The reaction of the 4-chloro-2H-chromene-derived thiosemicarbazone ligand (L1) with the Ru-benzene precursor [RuIICl2(eta 6-benzene)]2 gives different products depending on the reaction conditions. At room temperature, L1 coordinates through the sulfur donor to form the monodentate complex [(eta 6-benzene)RuII(eta 1-S-L1)Cl2] (P1), whereas refluxing in the presence of MeOH affords the four-membered N,S-chelated ruthenocycle [(eta 6-benzene) RuII(eta 2-N,S-L1)Cl] (P2). The origin of this coordination-mode switching was investigated by density functional theory calculations. The pathway through an activated mono-halo-bridged dimer is energetically preferred in P1 formation with an overall barrier of 5.9 kcal mol(-1). MeOH-assisted chloride abstraction and hydrogen-bond stabilization of the chloride ion favor Ru-N bond formation for P2 formation with a barrier of 13.4 kcal mol(-1). The alternative five-membered products [(eta 6-benzene)RuII(eta 2-S, N-L1)Cl]Cl (P1 ') and [(eta 6-benzene) RuII(eta 2-S, N-L1)Cl] (P2 ') are disfavored because of the need for energetically demanding ligand reorganization for their formation. These results rationalize the preference of the experimentally observed monodentate P1 and four-membered ruthenocycle P2 over the seemingly less strained five-membered alternatives.