The electrochemistry of dioxomolybdenum(VI) with the tetradentate S[sub 2](NH)[sub 2]-type ligand N,N[prime]-bis(2-mercaptophenyl)-2,3-diaminobutane (mPhabH[sub 4]), at neutral, acidic, and basic conditions, has been investigated. The molecular structure of (mPhabH[sub 2])Mo[sup VI]O[sub 2] (1) has been determined: space group P2[sub 1]2[sub 1]2[sub 1] (No. 19), a = 8.667(<1), b = 12.884(<1), c = 19.877(3) [Angstrom], V = 2220 [Angstrom][sup 3]; Z = 4. The cis-dioxo complex 1 displays a highly distorted octahedral structure. Electrochemical, one-electron reduction of the Mo(VI) compound 1 is followed by a fast dehydration reaction leading to [(mPhab)MoO][sup [minus]] (4), which can be further reduced in a subsequent one-electron reduction step to the Mo(IV) dianion. At neutral conditions this process appears as a single two-electron reduction. Upon addition of N-methylimidazole (NMI) the dehydration rate decreases and two separate reduction steps can be observed. Acidic conditions also lead to a decrease of the dehydration rate and to two separate one-electron reductions. In basic solution the Mo(VI) complex 1 is reduced by methanol to [(mPhab)Mo[sub V]O][sup [minus]] (4), which shows both a quasi-reversible one-electron reduction wave to form the Mo(IV) dianion and a quasi-reversible oxidation wave. Synthetically, [(mPhab)Mo[sup V]O][sup [minus]] (4) can also be obtained via deoxygenation of 1 by Ph[submore » 2]PMe in the presence of N-methylimidazole, presumably via formation of a Mo(IV)-NMI adduct that undergoes a comproportionation reaction with 1 and subsequent dehydration to the mononuclear Mo(V) complex 4. Neither [mu]-oxo-bridged Mo(V) dimers nor mononuclear Mo(IV) species can be detected. 30 refs., 5 figs., 4 tabs.« less