Mass spectrometry. Mass spectrometric (MS) experiments were carried out using the Thermo Scientific LTQ XL linear ion trap mass spectrometer with electrospray ionization (ESI) source. Energy-resolved collisioninduced dissociation spectra were measured with an LCQ Deca XP ion trap mass spectrometer. In the collision-induced dissociation (CID) experiments, bond dissociation energies (BDE) can be determined from the dependence of the relative cross-section of the given dissociation on the collision energy. The linear extrapolation of the rise of the sigmoid fit of the cross-section energy dependence gives the appearance energy of the given dissociation channel (see Figure and the data in the Supporting Information). The appearance energies are related to the bond dissociation energies associated with the given fragmentations. The collision energies in the LCQ instruments can be calibrated based on the measurements of the dissociation energies of a series of "thermometer" alkylpyridinium ions (Figure ). The ions of interest were generated by electrospray ionization from dichloromethane/acetonitrile (DCM/ACN) solution of the respective 1 mM ligand and 1 mM AuX3 stock solutions. The stock solutions were mixed in a 1:1 ratio and diluted to achieve the final 100 mM concentration. The electrospray voltage was 4-5 kV and the capillary was heated to 150-220 °C. The sheath gas, flow rate, and voltages of capillary and lenses were optimized in order to obtain the maximum ESI-MS signals of the required ions. Collision-induced dissociation (CID) experiments were performed for mass-selected ions with a trapping parameter qz = 0.25 and the excitation period of 30 ms.