Synthesis and structure of the incomplete cuboidal clusters [W3Se4H3(dmpe)3]+, [W3Se4H3-x(OH)x(dmpe)3]+ and [W3Se4(OH)3(dmpe)3]+, and the mechanism of the acid-assisted substitution of the coordinated hydrides

The novel incomplete cuboidal cluster [W3Se4H3(dmpe)(3)](PF6), [1](PF6), has been prepared by reduction of [W3Se4Br3(dmpe)3](PF6) with LiBH4 in THF solution. The trihydroxo complex [W3Se4(OH)(3)(dmpe)(3)](PF6), [2](PF6), was obtained by reacting [W3Se4Br3(dmpe)(3)](PF6) with NaOH in MeCN-H2O solution. The complexes [1](PF6) and [2](PF6) were converted to their BPh4- salts by treatment with NaBPh4. Recrystallisation of [1](BPh4) in the presence of traces of water affords the mixed dihydride hydroxo complex [W3Se4H2(OH)(dmpe)(3)](BPh4). The crystal structures of [1](BPh4), [2](BPh4) and [W3Se4H2(OH)(dmpe)(3)](BPh4) have been resolved. Although the [1](+) trihydride does not react with an excess of halide salts, reaction with HX leads to [W3Se4X3(dmpe)(3)](+) (X=Cl, Br). The kinetics of this reaction has been studied at 25degreesC in MeCN-H2O solution (1:1, v/v) and found to occur with two consecutive kinetic steps. The first step is independent of the nature and concentration of the X- anion but shows a first order dependence on the concentration of acid (k(1)=12.0 mol(-1) dm(3) s(-1)), whereas the second one is independent of the nature and concentration of both the acid and added salts (k(2)=0.024 s(-1)). In contrast, the reaction of [2]+ with acids occurs in a single step with k(obs)=0.63 s(-1) (HCl) and 0.17 s(-1) (HBr). These kinetic results are discussed on the basis of the mechanism previously proposed for the reactions of the analogous [W3S4H3(dmpe)(3)](+) cluster, with special emphasis on the effects caused by the change of S by Se on the rate constants for the different processes involved.