A novel mechanistic insight for the interconversion between two octahedral ruthenium isomers, trans-mer-[ (RuCl2)-Cl-II(bpea)(dmso)], 2a, and cis-fac-[ (RuCl2)-Cl-II(bpea)(dmso)], 2b, (bpea = N, N-bis(2-pyridylmethyl) ethylamine) is reported. The isomer 2a displaying the bpea ligand in a meridional fashion is kinetically favoured with respect to 2b, with the bpea ligand in a facial fashion, which is the thermodynamically favoured one. The dissociative mechanism with the cleavage of a chloro ligand as starting point becomes more feasible with respect to the intramolecular mechanism. However, DFT calculations make competitive the mechanism which dissociates first the dmso ligand. Thus, a further characterisation of all the reaction pathways for the interconversion 2a --> 2b has been fulfilled. (C) 2008 Elsevier B.V. All rights reserved.