Iridium complex anions (IrX62-, IrX5-, IrX4-, K+.IrX62- X = Cl, Br, and K+.IrCl52-) were subjected to high-energy collisions with Cs atoms in an accelerator mass spectrometer. The experiments probed the complex anions' propensity for electron attachment, which was only observed for IrCl5-, IrBr5- and K+.IrCl52-. These observations indicate that electron transfer is controlled by the relative stability of the Ir oxidation state and the availability of low-lying unoccupied molecular orbitals. Formation of IrX52- is notable since these ions are amongst the smallest molecular dianions observed in the gas-phase, while the K+.IrCl53- complex contains the smallest experimentally observed trianion. (c) 2007 Elsevier B.V. All rights reserved.