The reactions of the anionic cluster [NEt₄][Ir₄(CO)₁₁Br] with uni- and bi-dentate phosphines and arsines have been investigated. The bromide ligand is quantitatively displaced by 1 mol equivalent of phosphine or arsine at low temperature, the only complexes being formed under these mild conditions being the monosubstituted products [Ir₄(CO)₁₁L](L = PPh₃, PPh₂Me, PPhMe₂, or AsPh₃), [Ir₄(CO)₁₁(L–L)](L–L =trans-Ph₂PCHCHPPh₂), and [(OC)₁₁Ir₄(L–L)Ir₄(CO)₁₁][L–L =trans-Ph₂PCHCHPPh₂ or Ph₂P(CH₂)_nPPh₂(n= 3 or 4)]. Similar reactions with higher than stoicheiometric amounts of phosphine (L = PPh₃, PPh₂Me, or PPhMe₂) or diphosphine [L–L = Ph₂P(CH₂)_nPPh₂(n= 1–4), Me₂P(CH₂)₂PMe₂, cis-Ph₂PCHCHPPh₂, or o-Ph₂PCH₂C₆H₄CH₂PPh₂] give in good yields the disubstituted compounds [Ir₄(CO)₁₀(L–L)] respectively. The stereochemical arrangements of the phosphine ligands and the dynamic processes occurring in solutions of these complexes are discussed on the basis of i.r. and n.m.r, data. The structure of the diaxial isomer of [Ir₄(CO)₇(µ-CO)₃(Me₂PCH₂CH₂PMe₂)] has been determined by X-ray diffraction. The complex crystallizes in the monoclinic space group P2₁/c, with cell constants a= 15.694(2), b= 10.403(2), c= 15.706(2)Å, β= 92.63(2)°, and Z= 4. The structure has been solved from 2 289 diffraction intensities collected by counter methods, and refined by least-squares calculations to R= 0.087 (R′= 0.091). The four iridium atoms define a tetrahedron with three bridging CO ligands around a triangular face. All remaining carbonyls are terminally bonded and the two P atoms of the Me₂PCH₂CH₂PMe₂ ligand are found in axial positions, generating a six-membered ring.