[(NH3)(5)CoOP(H)(O)OCo(NH3)(5)](4+) and its conjugate acid; evidence for the P+-O- description of a terminal P-O bond

Bromine oxidation of the phosphite-bridged dimer [(NH3)(5)CoOP(H)(O)OCo(NH3 )(5)](4+) (1) in acidified aqueous solution (25 degrees C, I = 1.125 M, NaC lO4) gives the corresponding phosphato complex [(NH3)(5)CoOP(OH)(O)OCo(NH3) (5)](4+). The reaction follows the rate law k(obs) = kK(a)[dimer](T)/(K-a [H+])(1 + K-BR3[Br-]), with K-Br3(= [Br-3(-)]/[Br-2][Br-]) = 17.6 M-1, and with K-a (0.074 +/- 0.006 M) corresponding to the acidity constant of the acid conjugate [(NH3)(5)CoOP(H)(OH)OCo(NH3)(5)](5+) (2), and k (1.00 +/- 0. 05 M-1 s(-1)) to the rate constant for oxidation of 1. The reaction is inte rpreted as involving attack by free Br-2 on the O-atom of the terminal P-O group in 1 and rate-determining P-H bond breaking. The acidity constant of 2 was confirmed by independent spectrophotometric determination, K-a= 0.09 +/- 0.01 M, 25 degrees C, I= 1.0 M. Crystal structures are reported for bot h 1(ClO4)(4). 2H(2)O and 2(ClO4)(5). C2H5OH. The available evidence suggest s that the terminal P-O bond in 1 is best represented as a limiting case of the Pdelta+-Odelta- description. (C) 1999 Elsevier Science S.A. All rights reserved.