Oxidative addition of diorganyl diselenides to the coordinatively unsaturated, low-valent transition-metal-carbonyl fragment [Mn(CO)(5)](-) produced cis-[Mn(CO)(4)(SeR)(2)](-). The complex cis-[PPN][Mn(CO)(4)(SePh)(2)] crystallized in triclinic space group with a = 10.892(8) Angstrom, b = 10.992(7) Angstrom, c = 27.021(4) Angstrom, alpha = 101.93(4)degrees, beta = 89.79(5)degrees, gamma = 116.94(5)degrees, V = 2807(3) Angstrom(3), and Z = 2; final R = 0.085 and R(w) = 0.094 Thermolytic transformation of cis-[Mn(CO)(4)(SeMe)(2)](-) to [(CO)(3)Mn(mu-SeMe)(3)Mn(CO)(3)](-) was accomplished in high yield in THF at room temperature. Crystal data for [Na-18-crown-6-ether][(CO)(3)Mn(mu-SeMe)(3)Mn(CO)(3)] : trigonal space group , a = 13.533(3) Angstrom, c = 32.292(8) Angstrom, V = 5122(2) Angstrom(3), Z = 6, R = 0.042, R(w) = 0.041. Oxidation of Co2+ to Co3+ by diphenyl diselenide in the presence of chelating metallo ligands cis-[Mn(CO)(4)(SePh)(2)](-) and cis-[Mn(CO)(4)(TePh)(2)](-), followed by a bezenselenolate ligand rearranging to bridge two metals and a labile carbonyl shift from Mn to Co, led directly to [(CO)(4)Mn(mu-TePh)(2)Co(CO)(mu-SePh)(3)Mn(CO)(3)]. Crystal data : triclinic space group , a = 11.712(3) Angstrom, b = 12.197(3) Angstrom, c = 15.754(3) Angstrom, alpha = 83.56(2)degrees, beta = 76.13(2)degrees, gamma = 72.69(2)degrees, V = 2083.8(7) Angstrom(3), Z = 2, R = 0.040, R(w) = 0.040. Addition of fac-[Fe(CO)(3)(SePh)(3)](-) to fac-[Mn(CO)(3)(CH3-CN)(3)](+) resulted in formation of (CO)(3)Mn(mu-SePh)(3)Fe(CO)(3). This neutral heterometallic complex crystallized in monoclinic space group P2(1)/n with a = 8.707(2) Angstrom, b = 17.413(4) Angstrom, c = 17.541(4) Angstrom, beta = 99.72(2)degrees, V = 2621(2) Angstrom(3), and Z = 4; final R = 0.033 and R(w) = 0.030.