Reactivity of the lower phosphorus oxyanions toward oxidation by bromine: Reactions of free and Co(III)-coordinated hypophosphite and correlations with oxyanion basicity

Kinetic studies of acid-catalysed H/D-exchange at the phosphorus centre in [Co(NH3)(5)OP(H)(2)O](2+) (1) in D2O (k(ex) = 2 x 10(-4) M-1 s(-1), 25 degr eesC, I = 1.0 M NaClO4) shows that 1 exchanges approximately 20-fold less r apidly than does DH2PO2. Bromine oxidation of 1 in H2O (25 degreesC, 0.20, 0.40 M [H+], I = 1.0 M NaClO4) follows the rate law: k(obs) = (k(Br2) + kBr (3)KBr(3)[Br-] [1]/(1 + K-Br3[B-]), with KBr3 = 17.6 M-1 ( [Br-3(-)]/[Br-2] [Br-], and with K-Br2 ((1.03 +/- 0.05) x 10(-2) M-1 s(-1)) and k(Br3) ((1. 4 +/- 0.2) x 10(-3) M-1 s(-1)) respectively representing the specific rate constants for reaction of Br-2 and Br-3(-) with 1. Parallel studies with Na H2PO2 (25 degreesC, I = 1.0 M, NaClO4) show that oxidation by bromine follo ws the rate law: k(obs) = k(Br2)K(a)[hypophosphite](T)/(K-a + [H+])(1 + K-B r3[Br-]) with [hypophosphite](T) = [H2PO2-] + [H3PO2], and with K-a (0.16 /- 0.01) and k(Br2) (8.0 +/- 0.1 M-1 s(-1)) respectively representing the a cidity constant of H3PO2 and the specific rate constant for reaction of Br- 2 With H2PO2- The corresponding deuterated substrate, D2PO2- has k(Br2) = 3 .0 +/- 0.3 M-1 s(-1) for the same condition, thus k(H)/k(D) = 2.7. Rates of bromine oxidation of phosphito and hypophosphito species, extending over a reactivity range of 10(7), are shown to be in one to one correspondence wi th phosphorus oxyanion basicity and a common reaction mechanism is postulat ed. This involves formation of a cyclic 1:1 transition state complex, havin g both P . . .H . . .Bdelta-. . . Brdelta+ and P-O . . . Brdelta+. . . Brde lta- character.