The relative nucleophilic efficiency of water and the thiocyanate ion in the acid-catalysed substitution of azidopentacyanocobaltate(III)

Abstract

The acid-catalysed substitution of [Co(CN)₅(N₃)]^3– by NCS^– in water yields [Co(CN)₅(OH₂)]^2–, [Co(CN)₅(SCN)]^3–, and [Co(CN)₅(NCS)]^3–. Spectroscopic and high-performance liquid chromatographic analytical data are quantitatively consistent with an acid-catalysed dissociative mechanism [equations (1)–(5)], in which K₁= 4.47 ± 0.22 dm³ mol^–1, k₂=(3.46 ± 0.17)× 10³ s¹, [Co(CN)₅(N₃)]^3–+ H₃O⁺ [graphic omitted] [Co(CN)₅(N₃H)]^2–+ H₂O (1)[Co(CN)₅(N₃H)]^2– [graphic omitted] [Co(CN)₅]^2–+ HN₃(2)[Co(CN)₅(OH₂)]^2– [graphic omitted] [Co(CN)₅]^2–+ H₂O (3)[Co(CN)₅]^2–+ H₂O [graphic omitted] [Co(CN)₅(OH₂)]^2–(4)[Co(CN)₅]^2–+ NCS [graphic omitted] [Co(CN)₅(SCN)]^3–(5a)[Co(CN)₅]^2–+ NCS [graphic omitted] [Co(CN)₅(NCS)]^3–(5b)K₃= 6.07 × 10⁴ s¹, (K_5a+K_5b)/K₄[H₂O]= 0.14 ± 0.02 dm³ mol^–1, and K₅/K_5b≈ 4 at 40 °C and unit ionic strength. Equilibrium spectroscopic measurements of K₁(4.67 ± 0.09 dm³ mol¹) agree with the fitted kinetic result.