The paramagnetic even-electron cluster, [Et4N](2)[Se2Cr3(CO)(10)], was found to react readily with Mn(CO)(5)Br in acetone to produce two unprecedented mixed chromium-manganese selenide carbonyl complexes, [Et4N][Me-2-CSe2{Mn(CO)(4)} {Cr(CO)(5)}(2)] ([Et4N][1]) and [Et4N](2)[Se2Mn3(CO)(10){Cr(CO)(5)}(2)] ([Et4N](2)[2]). X-ray crystallographic analysis showed that anion 1 consisted of two Se-Cr(CO)(5) moieties, which were further bridged by one isopropylene group and one Mn(CO)(4) moiety. The dianionic cluster 2 was shown to display a Se2Mn3 square-pyramidal core with each Se atom externally coordinated by one Cr(CO)(5) group. The formation of complex 1, presumably via C=O activation of acetone, was further facilitated by acidification of the reaction of [Et4N](2)[Se2Cr3(CO)(10)] with Mn(CO)(5)Br in acetone. Complex 1 readily transformed into 2 upon treatment with Mn-2(CO)(10) in a KOH/MeOH/MeCN solution. Cluster 2 was a 51-electron species, which readily converted to the known 49-electron cluster [Se2Mn3(CO)(3)](2-) upon heating and bubbling with CO. Magnetic studies of the even-electron cluster, [Et4N](2)[Se2Cr3(CO)(10)], and the odd-electron species, [Et4N](2)[2] and [PPN](2)[Se2Mn3(CO)(9)], were determined by the SQUID measurement to have 2, 3, and 1 unpaired electrons, respectively. In addition, the nature and formation of complexes 1 and 2 are discussed, and the magnetic properties and electrochemistry of [Se2Cr3(CO)(10)](2-), 2, and [Se2Mn3(CO)(9)](2-) were further studied and elucidated by molecular orbital calculations at the PW91 level of density functional theory.