Organometallic complexes are widely used in homogenous catalysis. The reactivity of a complex is linked to the ligands with which it is bound. Ligands can influence the reactivity of the metal center through their electronic and steric effects. It is therefore essential to know of these effects to choose the most suitable ligand when synthesizing organometallic complexes for a given application [1,2]. Experimentally, ligand effects can be determined using indirect methods such as infrared spectroscopy of the elongation band CO in metal-carbonyl complexes (Tolman method) [3]. This approach has enabled the production of relative rankings of phosphine and NHC ligands according to their donor character [4]. However, these methods are limited to complexes with probe ligands such as CO (e.g. LMCO). Very few reliable experimental methods are available to determine the electronic properties of ligands in complexes without probe ligands. In this context, we have investigated the influence of ligands on [(C^C)Au(NHC)L]+ gold(III) complexes with L a phosphine ligand. The effect of phosphine substitution on the enrichment (in electron density) of the metal center was assessed by determining the bond dissociation energy (BDE) M-L -> M + L by mass spectrometry using higher energy collision dissociation (HCD) activation on an Orbitrap mass spectrometer. BDEs were then estimated from the experimental data using kinetic modeling of the fragmentation process. The results obtained were then rationalized by electronic structure calculations using density functional theory (DFT). Correlation between the lability of the phosphine ligands towards the catalytic efficiency of the complexes could be demonstrated in an intramolecular hydroarylation reaction of alkyne. [1] Constable, E.C., In Metals and Ligand Reactivity. 2005. 22-45. [2] Zecchina, A.C., S., Eds. , The Development of Catalysis: A History of Key Processes and Personas in Catalytic Science and Technology. John Wiley and Sons, 2017. [3] F. A. Tolman, Chem. Rev., 1977, 77, 313. [4] Moloy, K. G.; Petersen, J. L. N-Pyrrolyl Phosphines: An Unexploited Class of Phosphine Ligands with Exceptional .Pi.-Acceptor Character. Journal of the American Chemical Society 1995, 117 (29), 7696–7710.