Equilibration in the system : p-[Co(tren)(NH3)OH2](3+) + Cl- reversible arrow 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 degrees C is described by the following rate law : k(obs) = k(aq) + k(an)K(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 x 10(-5) s(-1)) and k(an) (1.2 x 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 (OH2)-O-17 from the labeled aqua complex is the same (k(OH2) similar to 1.2 x 10(-4) s(-1)) irrespective of the presence (2.0 M) or absence of NaClO4 ([H+] = 0.10 M, 37.5 degrees C) but increases with increasing [Cl-] to reach a limit (similar to 2.4 x 10(-4) s(-1)) at high concentrations (range 0-1.90 M; I = 2.0 M, NaClO4). Water exchange in the p-[Co(tren)(hTH(3))OH2](3+). Cl- ion pair (K-OH2 = 1.2 x 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-sire 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-d(6) 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]Cl-3, orthorhombic, Pmn2(1), a = 11.689(6) Angstrom, b = 8.215(3) Angstrom, c = 6.878(2) Angstrom, 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 Angstrom respectively). Heating the crystalline solid results in rapid Cl- entry.