H = -JS(Mn).S-rad) to give effective S = 2 ground state units. The S = 3 excited state is populated at high temperatures. At low temperatures a decrease in chi(M)T in both complexes is attributable primarily to inter- or intramolecular antiferromagnetic interactions rather than zero-field splitting (ZFS) of the S = 2 ground state. For the bimetallic compound, the magnetic data indicate that ligand-mediated interactions between the Mn(II) spin carriers are weak. The powder EPR spectra of both systems have been recorded and successfully simulated, giving a ZFS parameter D = 0.112 cm(-1). Crystals of 3 are triclinic, space group P (1) over bar with a = 10.6672(19) Angstrom, b = 13.270(6) Angstrom, c = 15.363(3) Angstrom, alpha = 93.84(2)degrees, beta = 108.054(16)degrees, gamma = 105.69(3)degrees, and Z = 2. Crystals of 4 are monoclinic, space group P2(1)/a with a = 12.463(6) Angstrom, b = 19.315(3) Angstrom, c = 17.084(9) Angstrom, alpha = 90 degrees, beta = 98.49(2)degrees, gamma = 90 degrees, and Z = 2.