The reaction of the hydride cluster [W3S4H3(dmpe)3]+ (1, dmpe=1,2‐bis(dimethylphosphanyl)ethane) with acids (HCl, CF3COOH, HBF4) in CH2Cl2 solution under pseudo‐first‐order conditions of excess acid occurs with three kinetically distinguishable steps that can be interpreted as corresponding to successive formal substitution processes of the coordinated hydrides by the anion of the acid (HCl, CF3COOH) or the solvent (HBF4). Whereas the rate law for the third step changes with the nature of the acid, the first two kinetic steps always show a second‐order dependence on acid concentration. In contrast, a single kinetic step with a first‐order dependence with respect to the acid is observed when the experiments are carried out with a deficit of acid. The decrease in the T1 values for the hydride NMR signal of 1 in the presence of added HCl suggests the formation of an adduct with a WH⋅⋅⋅HCl dihydrogen bond. Theoretical calculations for the reaction with HCl indicate that the kinetic results in CH2Cl2 solution can be interpreted on the basis of a mechanism with two competitive pathways. One of the pathways consists of direct proton transfer within the WH⋅⋅⋅HCl adduct to form WCl and H2, whereas the other requires the presence of a second HCl molecule to form a WH⋅⋅⋅HCl⋅⋅⋅HCl adduct that transforms into WCl, H2 and HCl in the rate‐determining step. The activation barriers and the structures of the transition states for both pathways were also calculated, and the results indicate that both pathways can be competitive and that the transition states can be described in both cases as a dihydrogen complex hydrogen‐bonded to Cl− or HCl2−.

The cluster [W(3)S(4)H(3)(dmpe)(3)](+) (1) (dmpe=1,2-bis(dimethylphosphino)ethane) reacts with HX (X=Cl, Br) to form the corresponding [W(3)S(4)X(3)(dmpe)(3)](+) (2) complexes, but no reaction is observed when 1 is treated with an excess of halide salts. Kinetic studies indicate that the hydride 1 reacts with HX in MeCN and MeCN-H(2)O mixtures to form 2 in three kinetically distinguishable steps. In the initial step, the W-H bonds are attacked by the acid to form an unstable dihydrogen species that releases H(2) and yields a coordinatively unsaturated intermediate. This intermediate adds a solvent molecule (second step) and then replaces the coordinated solvent with X(-) (third step). The kinetic results show that the first step is faster with HCl than with solvated H(+). This indicates that the rate of protonation of this metal hydride is determined not only by reorganization of the electron density at the M-H bonds but also by breakage of the H-X or H(+)-solvent bonds. It also indicates that the latter process can be more important in determining the rate of protonation.

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The [Mo3S4Cl3(dhmpe)3]Cl ([1]Cl) cluster has been prepared from [Mo3S7Cl6]2- and the water-soluble 1,2-bis(bis(hydroxymethyl)-phosphino)ethane (dhmpe, L) ligand. The crystal structure has been determined by X-ray diffraction methods and shows the incomplete cuboidal structure typical of the M3Q4 clusters (M=Mo, W; Q=S, Se), with a capping sulfide ligand to the three metal centers and the other three sulfides acting as bridges between two Mo atoms. The octahedral coordination around each metal center is completed with a chlorine and two phosphorus atoms of one L ligand. The chemistry of aqueous solutions of [1]Cl is dominated by the formation of the [Mo3S4L(L-H)2(H2O)]2+ complex ([2]2+), where the three chlorides have been replaced by one water molecule and two alkoxo groups of two different dhmpe ligands, thus leading to a solution structure where the three metal centers are not equivalent. A detailed study based on stopped-flow, 31P{1H} NMR, and electrospray ionization mass spectrometry techniques has been carried out to understand the behavior of [2]2+ in aqueous solution. In this way, it has been established that the addition of an excess of X- (Cl-, SCN-) leads to [Mo3S4X3(dhmpe)3]+ complexes in three resolved kinetic steps that correspond to the sequential coordination of X- at the three metal centers. However, whereas the first two steps involve the opening of the chelate rings formed with the alkoxo groups of the dhmpe ligands, the third one corresponds to the substitution of the coordinated water molecule. These results demonstrate that the asymmetry introduced by the closure of chelate rings at only two of the three Mo centers makes the kinetics of the reaction deviate significantly from the statistical behavior typically associated with M3Q4 clusters. The results obtained for the reaction of [2]2+ with acid and base are also described, and they complete the picture of the aqueous speciation of this cluster.

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