This work reports the results of a thermally driven cluster expansion of arachno-1-{η5-C5Me5IrH2}B 3H7, 1, with BH3•THF. In addition to the previously reported product, arachno-1-{η5-C5Me 5IrH}B4H9, 2, formed at lower temperatures, reaction at 100 °C permits the isolation of four new iridaboranes. Two products, nido-1-(η5-C5Me5Ir)B 5H9, 3, and nido-3-(η5-C5Me 5Ir)B9H13, 4, contain a single Ir atom and five and nine framework boron atoms, respectively. One, nido-3,4-(η5- C5Me5Ir)2B8H12, 5, contains two Ir atoms and eight framework boron atoms. Their structures are predicted by the electron counting rules to be a nido-iridahexaborane, 3, nido-iridadecaborane, 4, and nido-diiridadecaborane, 5. The accuracy of these predictions in each case is established experimentally by spectroscopic characterization in solution and structure determinations in the solid state. A less stable metallaborane has been identified and the available spectroscopic and crystallographic information are consistent with the formulation nido-3,4-(η5-C5Me5Ir)2B 8H13(μ-BH2), 6, i.e., a species containing an exopolyhedral bridging BH group. These new observations, along with earlier ones on ruthenaborane cluster systems, are used to fully define a general mechanism for a cluster expansion reaction, i.e., addition of borane to form an exopolyhedral adduct followed by cage insertion. © The Royal Society of Chemistry.