Compounds of the type Cp*Os(CO)LR (Cp**η 5 -C 5 Me 5 ; L=CO, PMe 2 Ph; R=Me, Et, i-Pr, CH 2 Ph), most of them new, have been prepared and their reactions with the electrophiles CF 3 CO 2 H, Br 2 , and HgBr 2 have been investigated. All of the electrophiles oxidatively add to give labile, formally osmium(IV) complexes of the type [Cp*Os(CO)(L)(R)(E)] + X − , one of which, [Cp*Os(CO)(PMe 2 Ph)(Me)(HgBr)] + , has been isolated as the PF 6 − salt