The electrochemical properties of the ions [Ir(CO)(2)X-2](-) (X=Cl, Br, and 1) have been studied in dichloromethane solutions using cyclic voltammetry, chronoamperometry, electrochemical quartz crystal microbalance (EQCM), X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and infrared spectroscopy. For the chloro and bromo salts, the anion [Ir(CO)(2)X-2](-) is oxidized initially to form [Ir(CO)(2)X-2](+). The standard rate constants for the two-electron oxidations of [Ir(CO)(2)X-2](-) are 7.8(+/- 0.6) x 10(-3) and 9.2(+/-0.9) x 10(-4) cm s(-1) for the redox couples, [Ir(CO)(2)Cl-2](-/+) and [Ir(CO)(2)Br-2](-/+), respectively. The processes following electrolysis lead to the formation of two types of crystalline deposits on the electrode Surface: needles and plates. The relative amounts of these solid phases that form depend mainly on the concentration of iridium complex in solution and on the time window of experiment. The strong intermetallic Ir-Ir interaction is responsible for the formation of the one-dimensional iridium complex chain. The crystal structures of the needle phases formed from [Ir(CO)(2)Cl-2](-) and [Ir(CO)(2)Br-2](-) are the same and belong to the space group CMCM (no. 63). The stoichiometry of the one-dimensional crystals depends on the constitution of the Supporting electrolyte: (TAA)(0.6)[Ir(CO)(2)Cl-2] and (TAA)(0.7)[Ir(CO)(2) Br-2] (TAA is tetra(alkyl)ammonium cation) salts are formed on the electrode surface. The formation of large three-dimensional crystal is responsible for the accumulation of electroactive materials on the electrode Surface. The irreversible oxidation of [Ir(CO)(2)I-2](-) leads only to the formation of large, plate-like crystals on the electrode surface, no needles are formed. (c) 2008 Elsevier Ltd. All rights reserved.