The reaction of CpMoCl2 with diazadiene ligands RN=CH-CH=NR (R-2-dad) affords the corresponding paramagnetic complexes CpMoCl2(R-2-dad) (R = Ph, 1; p-Tol, 2; C6H3Pr2i-2,6, 3; and Pr-i, 4). All compounds have been characterized by EPR spectroscopy and have been investigated by cyclic voltammetry. They display one-electron oxidation and reduction processes, these being reversible or irreversible depending on the nature of R. The irreversibility of the reduction wave is due to a chemical follow-up process which consists of chloride loss from the reduced product. This phenomenon is suppressed in the presence of excess chloride in solution. An X-ray structure of 3 verifies the mononuclear nature of the compound and the chelating mode of the dad ligand, the MoN2C2 ring being essentially planar. The N-C and C-C bond distance pattern suggests the important contribution of an enediamido Mo(V) limiting form. In the presence of 1-bromoethylbenzene, complexes 1-4 catalyze the controlled/'living' radical polymerization of styrene. Complex 3 also leads to a controlled/'living' radical polymerization of styrene in the presence of AIBN (alpha,alpha-azoisobutyronitrile) as a radical generator. Therefore, this is the second example of a compound which is capable of controlling the styrene radical polymerization under both ATRP and SFRP conditions.