Reactions of µ-hydroxo-dicobalt complexes. Part II. The kinetics of the µ-amido-µ-hydroxo-bis[tetra-amminecobalt(III)] complex with thiocyanate in aqueous perchloric acid and the characterization of the bisthiocyanate product

Abstract

The thiocyanate, nitrate, and perchlorate salts of the µ-amidobis[thiocyanatotetra-amminecobalt(III)] complex (NH₃)₄(NCS)Co·NH₂·Co(NCS)(NH₃)₄³⁺ have been prepared and characterized by analyses, ion-exchange, pH, and i.r. techniques. The kinetics of the formation of the complex from the µ-amido-µ-hydroxo-bis[tetra-amminecobalt(III)] complex in perchloric acid solutions, µ= 2·0M(NaClO₄), have been studied. A self-consistent interpretation is possible if the µ-amido-[aquotetra-amminecobalt(III)][thiocyanatotetra-amminecobalt(III)] complex which is first formed (k=ca. 2 × 10^–3 l. mole^–1 sec.^–1 at 25°) reacts to give the µ-amido-µ-thiocyanatobis[tetra-amminecobalt(III)] complex in the first stage. It is suggested tentatively that the nitrogen of the thiocyanate is co-ordinated to both cobalt atoms. Protonation (K_p= 0·38 ± 0·04 l. mole^–1) occurs before reaction with the second thiocyanate. At 25° the second-order rate constant for the reaction of the protonated form with thiocyanate is k= 1·22 ± 0·16 × 10^–3 l. mole^–1 sec.^–1.