In the presence of Cl2, [Ir(NO)Cl5]- can be extracted by TBP from 6 mol/L HCl solution, the distribution coefficient being 332. After storage in <5 mol/L HCl solns., the extration percentage decreased and [Ir(NO)Cl5]- was partially or completely converted to [Ir(NO2)Cl5]3- and its hydrate. The extent of decrease depended upon the duration of storage and the concentration of HCl. [Ir(NO2)Cl5]3- and its hydrate can be converted back to [Ir(NO)Cl5]- by adjusting the solns. to 9.5 mol/L HCl and standing for 2 h.

Key words: REACTION MECHANISM, HYDROCHLORIC ACID, PHOSPHORIC ACID TRIBUTYL ESTER, EXTRACTION, IRIDIUM, EXTRACTION SYSTEM, HYDROMETALLURGY, ACID RADICAL, NITROSYL COMPOUNDS, IRIDIUM COMPOUNDS