The syntheses of bis- and tris-[ (2-methoxyethyl)diphenylphosphine] ruthenium (11) complexes with chloro, acetato, and trifluoroacetato ligands are described. The complexes are mer-[RuCl2(P-O)(PO)2], fac-[RuCl(P-O)2(PO)]X (X = Cl, SbF6, or BPh,), fac-[Ru(P-O)3][SbF6]2, [RuCl(P-O)2]SbF6, fac-[Ru(O2CCH3)2(PO)3], fac-[Ru(O2CCH3)(P-O)(PO)2]X (X = O2CCH3 or BPh,), [RuX2(O,P)2](X = O2CCH3 or O2CCF3), [RuCl(O2CCH3)(O,P)2] and mer-[RuH(O2CCH3)(PO)3]; PO represents the ligand which is co-ordinated via phosphorus only (ether function free), P-O ligand which is co-ordinated in the bidentate chelating mode via phosphorus and oxygen, and O,P is used where the mode of co-ordination is not certain. The mechanism of the fluxional behaviour of these complexes in solution has been investigated by the use of temperature-programmed 31 P and 13C n.m.r. and by nuclear Overhauser enhancement spectroscopy 31 P n.m.r. studies. Fluxional processes occur through exchange between the bidentate (P- and O-bonded) and the monodentate (P-bonded) co-ordination modes of the ether phosphine ligands, as the labile metal-oxygen bonds are broken and reformed. A second type of fluxional process is observed in the six-co-ordinate tris(ether phosphine) complexes due to Berry-type rearrangements of five-co-ordinate intermediates formed upon opening of a metal-oxygen bond. In several of the complexes both types of fluxional process are operating simultaneously. Other complexes show different types of fluxional behaviour in polar and in non-polar solvents, due to ionic dissociation of chloride and acetate ligands in the polar solvents. Some reactions of the complexes are also discussed.