Preparation and Solution Properties of Chalcogenide-Rich Clusters [Mo(3)Y(7)(H(2)O)(6)](4+) (Y = S, Se): Kinetics of PR(3)(3-) Abstraction of Y from &mgr;-(Y(2)(2-)) and H(2)O Substitution by Cl(-) and Br(-).

The chalcogenide-rich trinuclear Mo(IV)(3) clusters [Mo(3)Y(7)(H(2)O)(6)](4+), containing single &mgr;(3)-(Y(2)(-)) and three &mgr;-(Y(2)(2)(-)) core ligands, have been obtained for the first time from polymeric {Mo(3)Y(7)Br(4)}(x)() via [Mo(3)Y(7)Br(6)](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 (Et(4)N)(2)[Mo(3)S(7)Br(6)], the structure of which is known, has been achieved. Abstraction of S and Se with PPh(3) (two-phase system), or the water-soluble phosphine (3-SO(3)C(6)H(4))(3)P(3)(-) (PR(3)(3)(-)), gives quantitative formation of [Mo(3)S(4)(H(2)O)(9)](4+) and [Mo(3)Se(4)(H(2)O)(9)](4+). With CN(-), both abstraction of S (or Se) and CN(-) replacement of H(2)O is observed, giving [Mo(3)S(4)(CN)(9)](5)(-) and [Mo(3)Se(4)(CN)(9)](5)(-) as products. It was possible to assign which atom of the sideways eta(2),eta(2) &mgr;-(S(2)(2)(-)) and &mgr;-(Se(2)(2)(-)) ligands is abstracted using the structurally characterized [Mo(3)S(4)Se(3)(H(2)O)(6)](4+) cluster. Thus it was demonstrated that with the phosphines the equatorial (to the Mo(3) plane) Se atoms of the three &mgr;-(SSe(2)(-)) groups are removed with formation of the Mo(3)S(4)(4+) core. Kinetic studies on the reactions of [Mo(3)S(7)(H(2)O)(6)](4+) and [Mo(3)Se(7)(H(2)O)(6)](4+) with PR(3)(3)(-) give approximately 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 &mgr;-(Y(2)(2)(-)) (k(1)) and an H(2)O conjugate-base form (k(-)(1)). Equilibration rate constants for X(-) = Cl(-) and Br(-) substitution of H(2)O on [Mo(3)S(7)(H(2)O)(6)](4+) and [Mo(3)Se(7)(H(2)O)(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 H(2)O cis to &mgr;(3)-(Y(2)(-)) at each Mo. At 25 degrees C formation rate constants 10(4)k(f)/M(-)(1) s(-)(1) are as follows for [Mo(3)S(7)(H(2)O)(6)](4+): Cl(-) (1.83); Br(-) (2.07). The same rate constants are as follows for [Mo(3)Se(7)(H(2)O)(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 [Mo(3)S(4)(H(2)O)(9)](4+) with Cl(-). Reactions of Mo(3)S(7)(4+) with three metals (Sn, Ni, In) were studied briefly.