Divalent [Yb(DippForm)(2)(thf)(n)] (n= 2 (1a), or 1 (1b), DippForm= N, N'-bis(2,6-diisopropylphenyl) formamidinate) complexes were treated with the ketones: 9-fluorenone (fn), or 2,3,4,5-tetraphenylcyclopentadienone (tpc, tetracyclone), giving ketyl complexes: [Yb(DippForm)(2)(fn(center dot)-CO)( thf)] (2), and [Yb(DippForm)(2)(tpc(center dot)-O)] (3), respectively (ketyl= a radical anion containing a C-center dot-O(-) group. By contrast, when perfluorobenzophenone (pfb) was treated with either 1a or 1b, transitory ketyl formation was followed by rapid decomposition through a C-F activation pathway, giving [YbF(DippForm)(2)(thf)] (4a) and a highly unusual fluoride/oxide-bridged species: [Yb5F6O2(DippForm)(5)] (4b). The reduction of diketones: 3,5-di-tert-butyl-1,2-benzoquinone (tbbq), 9,10-phenanthrenequinone (phen), or 1,2-acenaphthenequinone (acen), was also examined giving ketyl complexes: [Yb(DippForm)(2)(tbbq(center dot)-O-2)] (5), [Yb(DippForm)(2)(phenCO(2))] (6), and [Yb(DippForm)(2)(acen(center dot)-O-2)(thf)] (7). An unsolvated derivative of 7, namely [Yb(DippForm)(2)(acen(center dot)-O-2)] (8), was obtained from PhMe. All ketyl complexes had suitably elon gated C-center dot-O bonds, were stable in both polar and non-polar solvents-an uncommon trait for rare-earth ketyl complexes-and, with the exception of 3, showed radical signals in ESR spectra. To investigate the reactivity of the tpc(center dot)-O ketyl complex, 3 was treated with oxidants (CS2, Se) and reducing agents (Mg-0, KH, or [SmI2(thf)(2)]). Thus 3 was oxidised to tpc by Se. Treatment of 3 with KH led to a ligand exchange process giving an unusual diketyl species [Yb(DippForm)(tpc(center dot)-O)(2)(thf)(2)] (10), which has two cisoid tpc(center dot)-O- ligands in very close proximity. When treated with [SmI2(thf)(2)], the tpc(center dot)-O ketyl was further reduced to a dianion (1-oxido-2,3,4,5- tetraphenylcyclopentadianide(2 -)), (\C5Ph4\-O)(2-) by [SmI2(thf)(2)], giving dimeric [\SmI(\C5Ph4\-O)(thf)(2)\(2)] (Sm11) and monomeric complexes [YbI(DippForm)(2)(thf)] (11b) and [YbI2(DippForm)(thf)(2)] (11 c). Activated Sm metal reduced neutral tetracyclone to the dianion, (\C5Ph4\-O)(2-), in THF, giving tetranuclear [\Sm-2(II)(\C5Ph4\-O)(2)(thf)(3)\(2)] (Sm13). Treatment of Sm13 with iodine in situ provided access to [\SmI(\C5Ph4\-O)(thf)(2)\(2)] (Sm11), in good yield.