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Radical species resulting from the reaction of Mn2(CO)10 and [CO(CO)3P(OEt)3]2 with o-benzo- quinone have been generated and their e.s.r. spectra recorded. The reactions of Mn2(CO)lo with several other quinones and diketones have been similarly examined. In most cases the radicals were produced after a short period of ultraviolet irradiation. The radicals derived from Mn2(CO)10 and o-quinones or α-diketones are formulated as six-coordinate complexes (CO)4MnA· in which the manganese atom is chelated by the dicarbonyl compound A. The cobalt radical is similarly formulated as a five-coordinate complex (CO)2P(OEt)3CoA· (A = o-benzoquinone), The proton coupling constants in these adducts are very similar to those of the corresponding quinone or diketone radical anion; this indicates that the unpaired electron resides mainly on the quinone or diketone molecule. The radicals derived from the reaction of Mn2(CO)lo with p-quinones are much shorter lived than those of the corresponding o-quinones, but in the case of 2,3,5,6-tetrachlorobenzoquinone, a spectrum which might belong to the Mn(CO)5 adduct of this molecule was recorded.