Reaction of trans-[IrCl(CO)(PPh₃)₂] with [NMe₄][nido-6-CB₉H₁₂] in solution resulted in the formation in low yields of two isomers (sym and asym) of [9-(CO)-9,9-(PPh₃)₂-9-H-arachno-9,6-IrCB₈H₁₂]. These new air-stable yellow compounds differ in the arrangement of the exopolyhedral {(CO)(PPh₃)₂H} ligand group on the metal atom. Single-crystal X-ray diffraction analysis on the asym isomer confirmed the molecular constitution. Crystals were monoclinic, space group Cc, with a= 1 112.7(2), b= 2 184.7(3), c= 1 553.2(2) pm, β= 94.06(1)°, and Z= 4; 3 265 observed data [I > 2.0σ(I)]; R= 0.0183, R′= 0.0187. The cluster structure is of arachno ten-vertex ‘boat’ configuration, with the {Ir(CO)(PPh₃)₂} and {CH₂ centres occupying the “prow” positions 9 and 6, and with the Ir–H hydrogen atom being endo over the cluster open face. N.m.r. spectroscopy permits assignment of the cluster ¹¹B and ¹H parameters and thence comparison with the equivalent parameters of the analogues [B₁₀H₁₄]^2–, [6-CB₉H₁₄]^–, 6,9-C₂B₈H₁₄, and [5-(η⁵-C₅H₅)-5,6,9-CoC₂B₇H₁₁], in order to assess the comparative effects of replacement of boron by carbon and metal centres in the basic ten-vertex arachno[B₁₀H₁₄]^2– cluster type. Preparative details for [arachno-6-CB₉H₁₄]^– are given.