Treatment of (eta(6)-naphthalene)(eta(4)-1,5-cyclooctadiene)ruthenium(0), Ru(eta(6)-C10H8)(eta(4)-1,5-C8H12) (1), with a slight excess of trimethylphosphine, triethylphosphine, trimethyl phosphite, triethyl phosphite, or tert-butyl isocyanide below room temperature gives 1;eta(4)-naphthalene complexes Ru(eta(4)-C10H8)(eta(4)-1,5-C8H12)(L) [L = PMe3 (3), PEt3 (4), P(OMe)(3) (5), P(OEt)(3) (6), t-BuNC (7)], which provide the first examples of the often postulated eta(6) to eta(4) transformation of naphthalene induced by two-electron donor ligands. The eta(4)-naphthalene is easily displaced by an excess of the ligands to give RuL3(eta(4)-1,5-C8H12) At room temperature, complex 1 reacts with a deficiency of PMe3, PEt3, or P(OMe)(3) to give binuclear complexes containing bridging naphthalene, (eta(4)-1,5-C8H12)Ru(mu-eta(6):eta(4)-C10H8)Ru(eta(4)-1,5-C8H12)(L) [L = PMe3 (8), PEt3 (9), P(OMe)(3) (10)]. Single-crystal X-ray studies of complexes 3, 4,,5, 9, and 10 show the presence in each case of a folded naphthalene ring with a hinge angle of ca. 40 degrees. In 9 and 10 the metal atoms adopt an anti arrangement relative to the bridging naphthalene.