A phosphine gold(I) pi-alkyne complex: tuning the metal-alkyne bond character and counterion position by the choice of the ancillary ligand.

The intra- and interionic structures of a mononuclear phosphine gold(I) alkyne complex [(PAr(F)(3))Au(2-hexyne)]BF(4) [1BF(4); Ar(F) = 3,5-bis(trifluoromethyl)phenyl] and its analogous complex [(NHC)Au(2-hexyne)]BF(4) [2BF(4); NHC = 1,3-bis(diisopropylphenyl)imidazol-2-ylidene] have been investigated by combining 1D and 2D multinuclear NMR spectroscopy and density functional theory calculations. It has been found that alkyne in 1BF(4) is depleted of its electron density to a greater extent than that in 2BF(4). This correlates with the Deltadelta((13)C) NMR of the carbon-carbon triple bond. Instead, 2BF(4) is much more "kinetically stable" than 1BF(4). (19)F-(1)H HOESY NMR experiments indicate that the counterion locates close to the gold atom in 1BF(4) (differently from that previously observed in the few other gold(I) ion pairs studied), exactly where the computed Coulomb potential indicates that partial positive charge accumulates.