, 8), which were also obtained on heating the osmium(IV) precursor 3 or the labile 1:1 adducts of 3 and 4 with ethene in benzene under reflux. The dihydrido compounds 2 and 3 reacted with CO at room temperature to give initially the octahedral all-cis-configured complexes [OsCl 2 (CO) 2 {κ(P)-iPr 2 PCH 2 CH 2 OMe} 2 ] (11a) and [OsCl 2 (CO) 2 {κ(P)-iPr 2 PCH 2 CO 2 Me} 2 ] (12a), which rearrange to the more stable cis,cis,trans isomers 11b and 12b in benzene under reflux. The all-trans isomer 11c and the monocarbonyl complexes trans,trans-[OsCl 2 (CO){κ 2 (P,O)-