Reaction between Os(CO) 2 (PPh 3 ) 3 and Me 3 SnH produces Os(SnMe 3 )H(CO) 2 (PPh 3 ) 2 (1). Multinuclear NMR studies of solutions of 1 reveal the presence of four geometrical isomers, the major one being that with mutually cis triphenylphosphine ligands and mutually trans CO ligands. Os(SnMe 3 )H(CO) 2 (PPh 3 ) 2 undergoes a redistribution reaction, at the trimethylstannyl ligand, when treated with Me 2 SnCl 2 giving Os(SnMe 2 Cl)H(CO) 2 (PPh 3 ) 2 (2). Solutions of 2 again show the presence of four isomers but now the major isomer is that with mutually trans triphenylphosphine ligands and mutually cis CO ligands. The redistribution reaction of 1 with SnI 4 produces Os(SnMeI 2 )H(CO) 2 (PPh 3 ) 2 (3) which exists in solution as only one isomer, that with mutually trans triphenylphosphine ligands and mutually trans CO ligands. Treatment of 3 with I 2 cleaves the Os-H bond with retention of geometry giving Os(SnMeI 2 )I(CO) 2 (PPh 3 ) 2 (4). The crystal structure of 4 has been determined. No isomerization of the trans dicarbonyl complex 4 occurs when 4 is heated, instead there is a formal loss of ''MeSnI'' and formation of OsI 2 (CO) 2 (PPh 3 ) 2 (5).