Phosphato complexes of cobalt(III). III. Hydrolysis reactions in perchloric acid media

Abstract

Rates of hydrolysis of phosphato complexes of oobalt(111) in perchloric acid media ranging from 10^-3 to 11.4M have been measured by the rates of release of phosphate from ³²P-labelled phosphato complexes. First-order rate behaviour is exhibited under all conditions and half-times range from hours to minutes within the range 45-70º. 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. Rate constants have been measured for three distinct protonated complexes (PI, P2, P3). At 60º these rate constants, expressed in the order of aquo-tetraammine, -penhammine, and ?bisethylenediamine complexes, are PI: 74.0, 1.20, 5.5 X 10^-3 min^-1; P2: 2.5, 1.20, 0.40 x 10^-3 min^-1; P3: 20.0, 9.0, 5.5 x 10^-3 min^-1. A tetraprotonated complex (P4) is incompletely generated even in 11.4M HClO₄ where observed rate constants at 60º are 8.2, 3.4, 7.0 x 10^-2 min^-1 respectively. Intramolecular hydrogen bonding is considered important in the P1 and P2 species, especially in the cis-aquophosphato complexes. The rate of hydrolysis of P3 complexes is linearly dependent on the activity of the water solvent consistent with a mechanism involving S_N2 attack of water at a cobalt(111) centre. The P4 complex is considered to hydrolyse via an S_NICA mechanism.