The cluster {Cu-8(mu(8)-Se)[S2P(OEt)(2)](6)}(0.54){Cu-6[S2P(OEt)(2)](6)}(0.46) (2) was prepared in 78% yield from the reaction of Cu-8(Se)[Se2P(OPr)(2)](6) (1) and NH4S2P(OEt)(2) in toluene. The central selenide ion in 2 was characterized by Se-77 NMR at delta-976 ppm. The simulated solid-state P-31 NMR spectrum shows two components with an intensity ratio close to 55:45. The peak centered at 100.7 ppm is assigned to the P-31 nuclei in the hexanuclear copper cluster, and that at 101.1 ppm is due to the octanuclear copper cluster. The single-crystal X-ray diffraction analysis confirms the cocrystallization structures of Cu-8(Se)[S2P(OEt)(2)](6) (54%) and Cu-6[S2P(OEt)(2)](6) (46%) (2: trigonal, space group R (3) over bar, a = 21.0139(13) Angstrom, c = 11.404(3) Angstrom, gamma = 120degrees, Z = 3). While the octanuclear copper cluster possesses a 3-fold crystallographic axis which pass through the Cu(2), Se, and Cu(2A) atoms, the six copper atoms having the S-6 point group symmetry in Cu-6[S2P(OEt)(2)](6) form a compressed octahedron. The Cu-8(mu(8)-Se) cubic core in Cu-8(mu(8)-Se)[S2P(OEt)(2)](6) is larger in size than the metal core in Cu-8(mu(8)-Se)[Se2P(OPr)(2)](6) (1) although the bite distance of the Se-containing bridging ligand is larger than that of the S ligand. To understand the nature of the structure contraction of the metal core and metal-mu(8)-Se interaction, molecular orbital calculations have been carried out at the B3LYP level of density functional theory. MO calculations suggest that Cu-mu(8)-Se interactions are not very strong and a half bond can be formally assigned to each Cu-mu(8)-Se bond. Moderate (CuCu)-Cu-... repulsion exists, and it is the bridging ligands that are responsible for the observed (CuCu)-Cu-... contacts. Hence, the S-ligating copper clusters have greater (CuCu)-Cu-... separations because each Cu carries more positive charge in the presence of the more electronegative S-containing ligands.