Kinetic studies show that the reaction of [TpIr(CO)(2)] (1, Tp = hydrotris(pyrazolyl)borate) with water to give [TpIr(CO2H)(CO)H] (2) is second order (k = 1.65 x 10(-4) dm(3) mol(-1) s(-1), 25 degrees C, MeCN) with activation parameters Delta H-double dagger = 46 +/- 2 kJ mol(-1) and Delta S-double dagger = -162 +/- 5 J K-1 mol(-1). A kinetic isotope effect of k(H2O)/k(D2O)) 1.40 at 20 degrees C indicates that O-H/D bond cleavage is involved in the rate-determining step. Despite being more electron rich than 1, [Tp(*)Ir(CO)(2)](1(*), Tp(*) = hydrotris(3,5-dimethylpyrazolyl)borate) reacts rapidly with adventitious water to give [Tp(*)Ir(CO2H)(CO)H] (2(*)). A proposed mechanism consistent with the relative reactivity of 1 and 1(*) involves initial protonation of Ir(I) followed by nucleophilic attack on a carbonyl ligand. An X-ray crystal structure of 2(*) shows dimer formation via pairwise H-bonding interactions of hydroxycarbonyl ligands (r(O center dot center dot center dot O) 2.65 angstrom). Complex 2(*) is thermally stable but ( like 2) is amphoteric, undergoing dehydroxylation with acid to give [Tp(*)Ir(CO)(2)H](+) (3(*)) and decarboxylation with OH- to give [Tp(*)Ir(CO)H-2] (4(*)). Complex 2 undergoes thermal decarboxylation above ca. 50 degrees C to give [TpIr(CO)H-2] (4) in a first-order process with activation parameters Delta H-double dagger = 115 +/- 4 kJ mol(-1) and Delta S-double dagger = 60 +/- 10 J K-1 mol(-1).