The facile synthesis of a novel electron-precise paramagnetic hexamanganese carbonyl selenide cluster [Se6Mn6(CO)(18)](4-) (1) was discovered, which demonstrates contrasting reactivity toward O-2 and Se-8 under markedly mild conditions to afford the O- and Se-inserted clusters [Se6Mn6(CO)(18)(O)](4-) (2) and [Se10Mn6(CO)(18)](4-) (3), respectively. Clusters 1-3 represent the first examples of electron-precise paramagnetic main-group transition metal carbonyl clusters, and their formation and bonding properties are further elucidated by theoretical calculations.