The nitroxyl radical 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) reacts with diaquobis(hfac)iron(II) (hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dionato) in organic solvents to form the salt 2,2,6,6-tetramethylpiperidinium trifluoroacetate (1) as a major product. Crystals of 1 contain hydrogen-bonded dimers. At 123 K, 1 is monoclinic, C2/c, with a = 19.9528(12) Angstrom, b = 8.4100(5) Angstrom, c = 15.6966(9) Angstrom, beta = 91.4600(10)degrees, and Z = 8. Refinement with 3092 reflections gave R-1 = 0.063. From the reaction in hexane or heptane, crystals of the iron complex salt 2,2,6,6-tetramethylpiperidinium (bis(hfac-O,O')(2,2,6,6-tetramethylpiperidinium-1-oxo-O)ferrate(III))(mu-oxo)(bis-(hfac-O,O')(formato-O)ferrate(III)) (2) were also obtained. Crystals of 2 at 173 K are monoclinic, P2(1)/n, with a = 19.60790(10) Angstrom, b = 12.4155(2) Angstrom, c = 22.4199(4) Angstrom, beta = 90.9380(10)degrees, and Z = 4. Refinement with 12 993 reflections gave R-1 = 0.065. The mu-oxo iron dimer has an Fe-O-Fe angle of 146.28(14)degrees with asymmetric Fe-O distances of 1.802(2) and 1.783(2) Angstrom. TEMPO reduced to the hydroxylamine is coordinated to iron through oxygen. The nitrogen is protonated, forming an apparent hydrogen bond to the mu-oxo group.