Since the last 20 years, homogeneous gold catalysis is receiving considerable attention and great efforts have been made to disclose the mechanism of the catalytic cycle and the role of many variables: nature of the ligand, structure of the catalyst, effect of the additives, etc. Although a wealth of empirical information on ligand effects is now available on homogeneous gold catalysis, the development of new L-Au-X catalysts and reactions continues to rely upon trial and error, and the outcome of the reaction is often unpredictable. The mechanistic understanding of the gold(I)-catalysed nucleophilic addition to a carbon-carbon unsaturated bond has been pursued and appreciably extended in this thesis by experimental and theoretical investigations. We focused our study on weak interactions and counterion effects with the aim to better understand these often not considered variables. We explained that the anion is influencing each single step of the catalysis and that it is modulating its role depending on its nature (coordination power and basicity) and position. Of course, its effect is also depending on the type of reaction and on which is the rate determining step of the latter. This deep study on the ligands and counterions role in gold homogeneous catalysis, matching both experimental and theoretical studies, allow us to setup a green, room temperature, acid-free, solvent-free and sustainable methodology for the hydration of alkynes. This reaction is generally working only with acidic additives and at high temperature, thereby here is reported for the first time an innovative way to perform it. These preliminary studies open new avenues to consider and rationalize the homogeneous gold catalysis, spreading light into the weak interactions that were underestimated in this field for a long time. This study clearly demonstrates that the interplay between ligand nature and anion effect is crucial in different steps of the catalytic cycle. The multiple roles played by counterions and L-Au+ fragments in chemical transformations require more comprehensive computational and experimental studies of the ligand/anion correlation
