The structure of ([W3Q4X3(dmpe)3]+, Y-) ion pairs (Q = S, Se; X = H, OH, Br; Y = BF4, PF6, dmpe = Me2PCH2CH2PMe2) in dichloromethane solution and the effect of ion-pairing on the kinetics of proton transfer to the hydride cluster [W3S4H3(dmpe)3]+.

The 1H,19F 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 H2. 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.