A detailed mechanistic study is reported for the syn-insertion of alkynes and allenes in the Au-Si bonds of complexes (R3P)Au-SiR'Ph2 (R = Ph, Me and R' = t-Bu, Ph). Kinetic experiments indicate that (i) the reaction is first-order in alkyne and gold silyl complex, (ii) it requires a rather low enthalpy of activation and a relatively large negative entropy of activation [deltaH≠ = 13.7 (± 1.6) kcal.mol-1 and deltaS≠ = - 32.0 (± 5.0) cal.mol-1.K-1 for the reaction of (Ph3P)Au-Si(t-Bu)Ph2 with methyl propiolate], in line with a bimolecular associative transformation. The different mechanistic pathways have been explored by DFT calculations. Accordingly, the reaction is found to proceed via a two-step inner-sphere mechanism: (i) first, the alkyne coordinates to the gold silyl complex to form a pi-complex; (ii) the subsequent migratory insertion step is rate determining and occurs in a concerted manner. Provided dispersion effects are taken into account (B97D functional), the enthalpy of activation estimated theoretically [deltaH≠ = 11.5 kcal.mol-1] is in good agreement with that measured experimentally. The influence of the pi substrate (methyl propiolate, dimethyl acetylene dicarboxylate, phenyl acetylene, ethyl 2,3-butadienoate) has been analyzed theoretically and the regio-selectivity of the insertion has been rationalized. In particular, the unexpected selectivity observed experimentally with the allene is shown to result from the insertion of the terminal non-activated C=C double bond into the Au-Si bond of (Ph3P)Au-SiPh3, followed by an original isomerization (Au/Si exchange process). This study provides unambiguous evidence for coordination-insertion at gold and thereby strongly supports the possible occurrence of inner-sphere mechanisms during the functionalization of alkynes and allenes.