dianion. This molecule is a rhenium dimer with each metal coordinated to three CO ligands and a Se4'-chain. One end of the chelating tetraselenide chain is bridging both rhenium atoms, completing an octahedral coordination environment around each metal atom. The bridging distances are slightly asymmetric, leading to the suggestion that the bridging selenide has some dative character.Key words: selenides, rhenium carbonyls, Zintl ions, p-block clusters.