Tridentate ligands with [P,N,O] donor sets, prepared either by the condensation of 2-(diphenylphosphino)benzaldehyde with 1S,2R-norephedrine (HL1) or 2-aminophenol (HL2) or by the condensation of 2-(diphenylphosphino)aniline with salicylaldehyde (sal) (HL3), 5-(NO2)sal (HL4), 5-(Cl)sal (HL5), 5-(Br)sal (HL6), 5-(MeO)sal (HL7) or 3-(MeO)sal (HL8), reacted with [RuIICl2(dmso)4] in refluxing thf solution to yield complexes of the general formula cis-[RuIICl(η3-L1–8)(dmso)2]. Use of two equivalents of ligand HL1 resulted in the formation of mer-[RuII(η3-L1)2], a reaction not seen for HL2–8. Aerial oxidation of cis,mer-[RuIICl(η3-L3)(dmso)2] in a chloroform solution yielded cis, fac-[RuIIICl2(η3-L3)(dmso)], which has undergone an unexpected rearrangement of co-ordination geometry.