Iridium hydridophosphine complexes of general formula [Ir(PR₃)₂H₂(anion)] (PR₃ = PPh₃, PMe₂Ph; anion = [1-closo-CB₁₁H₆Cl₆]⁻, [1-closo-CB₁₁H₆I₆]⁻, [BAr^F₄]⁻) have been prepared by hydrogenation of cyclooctadiene precursor complexes. Solid-state structures of selected examples of these complexes reveal intimate contacts between the carborane anion and cation, with the anion binding through two lower-hemisphere halogen ligands. In CD₂Cl₂ solution the very weakly coordinating anions [1-closo-CB₁₁H₆Cl₆]⁻ and [BAr^F₄]⁻ are suggested to favour the formation of solvent complexes such as [Ir(PR₃)₂H₂(solvent)_n][anion], while the [1-closo-CB₁₁H₆I₆]⁻ anion forms a tightly bound complex with the cationic iridium fragment. Calculated ΔG‡ values for anion reorganisation in d₈-toluene reflect this difference in interaction between the anions and cation. With the bulky anion [1-closo-CB₁₁Me₅I₆]⁻ different complexes are formed: Ir(PPh₃)H₂(1-closo-HCB₁₁Me₅I₆) and [(PPh₃)₃Ir(H₂)H₂][1-closo-HCB₁₁Me₅I₆] which have been characterised spectroscopically. Diffusion measurements in CD₂Cl₂ are also consistent with larger, solvent coordinated, complexes for the more weakly coordinating anions and a tighter interaction between anion and cation for [1-closo-CB₁₁H₆I₆]⁻. All the complexes show some ion-paring in solution. Comparison with data previously reported for the [1-closo-CB₁₁H₆Br₆]⁻ anion shows that this anion—as expected—fits between [1-closo-CB₁₁H₆Cl₆]⁻ and [1-closo-CB₁₁H₆I₆]⁻ in terms of coordinating ability. Although not coordinating, the large [1-closo-CB₁₁H₆Cl₆]⁻ and [BAr^F₄]⁻ anions do provide some stabilisation towards the metal centre, as decomposition to the hydride bridged dimer [Ir₂(PPh₃)₄H₅]⁺ is retarded. This is in contrast to the [PF₆]⁻ salt where decomposition is immediate. As expected, complexes with the smaller phosphine PMe₂Ph form tighter interactions with the carborane anions. These observations on the interaction between anion and cation in solution are reflected in benchmark hydrogenation studies that show a significant attenuation in rate of hydrogenation of cyclohexane on using the [1-closo-CB₁₁H₆I₆]⁻ anion or complexes with the PMe₂Ph phosphine. We also comment on the reusability of the catalysts and their tolerance to water and oxygen impurities. Overall the catalyst with the [1-closo-CB₁₁H₆Br₆]⁻ anion shows the best combination of rate of hydrogenation, reusability and tolerance to impurities.