NEW PROCEDURES FOR THE PREPARATION OF [MO3S4(H2O)(9)](4- REDOX AND SUBSTITUTION STUDIES ON THE DOUBLE CUBE [MO7S8(H2O)(18)](8+)(), [MO4S4(H2O)(12)](5+) AND [MO7S8(H2O)(18)](8+) AND THEIR SE ANALOGS )

Alternative improved methods for the preparation of [Mo3S4(H2O)(9)](4), [Mo4S4(H2O)(12)](5+) and [Mo7S8(H2O)(18)](8+) are described from po lymeric {Mo3S7Br4}(x) (obtained by heating together the elements), via water soluble [Mo3S7Br6](2-). Abstraction of neutral sulfur with phos phines and aquation of terminal ligands in dilute acids gives [Mo3S4(H 2O)(9)](4+) in yields of up to 85%. The single cube [Mo4S4(H2O)(12)](5 +), and corner-shared double cube [Mo7S8(H2O)(18)](8+), are obtained b y treating [Mo3S4(H2O)(9)](4)+ with different reductants. The best pro cedure for [Mo7S8(H2O)(18)](8+) is with hypophosphorous acid (H3PO2), when yields of up to 20% are obtained. Yields of [Mo4S4(H2O)(12)](5+) are variable and in the range 10-65% depending on the reductant and pr ocedure employed. The reactions provide examples of Mo-III and Mo-IV-s ulfido reassembly from [Mo3S4(H2O)(9)](4+) following reduction. Surpri singly, the more direct approach for the preparation of [Mo7S8(H2O)(18 )](4+), involving addition of [Mo3S4(H2O)(9)](4+) to [Mo4S4(H2O)(12)]( 4+), gives only approximate to 3% yield, most likely due to the inertn ess of [Mo4S4(H2O)(12)](4+). The procedures described are effective al so for [Mo3Se7Br6](2-), and in exploratory studies the double cube [Mo 7Se8(H2O)(18)](8+) has been prepared for the first time. The kinetics of substitution of H2O on [Mo7S8(H2O)(18)]8+ by NCS- have been investi gated, and two stages identified. One of these is a [NCS-]-dependent e quilibration, with rate constants (25 degrees C) for formation k(f) = 0.173 M-1 s(-1) and aquation k(aq) = 0.20 x 10(-3) s(-1) at [H+] = 1.9 6 M, I = 2.00 M (Lipts), pts(-) = p-toluenesulfonate. The other is a [ NCS-]-independent step assigned as isomerisation of the S-bonded thioc yanato product (k approximate to 1.5 x 10(-3) s(-1)). With [Co(dipic)( 2)](-) (dipic = pyridine-2,6-dicarboxylate) as oxidant, second-order k inetics are observed with the rate constant 0.31 M-1 s(-1) at 25 degre es C independent of [H+] in the range 0.87-2.00 M, I = 2.00 M (Lipts). The stoichiometry indicates higher than expected consumption (approxi mate to 7 equivalents) of [Co(dipic)(2)](-). A mechanism involving tra nsient formation of [Mo7S8(H2O)(18)](9+), which fragments to [Mo3S4(H2 O)(9)](4+) and [Mo4S4(H2O)(12)](5+), is proposed. In separate experime nts the latter was found to be oxidised by [Co(dipic)(2)](-) yielding first [Mo4S4(H2O)(12)](6+), which itself fragments with formation of [ Mo3S4(H2O)(9)](4+), the Mo-2(v) dimer [Mo2O2S2(H2O)(6)](2+), and other products.