Reactivity of a Regiospecific Ion Pair. Comparisons of Cl^- and H₂O Entry into the Five-Coordinate Intermediate Generated from p-[Co(tren)(NH₃)OH₂]³⁺·Cl^-

Equilibration in the system :  p-[Co(tren)(NH₃)OH₂]³⁺ + Cl^- ⇄ p-[Co(tren)(NH₃)Cl]²⁺ + H₂O (p-stereochemistry has the labile group (OH₂, Cl^-) trans to a tren NH₂) in aqueous solution of variable (I = 0−2.0 M) and constant (I = 2.0 M, NaClO₄) ionic strength at 37.5 °C is described by the following rate law:  k_obs = k_aq + k_anK_Cl[Cl^-]/(1 + K_Cl[Cl^-] + K_ClO4[ClO₄^-]); K_Cl (0.64 M^-1) and K_ClO4 (0.21 M^-1) correspond to ion pairing of Cl^- and ClO₄^- to the aqua complex and k_aq (1.4 × 10^-5 s^-1) and k_an (1.2 × 10^-4 s^-1) to the rate constants for aquation of the chloro complex and anation of the ion-paired (Cl^-) aqua complex respectively. The rate constant for loss of coordinated ¹⁷OH₂ from the labeled aqua complex is the same (k_OH2 ∼1.2 × 10^-4 s^-1) irrespective of the presence (2.0 M) or absence of NaClO₄ ([H⁺] = 0.10 M, 37.5 °C) but increases with increasing [Cl^-] to reach a limit (∼2.4 × 10^-4 s^-1) at high concentrations (range 0−1.90 M; I = 2.0 M, NaClO₄). Water exchange in the p-[Co(tren)(NH₃)OH₂]³⁺·Cl^- ion pair (k‘_OH2 = 1.2 × 10^-4 s^-1) occurs just as readily as anation. A mechanism for anation is suggested whereby entry of Cl^- ion and OH₂ molecules from positions adjacent to the p-site affects reentry of the original OH₂ ligand such that all processes are partly rate determining. A combination of 1D nOe and 2D COSY spectroscopy (DMSO-d₆ solvent) has allowed all the H atoms in p-[Co(tren)(NH₃)OH₂]³⁺ and p-[Co(tren)(NH₃)Cl]²⁺ to be assigned. Added Cl^- ion strongly influences the chemical shifts of the OH₂, NH₃, and NH hydrogens in the aqua complex (in decreasing order of importance) and appears to associate more strongly than ClO₄^-. Anation in DMSO occurs without the intermediacy of the DMSO complex. The crystal structure of p-[Co(tren)(NH₃)OH₂]Cl₃, orthorhombic, Pmn2₁, a = 11.689(6) Å, b = 8.215(3) Å, c = 6.878(2) Å, Z = 2, and R = 0.0490 shows one unique Cl^- ion (Cl(1)) H-bonded to the OH₂ and NH₃ ligands of the complex cation (distances 2.89 and 3.96 Å respectively). Heating the crystalline solid results in rapid Cl^- entry.