Reaction of the arachno-[B₉H₁₄]^– or nido-[B₉H₁₂]^– anions with [IrCl(PPh₃)₃] at ca. 298 K gives, in addition to previously reported species, pale violet [7,7,9-(PPh₃)₃-isonido-7-IrB₉H₁₀]2 in yields of ca. 2%. Single crystal X-ray diffraction analysis and multinuclear NMR spectroscopy reveal a ten-vertex {IrB₉} cluster of an ‘isonido’ type that is based upon the closo eleven-vertex structure of [B₁₁H₁₁]^2– from which a four-connected vertex is removed to generate a four-membered open face. There are two fluxional bridging hydrogen atoms associated with the open face. Thermolysis at ca. 355 K of solutions in 1,1,2,2-tetrachloroethane of yellow [5,7-(PPh₃)₂-5-H-5-(o-Ph₂P[graphic omitted]]4 result in loss of hydrogen and the formation of deep purple [8-Cl-7,9-(PPh₃)₂-7-(o-Ph₂P[graphic omitted]0]5(0–5%) and royal blue [7,9-(PPh₃)₂-7-(o-Ph₂P[graphic omitted]0]6(0–5%). Single-crystal X-ray diffraction shows that compounds 5 and 6 also have isonido cluster structures, although 5 does not have bridging hydrogen atoms on the open face, suggesting that any extra electrons required for it to be isoelectronic with 2 and 6 may originate from the metal vertex. The structure of the crystal of compound 6 examined was found to contain ca. 25 mole % of the isostructural 3-chloro derivative 6a. The ten-vertex clusters 2, 5 and 6 are part of a structural continuum ranging from closo through isonido to isocloso. It is proposed that these isonido compounds represent intermediates in a variety of reactions involving nido to isocloso cluster oxidations and nido to nido rearrangements.