Treatment of the dimers [RuX2(eta(6)-cymene)](2) With PF3 in hot heptane produces the compounds [RuX2(eta(6)-cymene)(PF3)] (X = Cl, Br, I) in good yield. Difluoro(dimethylamino)phosphine and tris(dimethylamino)phosphine react similarly to produce the compounds [RuX2(eta(6)-cymene)(PF2{NMe2})] and [RuX2(eta(6)-cymene)(P{NMe2}(3))]. Reaction of the dimers (RuX2(eta(6)-cymene)](2) with PCl3 and PBr3 proceeded with the production of mononuclear products which had undergone halogen exchange at ruthenium in some cases. H-1, C-13, P-31, and F-19 NMR spectra have been obtained where appropriate together with (H-1-H-1) correlation spectroscopy (COSY) and (C-13-H-1)-HETCORR spectra of selected compounds. The variation of (1)J(P-31-F-19) with the nature of the auxiliary ligand (X) in the PF3 and PF2(NMe2) complexes has been examined both experimentally and computationally using a natural localized molecular orbital-natural bond order approach. The single crystal X-ray structure of [RuBr2(eta(6)-cymene)(PF3)] has been determined at 223 K and those of [RuBr2(eta(6)-cymene)(PF2{NMe2})] and [RuI2(eta(6)-cymene)(P{NMe2}(3))] at 294 K.