Aqueous solutions of group nine metal(119 (M = Co, Rh, Ir) complexes of tetra(3,5-disulfonatomesityl)porphyrin [(TMPS)M-III] form an equilibrium distribution of aquo and hydroxo complexes ([(TMPS)M-III(D2O)(2-n)(OD)(n)]((7+n)-)). Evaluation of acid dissociation constants for coordinated water show that the extent of proton dissociation from water increases regularly on moving down the group from R cobalt to iridium, which is consistent with the expected order of increasing metal-ligand bond strengths. Aqueous (D2O) solutions of [(TMPS)Ir-III(D2O)(7-) react with dihydrogen to form an iridium hydride complex ([(TMPS)Ir-D(D2O)](8-)) with an acid dissociation constant of 1.8(0.5) x 10(-12) (298 K), which is much smaller than the Rh D derivative (4.3 (0.4) x 10(-8)), reflecting a stronger Ir D bond. The iridium hydride complex adds with ethene and acetaldehyde to form organometallic derivatives [(TMPS)Ir-CH2CH2D(D2O)](8-) and [(TMPS)Ir-CH(OD)CH3(D2O)](8-). Only a six-coordinate carbonyl complex [(TMPS)Ir-D(CO)](8-) is observed for reaction of the Ir-D with CO (P-CO = 0.2-2.0 atm), which contrasts with the (TMPS)Rh-D analog which reacts with CO to produce an equilibrium with a rhodium formyl complex (RTMPS)Rh-CDO(D2O)]8(-)). Reactivity studies and equilibrium thermodynamic measurements were used to discuss the relative M X bond energetics (M = Rh, Ir; X = H, OH, and CH2-) and the thermodynamically favorable oxidative addition of water with the (TMPS)Ir(II) derivatives.