CHARACTERIZATION OF OXOPEROXO(2,6-PYRIDINEDICARBOXYLATO)MOLYBDENUM(VI) AND OXOPEROXO(NITRILOTRIACETATO)MOLYBDATE(VI) IN AQUEOUS-SOLUTION AND A KINETIC-STUDY OF THEIR REDUCTION BY A (THIOLATO)COBALT(III) COMPLEX, DIMETHYL-SULFOXIDE, AND IRON(II)

Formation constants have been determined for the complexes MoO(O-O)(di pic) and MoO(O-O)(nta)- at 25-degrees-C with I = 0.10 M (NaCl). For th e general formula (MoO4)(L)(H+)4(H2O2).log K(f) values of 23.48 and 23 .0 were determined for L = dipic by potentiometry and spectrophotometr y, respectively; for L = nta, the corresponding log K(f) values were 2 7.87 and 28.8. A single-crystal X-ray structure was determined for the nta complex, and Mo-95 NMR spectra were collected. The dipic complex is much more labile than the nta analogue. The rate of loss of peroxid e from the complexes was measured by use of the peroxide ''trapping ag ents' S(IV) and Zr(IV); first-order rate constants (s-1) at 25-degrees -C were, for dipic, 0.03. at pH 1 and, for nta, 1 X 10(-4) at pH 1 and 5 X 10(-6) at pH 4. Rate constants (M-1 s-1, at 25-degrees-C) for the oxygen atom transfer reactions with (en)2Co(SCH2CH2NH2)2+ and (CH3)2S O were, for MoO(O-O)(dipic), 8.6 X 10(2) and 7.6 X 10(-3) and, for MoO (O-O)(nta)-, 2.5 X 10(3) and 5.8 X 10(-3), respectively. These reactiv ities are approximately the same as observed previously with the oxo d iperoxo complex MoO(OH)-(O-O)2-. Activation of coordinate peroxide was also observed in the kinetics of reduction by iron(II), with rate con stants of 260 and 3.0 X 10(3) M-1 s-I at 25-degrees-C for the dipic an d nta oxo monoperoxo complexes, respectively. No intermediate was dete cted in either reaction, whereas one formulated as a superoxo complex is formed in the reduction of oxodiperoxomolybdates by iron(II).