“…Preparation and single-crystal X-ray crystallographic chracterization of deeply reduced Mo 3 IV -POMs have been long-term challenging issues faced by POMs’ synthetic chemists for several reasons: (a) single Mo IV is extremely easily oxidized to higher oxidation states; (b) all electrochemical reduction in the solution stops at the Mo VI → Mo V stage and the further reduction to Mo IV cannot be achieved; and (c) different from W V , − d 1 -Mo V is stabilized by the single Mo V –Mo V metal–metal bond and hence is not disproportionate to Mo IV and Mo VI . With the above concerns in mind, a strategy employing low-valence oxoclusters containing the incomplete cuboidal [Mo 3 O 4 ] moiety as precursor, which are the fundamental building units of POMs, was pioneered by our group. , The triangular Mo–Mo bonded [Mo 3 IV O 4 (H 2 O) 9 ] 4+ cluster has been well known as a real Mo IV aqua ion after a long-term dispute about the nature of Mo IV in aqueous solution. , Its synthesis, structure, kinetic substitution reactions, and multiple redox properties have been the subjects of numerous studies. − In 2013, the partial oxidation aggregation of the [Mo 3 IV O 4 (Hnta) 3 ] 2– (where Hnta = nitrilotriacetic acid) precursor produced the first Mo 3 IV -POMs, namely the Mo VI O 2 -Keggin adduct [Mo 6 IV Mo 7 VI O 32 (OH) 4 py 6 ] 2– . However, the serious drawback of [Mo 3 IV O 4 ]-type precursors, namely the uncontrolled complete oxidation of all Mo IV atoms to form fully oxidized POMs of the highest oxidation state, was commonly encountered for [Mo 3 IV O 4 L 9 ] precursors under solvothermal (py/H 2 O) conditions, which led to the very low and unrepeatable yields of Mo 3 IV -POMs, and hence restrained the further development of Mo 3 IV -POM chemistry in the following five years.…”