Electron transfer reactions between penta-ammine(nitrilotriacetato)-cobalt(III) and chromium(II) and between acetatopenta-amminecobalt(III) and N-methyliminodiacetato-chromium(II)

Abstract

The complex [Co(NH₃)₅(H₂nta)]²⁺(nta^3–= nitrilotriacetate) reacts with Cr^II with the rate law –d[Co^III]/dt=k′[Co(NH₃)₅(H₂nta)²⁺][Cr²⁺]a_H^–2 with k′= 2.45 mol dm^–3 s^–1 at 25 °C and ionic strength 1.0 mol dm^–3(Na[ClO₄]). A binuclear intermediate [Co(NH₃)₅(nta)Cr]²⁺ is proposed and the specific rate of internal electron transfer is estimated to be ca. 600 s^–1. The rate of reaction of [Co(NH₃)₅(O₂CMe)]²⁺ with Cr^II is enhanced by the presence of N-methyliminodiacetic acid (H₂mida). The rate law is –d[Co^III]/dt=k₂′[Co(NH₃)₅(O₂CMe)²⁺]-[Cr²⁺][HL^–]a_H^–1 with k₂′= 0.24 dm³ mol^–1 s^–1 at 25 °C, and ionic strength 1.0 mol dm^–3(Na[ClO₄]). The specific rate for the bimolecular reaction [Co(NH₃)₅(O₂CMe)]²⁺+[Cr(mida)] is estimated to be 18 dm³ mol^–1 s^–1. The results are discussed in relation to the analogous reactions involving Fe^II as reductant.