The structural and spectroscopic properties of a series of tetranuclear chromium-oxo complexes are reported. One example, [Cr4O2(O(2)CMe)(7)(phen)(2)]Cl.1.5CH(2)Cl(2).6H(2)O (1) crystallizes in the orthorhombic space group C222(1) with a = 18.652(4) Angstrom, b = 25.200(7) Angstrom, c = 12.559(3) Angstrom, V = 5903.23 Angstrom(3), and Z = 4. The structure was refined with 2043 reflections having F > 3.0 sigma(F), giving final R factors of 0.0609 and 0.0623 for R and R(w), respectively. The [Cr4O2](8+) core consists of a "butterfly" arrangement and is structurally akin to other recently reported [M(4)O(2)](8+) (M = Fe, Mn) complexes. These complexes were synthesized by heating a PhCN solution of a bidentate ligand and a trinuclear complex of the composition [Cr3O(O(2)CR)(6)L(3)](n+). A combination of H-1 and H-2 NMR studies allows for assignment of all proton (deuteron) resonances. A pi-spin delocalization mechanism is the dominant contributor to the H-1 and H-2 chemical shifts. Fast atom bombardment mass spectrometry has been shown to be a valuable tool in the characterization of the chromium-oxo assemblies. The results of electronic and room temperature magnetic studies and the biological relevance of these materials is discussed.