Oxygen exchange and glycinate ring opening in [Co(en)₂(gly)]²⁺

Abstract

Over the pH range 0-2 chelated glycine in [Co(en)₂(gly)²⁺ exchanges one of its oxygens according to the rate law k_obs = k₍₁₎,[H⁺] with k₍₁₎, (1.0+0.1) × 10^-5 mol^-1 dm³ s^-1. The second oxygen exchanges much more slowly with k₍₂₎ c. 7 × 10^-8 s^-1 in 0.1 mol dm^-3 acid. In alkaline solution, [OH^-] being 1.0-10^-3 mol dm^-3, the two oxygen atoms exchange according to the expression k_obs = k_(1),(2)[OH^-] with k₍₁₎ 3.0 × 10^-3mol^-1 dm³ s^-1 and k₍₂₎ c. 1.7 × 10^-6 mol^-1 dm³ s^-1 both at 25.0°C and ionic strength, μ 1.0 (NaClO₄). The latter process occurs with C-O bond fission to form cis-[Co(en)₂(gly)(OH)]⁺, with k_d 1.8 × 10^-5 mol^-1 dm³ s^-1, which subsequently undergoes a more rapid OH^--catalysed loss of glycine, with k'_d, 6 × 10^-4 mol^-1 dm³ s^-1, to form cobalt(III) and cobalt(II) products.