Two new dirhodium(II) catalysts of general formula Rh-2(N-O)(2)[(C6H4)P(C6H5)(2)](2) (N-O = C4H4NO2) are prepared, starting from Rh-2(O2CCH3)(2)(PC)(2)L-2 [PC = (C6H4)P(C6H5)(2) (head-to-tail arrangement); L = HO2CCH3]. The thermal reaction of Rh-2(O2CCH3)(PC)(2).L-2 with the neutral succinimide stereoselectively gives one compound that according to the X-ray structure determination has the formula Rh-2(C4H4NO2)(2)[(C6H4)P(C6H5)(2)](2) (1). It corresponds to the polar isomer with two bridging imidate ligands in. a head-to-head configuration. However, stepwise reaction of Rh-2(O2CCH3)(2)(PC)(2).L-2 with (CH3)(3)SiCl and potassium succinimidate yields a mixture of 1 and one of the two possible isomers (structure B) with a head-to-tail configuration of the imidate ligands, Rh-2(C4H4NO2)(2)[(C6H4)P(C6H5)(2)](2) (2), also characterized by X-ray methods. In solution, compound 2 undergoes slow isomerization to 1; the rate of this process is enhanced by the presence of acetonitrile. Compounds 1 and 2 are obtained as pure enantiomers starting from (M)- and (P)-Rh-2(O2CCH3)(2)(PC)(2).L-2 rather than from the racemic mixture. Their enantio selectivities in cyclopropanation of 1-diazo-5-penten-2-one are similar to those reported for the dirhodium amidate catalysts.