Phosphato complexes of cobalt(III). IV. Hydrolysis in basic solution

Abstract

The rates of hydrolysis of phosphato complexes of cobalt(111) in sodium hydroxide concentrations ranging from 0.02M to 0.37M, and at several ionic strengths, have been measured with a tracer technique. Bidentate phosphato complexes exhibit the same rates of hydrolysis as the corresponding monodentate complexes, due to a rapid conversion of the bidentate into the monodentate form. The general rate law for base hydrolysis of all the phosphato complexes is: d[PO³₄]/dt = {k_H2O + k_OH[OH^-]}[complex] At 60º and at unit ionic strength, the rate constants for the complexes cis-[Co(NH₃)₄OH.PO₄]^-, cis-[Co en₂OH.PO₄]^-, and [Co(NH₃)₅PO₄] respectively are: 10³k_H2O (min^-l) 85.0, 2.0, <1; and 10³k_OH (1. mole^-1 min^-l) 42.7, 12.0, 69.5. Mechanistic conclusions have been based on the measured enthalpies and entropies of activation and deuterium solvent isotope effects. For all complexes, kH₂O is identified with an aquation mechanism involving synchronous interchange of the phosphate and solvent water between the first and second coordination spheres of the complexes. In the case of the tetrammine and bis(ethylenediamine) complexes, kOH is identified with a process involving synchronous interchange of phosphate and hydroxide ion between the first and second coordination spheres of the complexes. In the case of the pentammine complex, an S_N2CB mechanism is considered to be more probable. A comparison with the base hydrolysis of halogen complexes of cobalt(111) is presented.