Synthetic procedures are described that allow access to an octanuclear, mixed-valence 4Co(II),4Co(III) molecular aggregate. Oxidation of Co(O(2)CPh)(2) in DMF with aqueous H2O2 leads to a dark emerald-green solution from which can be isolated ] (1a) in similar to 60% yield. The same reaction in MeCN leads to [Co8O4(O(2)CPh)(12)(MeCN)(3)(H2O)] (1b) Complex 1b . 2MeCN crystallizes in triclinic space group P $($) over bar$$ 1 with the following unit cell dimensions at -153 degrees C : a = 15.056(4) Angstrom, b = 23.548(6) Angstrom, c = 14.697(4) Angstrom, alpha = 90.74(1)degrees, beta = 114.75(1)degrees, gamma = 75.12(1)degrees, V = 4546.5 Angstrom(3), Z = 2. A total of 10 207 unique data with F > 3 sigma(F) were used to solve the structure, which refined to R and R(w) values of 4.01 and 4.24%, respectively. The structure consists of a central [(C4O4)-O-III](4+) cubane unit, with each mu(3)-O2- ion becoming mu(4) by attachment to a Co-II center to give a [Co8O4](12+) core; the [Co4O4](4+) cubane is thus bridging the four Co-II ions. Twelve PhCO(2)(-) groups bridge the Co-II.. Co-III pairs, and terminal MeCN/H2O groups complete the peripheral ligation at the five-coordinate, high-spin Co-II centers. The Co-III centers are low-spin. Excluding differences in terminal ligation (MeCN or H2O), the molecule has idealized Td symmetry. Variable-temperature, solid-state magnetic susceptibility studies on in have been carried out in the range 4.00-320 K. : a = 15.056(4) Angstrom, b = 23.548(6) Angstrom, c = 14.697(4) Angstrom, alpha = 90.74(1)degrees, beta = 114.75(1)degrees, gamma = 75.12(1)degrees, V = 4546.5 Angstrom(3), Z = 2. A total of 10 207 unique data with F > 3 sigma(F) were used to solve the structure, which refined to R and R(w) values of 4.01 and 4.24%, respectively. The structure consists of a central [(C4O4)-O-III](4+) cubane unit, with each mu(3)-O2- ion becoming mu(4) by attachment to a Co-II center to give a [Co8O4](12+) core; the [Co4O4](4+) cubane is thus bridging the four Co-II ions. Twelve PhCO(2)(-) groups bridge the Co-II.. Co-III pairs, and terminal MeCN/H2O groups complete the peripheral ligation at the five-coordinate, high-spin Co-II centers. The Co-III centers are low-spin. Excluding differences in terminal ligation (MeCN or H2O), the molecule has idealized Td symmetry. Variable-temperature, solid-state magnetic susceptibility studies on in have been carried out in the range 4.00-320 K.