The reactivity of [HMCo3(CO)(12)] and [Et4N][MCo3(CO)(12)] (M = Fe, Ru) toward phosphine selenides such as Ph3PSe, Ph2P(Se)CH2PPh2, Ph-2(2-C5H4N)PSe, Ph-2(2-C4H3S)PSe, and Ph-2{(2-C5H4N)(2-C4H2S)}PSe has been studied with the aim to obtain new selenido-carbonyl bimetallic clusters. The reactions of the hydrido clusters give two main classes of products: (i) triangular clusters with a mu(3)-Se capping ligand of the type [MCo2(mu(3)-Se)(CO)(9-x)L-y] resulting from the selenium transfer (x = y = 1, 2, with L = monodentate ligand; x = 2, 4, and y = 1, 2, with L = bidentate ligand) (M = Fe, Ru) and (ii) tetranuclear clusters of the type [HMCo3(CO)(12-x)L-y] obtained by simple substitution of axial, Co-bound carbonyl groups by the deselenized phosphine ligand. The crystal structures of [HRuCo3(CO)(7)(mu-CO)(3)(mu-dppy)] (1), [MCo2(mu(3)-Se)(CO)(7)(mu-dppy)] (M = Fe (16) or Ru (2)), and [RuCo2(mu(3)-Se)-(CO)(7)(mu-dppm)] (12) are reported [dppy = Ph-2(2-C5H4N)P, dppm = Ph2PCH2PPh2]. Clusters 2, 12, and 16 are the first examples of trinuclear bimetallic selenido clusters substituted by phosphines. Their core consists of metal triangles capped by a mu(3)-selenium atom with the bidentate ligand bridging two metals in equatorial positions. The core of cluster 1 consists of a RuCo3 tetrahedron, each Co-Co bond being bridged by a carbonyl group and one further bridged by a dppy ligand. The coordination of dppy in a pseudoaxial position causes the migration of the hydride ligand to the Ru(mu-H)Co, edge. In contrast to the reactions of the hydrido clusters, those with the anionic clusters [MCo3(CO)(12)](-) do not lead to Se transfer from phosphorus to the cluster but only to CO substitution by the deselenized phosphine.