The H-1, F-19 HOESY spectra of the title compounds in CD2Cl2 solution indicate that the cluster cations form ion pairs with the BF4- and PF6-anions with a well-defined interionic structure that appears to be basically determined essentially by the nature of the X- ligand. For the clusters with X) H and OH, the structure of the ion pairs is such that the counteranion (Y-) and the X(-)ligands are placed close to each other. However, when the size and electron density of X- increase (X = Br), Y- is forced to move to a different site, far away from X-. The relevance of ion-pairing on the chemistry of these compounds is clearly seen through a decrease in the rate of proton transfer from HCl to the hydride cluster [W3S4H3(dmpe)(3)](+) in the presence of an excess of BF4-. The kinetic data for this reaction can be rationalized by considering that the ([W3S4H3(dmpe)(3)](+), BF4-) ion pairs are unproductive in the proton-transfer process. Theoretical calculations indicate that the real behavior can be more complex. Although the cluster can still form adducts with HCl in the presence of BF4-, the structures of the most-stable BF4--containing HCl adducts show H...H distances too large to allow the subsequent release of H-2. In addition, the effective concentration of HCl is also reduced because of the formation of adducts as ClH..BF4-. As a consequence of both effects, the proton transfer takes place more slowly than for the case of the dihydrogen-bonded HCl adduct resulting from the unpaired cluster.