Evidence for direct hydride ion transfer from phosphorus to manganese(VII)in the permanganate oxidation of hypophosphite ion

The reaction between permanganate ion and hypophosphorous acid, P(I), in ac id perchlorate medium exhibits P(I)/MnO4- stoichiometry of 2:1. and is firs t order in MnO4- and an order < 1 in [H3PO2]. Kinetic studies revealed inhi bition of reaction at high [H2PO2-]. 2H(2)PO(2)(-) + MnO4- + 4H(+) --> 2H(2)PO(3)(-) + Mn3+ + 2H(2)O A weak 1:1 complex is formed on the msec time scale which enhances the MnO4 - a spectrum in rapid scanning spectrophotometry( 10-100msec). The complex between MnO2- and H2PO2- is surprisingly stronger than that formed between MnO4- and H3PO2. The dependence of the rate on [H+] indicates that the self decomposition of [MnO4H3PO3](-) to the products in the rate determining st ep is also assisted by a proton in a simultaneous rate determining step. No evidence is round for the formation ol. a radical intermediate nor partici pation of Mn3+ ion in the rate determining step ar has been observed in the oxidation of oxalic of acid by MnO4- ion. The reaction exhibits a large pr imary deuterium-isotope effect (k(H)/k(D) = 4.3). The empirical rate law is found rate = k(0)[MnO4- ][P(I)]/1 + K-c[P(I)] where both k and K are complicated functions of [H+] and in ionization cons tant of H3PO2). From the kinetic data it has been deduced that MnO4- forms complexes with both H3PO2 and H2PO2- in which it abstracts a hydride ion or equivalent to form HPO2(meta-phosphorous acid) which hydrolyzes to form H3 PO3. The redox reactions are catalyzed by H+ ion. Results are compared with those suggesting hydride (H+) transfer from alkoxide and formulate ions to MnO4- and with other results suggesting 2-electron transfer in a chromate ester, HP(OH)(OCrO3)(2-), From a lone pair oil tautometric HP(OH)(2) to Cr( VI). The complexes [O2PH2(O2MnO2)](2-) are thought to be held together by P -H . . .O hydrogen bonds which provide a pathway for hydride ion transfer f rom H2PO2- at high pH and proton plus two electron transfer from H3PO2 at l ow pH.