Formation constants have been determined for the complexes MoO(O-O)(dipic) and MoO(0-0)(nta)" at 25 °C with / = 0.10 M (NaCl). For the general formula ( 4)( )( +)4( 2 2), log K¡ values of 23.48 and 23.0 were determined for L = dipic by potentiometry and spectrophotometry, respectively; for L = nta, the corresponding log Rvalues were 27.87 and 28.8. A single-crystal X-ray structure was determined for the nta complex, and 95Mo NMR spectra were collected. The dipic complex is much more labile than the nta analogue. The rate of loss of peroxide from the complexes was measured by use of the peroxide "trapping agents" S(IV) and Zr(IV); first-order rate constants (s-1) at 25 °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 °C) for the oxygen atom transfer reactions with (en)2Co(SCH2CH2NH2)2+ and (CH3)2SO were, for MoO(0-0)(dipic), 8.6 X 10* 12and7.6X 10~3and,forMoO(O-O)(nta)~,2.5 X 103 56and5.8X 10"3, respectively. These reactivities are approximately the same as observed previously with the oxo diperoxo complex MoO(OH)-(0-0)2". Activation of coordinate peroxide was also observed in the kinetics of reduction by iron(II), with rate constants of 260 and 3.0 X 103 M-1 s~' at 25 °C for the dipic and nta oxo monoperoxo complexes, respectively. No intermediate was detected in either reaction, whereas one formulated as a superoxo complex is formed in the reduction of oxodiperoxomolybdates by iron(II).