The chalcogenide-rich trinuclear Mo-3(IV) clusters [Mo3Y7(H2O)(6)](4+), containing single mu(3)-(Y2-) and three mu-(Y-2(2-)) core ligands, have been obtained for the first time from polymeric {Mo3Y7Br4}(x) via [Mo3Y7Br6](2-) (Y = S, Se). ICP analyses of 2 M HCl solutions give Mo:S and Mo:Se ratios consistent with the formulas indicated, and on reaction with concentrated HBr, 85% recovery of (Et4N)(2)[Mo3S7Br6], the structure of which is known, has been achieved. Abstraction of S and Se with PPh3 (two-phase system), or the water-soluble phosphine (3-SO3C6H4)(3)P3-(PR33-), gives quantitative formation of [Mo3S4(H2O)(9)](4+) and [Mo3Se4(H2O)(9)](4+), With CN-, both abstraction of S (Or Se) and CN- replacement of H2O is observed, giving [Mo3S4(CN)(9)](5-) and [Mo3Se4(CN)(9)](5-) as products. It was possible to assign which atom of the sideways eta(2),eta(2) mu-(S-2(2-)) and mu-(Se-2(2-)) ligands is abstracted using the structurally characterized [Mo3S4Se3(H2O)(6)](4+) cluster. Thus it was demonstrated that with the phosphines the equatorial (to the Mo-3 plane) Se atoms of the three mu-(SSe2-) groups are removed with formation of the Mo3S44+ core. Kinetic studies on the reactions of [Mo3S7(H2O)(6)](4+) and [Mo3Se7(H2O)(6)](4+) With PR33- give similar to 10(3) faster abstraction rate constants (k(a)/M-1 s(-1)) for S than Se. The rate law k(a) = k(1)[H+] + k(-1)[H+](-1) is explained by the involvement of protonated mu-(Y-2(2-)) (k(1)) and an H2O conjugate-base form (k(-1)). Equilibration rate constants for X- = Cl- and Br- substitution of H2O on [Mo3S7(H2O)(6)](4+) and [Mo3Se7(H2O)(6)](4+) are however independent of [H+] in the range 0.5-2.0 M investigated. With X- concentrations up to 1.3 M (S cluster) and 0.3 M (Se), the uniphasic reactions are assigned as substitution of the H2O cis to mu(3)-(Y2-) at each Mo. At 25 degrees C formation rate constants 10(4)k(f)/M-1 s(-1) are as follows for [Mo3S7(H2O)(6)](4+) : Cl- (1.83); Br- (2.07). The same rate constants are as follows for [Mo3Se7(H2O)(6)](4+) : Cl- (6.7); Br- (33). Formation rate constants for Cl- are surprisingly 2 x 10(6) times slower than for the reaction of [Mo3S4(H2O)(9)](4+) With Cl-. Reactions of Mo3S74+ With three metals (Sn, Ni, In) were studied briefly.