Kinetics and mechanism of anation reactions of [M(NH3)5(H2O)]3+ by H3POn/H2POn– Systems (M = CrIII, n= 3; M = CoIII, n= 2)

Abstract

The kinetics of anation of [Cr(NH₃)₅(H₂O)]³⁺ with H₃PO₃/H₂PO₃^– and of [Co(NH₃)₅(H₂O)]³⁺ with H₃PO₂/H₂PO₂^– has been studied at different temperatures and at I= 1.0 mol dm^–3(LiClO₄). The results are interpreted in terms of an I_a(near the I_a/I_d border) anation mechanism for the chromium complex and an I_d one for the cobalt complex. Comparison of the anation rate constants with those of other anating systems and the same transition-metal complexes confirms this assignment. A comparison of all the anation reactions [M(NH₃)₅(H₂O)]₃₊+ H₃PO_n/H₂PO_n^–(M = Co^III or Cr^III; n= 2–4) is made, and two unexpected facts are examined: (a) for M = Cr^III, first-order rate constants for the neutral (k₁) and anionic (k₂) species of the series H₃PO_n/H₂PO_n^–(n= 2–4) are the same (k₁=k₂); and (b) for M = Co^III, even though the H₂PO_n^– species are always reactive, the conjugated species, H₃PO_n, are not reactive, except for n= 2.