Novel neutral and cationic Rh-I and Ir-I complexes that contain only DMSO molecules as dative ligands with S-, O-, and bridging S,O-binding modes were isolated and characterized. The neutral derivatives [RhCl(DMSO)(3)] (1) and [IrCl(DMSO)(3)] (2) were synthesized from the dimeric precursors [M2Cl2(coe)(4)] (M=Rh, Ir; COE=cyclooctene). The dimeric Ir-I compound [Ir,Cl,(DMSO)41 (3) was obtained from 2. The first example of a square-planar complex with a bidentate S,O-bridging DMSO ligand, [(coe)(DMSO)Rh-(mu-Cl)(mu-DMSO)RhCl(DMSO)] (4), was obtained by treating [Rh2Cl2(coe)(4)] with three equivalents of DMSO. The mixed DMSO-olefin complex [IrCl(cod)(DMSO)] (5, COD=cyclooctadiene) was generated from [lr(2)Cl(2)(cod)(2)]. Substitution reactions of these neutral systems afforded the complexes [RhCl(py)(DMSO)(2)] (6), [IrCl(py)(DMSO)(2)] (7), [IrCl(iPr(3)P)(DMSO)(2)] (8), [RhCl(dmbpy)(DMSO)] (9, dmbpy = 4,4'-dimethyl-2,2'-bipyridine), and [lrCl(dmbpy)(DMSO)] (10). The cationic O-bound complex [Rh(cod)(DMSO)(2)]BF4 (11) was synthesized from [Rh(cod)(2)]BF4. Treatment of the cationic complexes [M(coe)(2)(O=C-Me-2)(2)]PF6 (M=Rh, Ir) with DMSO gave the mixed S- and O-bound DMSO complexes [M(DMSO)(2)(DMSO)(2)]PF6 (Rh=12; Ir=in situ characterization). Substitution of the O-bound DMSO ligands with dmbpy or pyridine resulted in the isolation of [Rh(dmbpy)(DMSO)(2)]PF6 (13) and [Ir(py)(2)(DMSO)(2)]PF6 (14). Oxidative addition of hydrogen to [IrCl(DMSO)(3)] (2) gave the kinetic product fac-[Ir(H)(2)Cl(DMSO)(3)] (15) which was then easily converted. to the more thermodynamically stable product mer-[Ir(H)(2)Cl(DMSO)(3)] (16). Oxidative addition of water to both neutral and cationic Ir-I DMSO complexes gave the corresponding hydrido-hydroxo addition products syn-(DMSO)(2)HIr(mu-OH)(2)- (mu-Cl)IrH(DMSO)(2)][IrCl2(DMSO)(2)] (17) and anti-[(DMSO)(2)(DMSO)HIr(mu-OH)(2)- IrH(DMSO)(2)(DMSO)][PF6](2) (18). The cationic [Ir(DMSO)(2)(DMSO)(2)]PF6 complex (formed in situ from [Ir(coe)(2)(O=CMe2)(2)]PF6) also reacts with methanol to give the hydrido-alkoxo complex syn[(DMSO)(2)HIr(mu-OCH3)(3)IrH(DMSO)(2)]- PF6 (19). Complexes 1, 2, 4, 5, 11, 12, 14, 17, 18, and 19 were characterized by crystallography.