Carbonylation of the triply bonded dirhenium(II) complexes Re2X4(dppm)2 (X = chloro, bromo; dppm = bis(diphenylphosphino)methane) to give [Re2X3(dppm)2(CO)3]PF6 and [Re2X3(dppm)2(CO)2]PF6. Structural characterization of [(CO)ClRe(.mu.-Cl)(.mu.-CO)(.mu.-dppm)2ReCl(CO)]PF6