The 16-electron dithiolene complexes (p-cymene)M[S2C2(B10H10)] (M=Ru, Os) containing both η6-(p-cymene) and η2-(ortho-carborane-dithiolate): adduct formation with Lewis bases, and X-ray crystal structures of (p-cymene)Ru[S2C2(B10H10)](L) (L=PPh3) and {(p-

The reaction of the η6-arene complexes [(p-cymene)MCl2]2 (M=Ru, Os; p-cymene=4-isopropyl–toluene) with dilithium 1,2-dicarba-closo-dodecaborane-1,2-dithiolate (a) leads to the new 16e dithiolene complexes (p-cymene)M[S2C2(B10H10)] (M=Ru (1), Os (1A)). Addition of monodentate Lewis bases (L) to 1 gives 18e dithiolate complexes of the type (p-cymene)Ru[S2C2(B10H10)](L) (L=PPh3 (2), P(OMe)3 (3), NH3 (4), NC5H5 (5), CO (6), CNtBu (7), SEt2 (8), SC4H8 (9), CN− (10) and SCN− (11)), whereas bidentate bridging Lewis bases (LL) give centrosymmetric binuclear analogues, {(p-cymene)Ru[S2C2(B10H10)]}2(LL) (LL=Ph2PCH2CH2PPh2 (12), N2H4 (13) and 4,4′-dipyridine (14)). The stability of the Lewis base adducts depends on the nature of the ligating atom and decreases in the order C>P>N>S>O. The adducts were characterized by their 1H-, 13C- and 11B-NMR spectra, and X-ray crystal structures were determined for 2, 12 and 13. The phosphane ligands in 2 and 12 cause stronger folding of the planar dithiolene ring RuS2C2 in 1 along the S⋯S vector (by 25.6° in 2 and 21.0° in 12) than the hydrazine ligand in 13 (7.5°).