A wide range of osmium-polyhydride complexes stabilized by the POP-pincer ligand xant((PPr)-Pr-i,), (9,9-dimethyl-4,5-bis(diisopropylphosphino)-xanthene) have been synthesized through cis-OsCl2{k-S-(DMSO)(4)} (1, DMSO = dimethyl sulfoxide). Treatment of toluene solutions of this adduct with the diphosphine, under reflux, leads to OsCl2{xant((PPr2)-Pr-i)(2)}-(k-S-DMSO) (2). The reaction of 2 with H-2 in the presence of Et3N affords OsH3Cl{xant(PiPr2)2} (3), which can be also prepared by addition of xant(PiPr2)2 to toluene solutions of the unsaturated d(4)-trihydride OsH3Cl((PPr3)-Pr-i)(2) (5). Complex 3 reductively eliminates H-2 in toluene at 90 degrees C. In the presence of dimethyl sulfoxide, the resulting monohydride is trapped by the S-donor molecule to give OsHCl{xant((PPr2)-Pr-i)(2)}(K-S-DMSO) (6). The reaction of 2 with H-2 is sensible to the Bronsted base. Thus, in contrast to Et3N, NaH removes both chloride ligands and the hexahydride OsH6{xant((PPr2)-Pr-i)(2)} (7), containing a k(2)-P-binding diphosphine, is formed under 3 atm of hydrogen at 50 degrees C. Complex 7 releases a H-2 molecule to yield the tetrahydride OsH4{xant((PPr2)-Pr-i)(2)} (8), which can be also prepared by reaction of OsH6((PPr3)-Pr-i)(2) (9) with xant(PiPr(2))(2). Complex 8 reduces H to give, in addition to H-2., the oxidized OsH4-species [OsH4(OTf){xant((PPr2)-Pr-i)(2)})(+) (10, OTf = trifluoromethanesulfonate). The redox process occurs in two stages via the OsH5-cation [OsH5{xant((PPr2)-Pr-i)(2)}](+) (11). The metal oxidation state four can be recovered. The addition of acetonitrile to 10 leads to [OsH2(eta(2)-H-2)(CH3CN){xant((PPr2)-Pr-i)(2)}](2+) (12). The deprotonation of 12 yields the osmium(IV) trihydride [OsH3(CH3CN){xant((PPr2)-Pr-i)(2)}](+) (13), which is also formed by addition of HOTf to the acetonitrile solutions of 8. The latter is further an efficient catalyst precursor for the head-to-head (Z)-dimerization of phenylacetylene and tert-butylacetylene. During the activation process of the tetrahydride, the bis(allcynyl)vinylidene derivatives Os(C CR)(2)(=C=CHR){xant((PPr2)-Pr-i)(2)} (R = Ph (14), Bu-t (15)) are formed.