Oxidation of Co2+ by diphenyl diselenide in the presence of cis-[PPN][Mn(CO)(4)(SePh)(2)], followed by carbonyl shift from Mn(I) to Co(III) and a benzeneselenolate group rearranging to bridge two metals, led directly to the thermally unstable (CO)(4)Mn(mu-SePh)(2)Co(CO)(mu-SePh)(3)Mn(CO)(3). Dropwise addition of [PPN][SePh] to the neutral (Co)(4)Mn(mu-SePh)(2)Co(CO)(mu-SePh)(3)Mn(CO)(3) resulted in formation of a linear trinuclear complex possessing a hexaselenolatecobalt(III) core, [PPN] [(CO)(3)Mn(mu-SePh)(3)Co(mu-SePh)(3)Mn(CO)(3)]. This complex crystallized in the triclinic space group with a = 10.878(1) Angstrom, b = 15.095(2) Angstrom, c = 25.372(4) Angstrom, alpha = 95.04(1)degrees, beta = 95.00(1)degrees, gamma = 91.52(2)degrees, V = 4132(2) Angstrom(3), and Z = 2; final R = 0.042 and R(W) = 0.042. In contrast, the thermally unstable cis-[PPN][Mn(CO)(4)(SeMe)(2)], which was reacted with Co(ClO4)(2) . 6H(2)O and (MeSe)(2) in THF under a nitrogen atmosphere, led to the isolation of the stable heterometallic selenolate (CO)(4)Mn(mu-SeMe)(2)Co(CO)(mu-SeMe)(3)Mn(CO)(3). Crystal data : monoclinic space group C2/c, a = 28.413(7) Angstrom, b = 11.091(3) Angstrom, c = 22.849(6) Angstrom, beta = 125.06(3)degrees, V = 5894(3) Angstrom(3), and Z = 8; final R = 0.047 and R(W) = 0.048. The results indicated that the distinct electronic effects between methaneselenolate and benzeneselenolate play a key role in stabilizing the neutral Mn(I)-Co(III)-Mn(I)-selenolate complexes.