Synthesis and structure of theincomplete cuboidal clusters [W_3Se_4H_3(dmpe)_3]~+,[W_3Se_4H_(3-x)(OH)_x(dme)_3]and [W_3Se_4(OH)_3(dmpe)_3]~+,and the mechanism of the acid-assisted substitution of the coordinated hydrides

摘要

The novel incomplete cuboidal cluster [W_3Se_4H_3(dmpe)_3](PF_6),[1](PF_6),has been prepared by reduction of [W_3Se_4Br_3(dme)_3](PF_6) with LiBH_4 in THF solution.the trihydroxo complex [W_3Se_4(OH)_3(dmpe)_3](PF_6),[2](PF_6),was obtained by reacting [W_3Se_4Br_3(dmpe)_3](PF_6) withNaOH inMeCN-H_2Osolution.The complexes [1](F_6) and [2](PF_6) were convertedto their BPh~-_4 salts by treatment with NaBPh_4.Recrystallisation of [1](BPh_4) in the prescence of tracesof water affords the mixed dihydride hydroxo complex [W_3Se_4H_2(OH)(dmpe)_3](BPh_4).The crystal structures react with an excess of halide salts,reaction with HX leads to [W_3Se_4X_2(dmpe)_3]~+ (X=Cl,Br).The kinetics of this reacton has beenstudied at 25 deg C in MeCN-H_2O solution (1:1,v/v) and found to occur with two consecutive kinetic steps.The first step is independent of the nature and concnetration of the X~- anion but shows a first order dependence on the concentration of acid (k_1=12.0mol~(-1)dm~3s~(-1)),whereas the second one is independent of th ennature and concentraton of both the acid and added salts(K_2=0.024s~(-1).In contrast,the reaction of [2]~+ with acids occurs in a signle step with K_(obs)=0.63s~(-1),(HCl) and 0.17s~(-1)(HBr).These kinetic results are discussed on the basis of the mechanism previously proposed for th ereactonsof the analogous [W_3S_4H_3(dmpe)_3]~+ cluster,with special emphasis on the effects caused by the change of S by Se on the rate constants for the different processesinvolved.