A new family of CuX-, Cu2X2-, and Cu4X2-incorporated mono- or di-SeFe3-based carbonyl clusters were constructed and structurally characterized. When the selenium-capped thiron carbonyl cluster [Et4N](2)[SeFe3(CO)91 was treated with 1-3 equiv of CuX in tetrahydrofuran (THF) at low or room temperatures, CuX-incorporated SeFe3 complexes [Et4N](2)[SeFe3(CO)(9)CuX] (X = Cl, [Et4N](2)[1a]; Br, [Et4N](2)[1b]; I, [Et4N](2)[1c]), Cu2X2-incorporated SeFe3 clusters [Et4N](2)[SeFe3(CO)(9)Cu2X2) (X = Cl, [Et4N](2)[2a]; Br, [Et4N](2)[2b)), and Cu4X2-linked di-SeFe3 clusters [Et4N](2)[{SeFe3(CO)(9)}(2)Cu4X2] (X = Cl, [Et4N](2)[3a]; Br, [PPh4](2)[3b]) were obtained, respectively, in good yields. SeFe3CUX complexes 1a and 1b were found to undergo cluster expansion to form SeFe3Cu2X2 complexes 2a and 2b, respectively, upon the addition of 1 equiv of CuX (X = Cl, Br). Furthermore, complexes 2a and 2b can expand further to form Cu4X2-linked di-SeFe3 clusters 3a and 3b, upon treatment with 1 equiv of CuX (X = Cl, Br). [Et4N](4)[(SeFe3(CO)(9)(CuCl)(2)}(2)] ([Et4N](4)[4a]) was produced when the reaction of [Et4N](2)[SeFe3(CO)(9)] with 2 equiv of CuCl was conducted in THF at 40 degrees C. The Cu2Cl2-linked di-SeFe3CuCl cluster 4a is a dimerization product derived from complex 2a. Further, it is found that complex 4a can convert to the Cu4Cl2-linked di-SeFe3 cluster 3a upon treatment with CuCl. The nature, formation, stepwise cluster expansion, and electrochemical properties of these CuX-, Cu2X2-, and Cu4X2-incorporated mono- or di-SeFe3-based clusters are elucidated in detail by molecular calculations at the B3LYP level of the density functional theory in terms of the effects of selenium, iron, copper halides, and the size of the metal skeleton.