A new range of pendent diphosphine (η⁶-p-cymene)ruthenium(II) complexes, [RuCl(PPh₃)(η¹-(P-P))(η⁶-p-cymene)]PF₆ (P-P = dppm, cis-PPh₂CHCHPPh₂ (dppv), dppe, dppp, dppf), have been prepared by substitution of the labile acetonitrile ligand in [RuCl(CH₃CN)(PPh₃)(η⁶-p-cymene)]PF₆. The formation of chelate complexes, [RuCl(η²-(P-P))(η⁶-p-cymene)]⁺, from these pendent phosphine complexes and from the related neutral complexes, [RuCl₂(η¹-(P-P))(η⁶-p-cymene)] (P-P = dppm, dppv), has been investigated, including determination of activation enthalpies (ΔH^‡) and entropies (ΔS^‡). A concerted substitution mechanism is proposed for the latter complexes, in which methanol plays an important role in the ring-closing process by formation of hydrogen bonds with the chloride ligands. This proposal is supported by volumes of activation (ΔV^‡) determined by variable-pressure UV-visible spectroscopy. In contrast, a dissociative mechanism is proposed for the series of cationic pendent phosphine complexes, which generally require higher temperatures to effect ring closure. Secondary reaction pathways can be observed in some cases and are discussed in terms of differences between the phosphine complexes and supplemented by investigations using electrospray ionization mass spectrometry (ESI-MS). The X-ray structures of [RuCl(PPh₃)(η¹-(P-P))(η⁶-p-cymene)]PF₆ (P-P = dppm, dppv, dppp) and [RuCl(η²-dppv)(η⁶-p-cymene)]PF₆ are also reported.