Reaction of the cyclic organophosphorus ligand P₄Bu^t₄1 with [Ru₃(CO)₁₂] or [Ru₄H₄(CO)₁₂] under forcing conditions yields a number of novel phosphido- and phosphinidene-substituted ruthenium carbonyl clusters. Three products have been structurally characterised in reasonable yield: [Ru₄(CO)₈(PBu^t)₄]2, where a square-planar arrangement of four ruthenium atoms is capped above and below the metal-atom plane by two tert-butylphosphinidene ligands, and additionally two µ-phosphinidene ligands bridge two Ru–Ru edges; [Ru₅(CO)₁₅(PBu^t)]3 in which the five ruthenium atoms adopt a square-pyramidal geometry with the square-basal plane capped by a tert-butylphosphinidene ligand; and [Ru₆H₂(CO)₁₂(PBu^t)₃]4, where the six ruthenium atoms define an octahedron with triply-bridging phosphinidene groups capping three triangular faces. Clusters 2 and 4 were isolated from the thermolysis of compound 1 with the tetraruthenium substrate [Ru₄H₄(CO)₁₂]; cluster 3 is a product of the thermolysis of 1 with [Ru₃(CO)₁₂]. In addition, the cluster [Ru₄(µ₄-Se)₂(CO)₈(µ-CO)₃]5 has been isolated by the vacuum pyrolysis of [Ru₃(CO)₁₂] with Ph₂Se₂ at 185 °C. Reaction of this cluster with 1,3-bis(diphenylphosphino)propane (dppp) at room temperature affords the triruthenium phosphine-substituted cluster [Ru₃(µ₃-Se)₂(CO)₇(dppp)]6. The structures of clusters 5 and 6 have also been established by single crystal X-ray analysis.