AbstractThe dimer [{(η6‐p‐cymene)RuCl}2(μ‐Cl)2] (cymene=MeC6H4iPr) reacts with N,N′‐bis(p‐tolyl)‐N′′‐(2‐pyridinylmethyl)guanidine (H2L1) and N,N′‐bis(p‐tolyl)‐N′′‐(2‐diphenylphosphanoethyl)guanidine (H2L2), in the presence of NaSbF6, giving rise to chlorido compounds of formula [(η6‐p‐cymene)RuCl(H2L)][SbF6] (H2L=H2L1 (1), H2L2 (2)) in which the guanidine ligand adopts a κ2 chelate coordination mode. The related ligand (S)‐N,N′‐bis(p‐tolyl)‐N′′‐(1‐isopropyl, 2‐diphenylphosphano ethyl)guanidine (H2L3) affords mixtures of the corresponding chlorido compound [(η6‐p‐cymene)RuCl(H2L3)][SbF6] (3) together with the complexes [(η6‐p‐cymene)RuCl2(H3L3)][SbF6] (4) and [(η6‐p‐cymene)Ru(κ3N,N′,P‐HL3)][SbF6] (10) which contain phosphano‐guanidinium and phosphano‐guanidinate ions acting as monodentate and tridentate ligand, respectively. Compounds 1, 2 and mixture of 3/4/10 react with AgSbF6 rendering the cationic aqua‐complexes [(η6‐p‐cymene)Ru(H2L)(OH2)][SbF6]2 (H2L=H2L1 (5), H2L2 (6), H2L3 (7)). These aqua‐complexes exhibit a temperature‐dependent fluxional process in solution. Experimental NMR studies and DFT theoretical calculations on complex 6 suggest that the process involves the exchange between two rotamers around one of the C−N guanidine bonds. Treatment of 5–7 with NaHCO3 renders the complexes [(η6‐p‐cymene)Ru(κ3N,N′,N′′‐HL1)][SbF6] (8) and [(η6‐p‐cymene)Ru(κ3N,N′,P‐HL)][SbF6] (HL=HL2 (9), HL3 (10)), respectively, in which the HL ligand adopts a fac κ3 coordination mode. The new complexes have been characterized by analytical and spectroscopic means, including the determination of the crystal structures of the compounds 1, 2, 5, 9 and 10, by X‐ray diffractometric methods.