The differences in the reactivities of the square-planar complexes cis-[Rh(CO)(2)I-2](-) (1) and cis-[Ir(CO)(2)I-2](-) (2), involved in the catalytic carbonylation of olefins, are investigated, with P(C6H5)(4)(+) as the counterion, by ambient-and high-pressure NMR and IR spectroscopy. Under an elevated pressure of CO, 1 and 2 form the [M(CO)(3)I] complexes with the equilibrium constants K-Ir approximate to 1.8 x 10(-3) and K-Rh approximate to 4 X 10(-5). The ratio K-Ir/K-Rh close to 50 shows that, under catalytic conditions (a few megapascals), only complex 1 remains in the anionic form, while a major amount of the iridium analogue 2 is converted to a neutral species. The oxidative addition reactions of HI with 1 and 2 give two monohydrides of different geometries, mer,trans-[HRh(CO)(2)I-3](-) (3) and fac,cis-[HIr-(CO)(2)I-3](-) (4), respectively. Both hydrides are unstable at ambient temperature and form, within minutes for Rh and within hours for Ir, the corresponding cis-[M(CO)(2)I-2](-) (1 or 2) and [M(CO)(2)O-4](-) (5 or 6) species and H-2 When an H-2 pressure of 5.5 MPa is applied to a nitromethane solution of complex 2, ca. 50% of 2 is transformed to cis-dihydride complexes. The formation of cis,cis,cis-[IrH2(CO)(2)I-2](-) (8a) is followed by intermolecular rearrangements to form cis,trans,cis-[IrH2(CO)(2)I-2](-) (8b) and cis,cis,trans-[IrH2(CO)(2)I-2](-) (8c). A Small amount of a dinuclear species, [Ir2H(CO)(4)I-4](x-) (9), is also observed. The formation rate constants for 8a and 8b at 262 K are k(1)(262) = (4.42 +/- 0.18) x 10(-4) M-1 s(-1), k(-1)(262) = (1.49 +/- 0.07) x 10(-4) s(-1), k(2)(262) = (2.81 +/- 0.04) X 10(-5) s(-1) land k(-2)(262) = (5.47 +/-0.16) x 10(-6) s(-1). The two equilibrium constants K-1(262) = [8a]/([2][H-2]) = 2.97 +/- 0.03 M-1 and K-2(262) = [8b]/[8a] = 5.13 +/- 0.10 show that complex 8b is the thermodynamically stable addition product. However, no similar H-2 addition products of the rhodium analogue 1 are observed. The pressurization with Hz of a solution containing 2 and 6 give the monohydride 4, the dihydrides 8a and 8b, the dinuclear complex 9, and the two new complexes [Ir(CO)(2)I-3] (10) and [HIr(CO)(2)I-2] (11). The reactions of the iridium complexes with H2 and HI are summarized in a single scheme.