An improved preparation of mixed ortho-metalated phosphine/succinimidato dirhodium(II) complexes, [Rh-2(P(C5CH4)-Ph-2)(2)(OC4NH4O)(2)], allowed the isolation and characterization of a new isomeric form having both imidato N donors trans to P, 1', that adds up to the two already known having both, 1, or only one, 2, of the imidato N donors trans to the metalated C (Chart 3). The new complex, 1', isomerizes to the thermodynamically stable complex 2 similarly to what had already been observed for isomer 1. Stoichiometric and kineticomechanistic studies of both isomerization processes have been carried out. The reactions have been shown to occur via an intramolecular dissociatively activated process, despite the involvement of the labile axial Rh-2 coordination sites in the formation of intermolecular adducts in solution that do not affect the processes. Density functional theory calculations show two transition states with similar energies for the isomerizations, in very good agreement with the kineticomechanistic measurements. The calculation of the charge generation in the two distinct transition states, TS1 and TS1', indicates an important increase in the N negative charge from the reactants, more pronounced for TS1'. This fact agrees very well with the acceleration observed for the processes in polar solvents, especially for the 1' to 2 reaction, when compared that for the reactions carried out in toluene.