The hydroperoxocobalt complex [L-2(CH3CN)CoOOH](ClO4)(2). CH3CN (L-2 = meso-5,7,7,12,14,14-Me-6-[14]aneN(4)) crystallizes with discrete anions, cations, and solvate acetonitrile molecules in the lattice. The complex crystallizes in the monoclinic space group P2(1)/n, a = 10.4230(5) Angstrom, b = 16.1561(8) Angstrom, c = 17.4676(9) Angstrom, beta = 92.267(1)degrees, V = 2939.2(3) Angstrom (3), Z = 4. The O-O bond length is 1.397(4) Angstrom, and the O(2)-O(1)-Co angle spans 117.7 degrees. The O-O stretch in the infrared spectrum appears at 815 cm(-1). The 355- and 266-nm photolysis of acidic aqueous solutions of L-2(H2O)CoOOH2+ results in homolytic splitting of the Co-O bond and yields L2Co(H2O)(2)(2+) and HO2./O-2(.-) as the only products. The two fragments were scavenged selectively in separate experiments with Oz and C(NO2)(4). There is no evidence for photochemical O-O bond homolysis, presumably because the appropriate optical transition is masked by the HO2-to-Co LMCT transition.