First, we investigate the reactivity of 1-Ir towards tris(tert-butoxy)silanol (H-TBOS), a molecular analog of isolated surface silanols found on silica. Cooling a concentrated 1:1 pentane solution of orange 1-Ir and colorless H-TBOS affords yellow crystals in 80% yield (Scheme 1). Single-crystal X-Ray diffraction reveals that the reaction yields a product having a TBOS anionic ligand and a protonated neutral amidine ligand remaining coordinated to Ir (Ir-TBOS), with an overall square pyramidal geometry, paralleling the proposed structure for the corresponding silica-supported species prepared by grafting on silica. due to breaking of the symmetry upon protonation of the amidinate ligand. The signal attributed to the quaternary amidine carbon is considerably more shielded upon grafting (160 ppm vs. 183 ppm in 1-Ir), which supports a change in electronic environment consistent with protonation of one amidinate nitrogen. It is, however, still more shielded than free amidine, reported at 142 ppm, consistent with the amidine remaining coordinated to Ir. Moreover, the quaternary carbon of the amidine ligand in Ir-TBOS appears at 158.8 ppm (T = 268 K, see ESI for details), supporting the proposed surface structure. To our surprise though, measuring the solution NMR spectrum of single crystals of Ir-TBOS dissolved in C6D6 only reveals the presence of both starting materials, 1-Ir and H-TBOS, indicating that proton transfer must be (completely) reversible. Considering that the crystallization of Ir-TBOS was performed at low temperature (-30 °C), we have next investigated the reaction of a 1:1 mixture of 1-Ir and H-TBOS using variable-temperature NMR spectroscopy (VT-NMR) (Figure ). Notably, we observe that lowering the temperature yields NMR spectra consistent with the partial conversion of 1-Ir and H-TBOS to Ir-TBOS. In the range of 25 °C to ca. -5 °C, the relative concentration of Ir-TBOS increases linearly with decreasing temperature. However, the distribution of product and starting materials reaches a plateau below -25 °C, at ca. 30% conversion to Ir-TBOS, suggesting a kinetic barrier blocking any further reaction below this temperature. Notably, this process is fully reversible upon warming the temperature of the reaction mixture. Such temperature dependence reflects the loss of entropy upon formation of the adduct. The driving force for the reaction is the protonation of the basic amidinate ligand, and likely a release of strain from the k 2coordination of the amidinate ligand, while forming a rather weak Ir-O bond (by comparison to what is expected for the earlier transition-metal series often used in SOMC). DFT calculations confirm an exothermic and exergonic adduct formation, with a negative entropic contribution to the Gibbs free energy of the reaction (Figure ). Formation of the intermolecular is exothermic by DH = -17 kcal/mol (DG = -3 kcal/mol), the proton transfer is associated with an energy barrier of DH ‡ = 11 kcal/mol (DG ‡ = 12 kcal/mol) resulting in a structure with an intramolecular H-bond, which relaxes to the non-H-bonded structure of Ir-TBOS, found by single crystal XRD. Note that similar VT-NMR experiments on 1-Ir/ SiO2-700 (using an internal standard) show marginal effect of temperature on the amount of deposited 1-Ir in the temperature range of -10 °C to +40 °C, suggesting that the effect of entropy loss upon immobilization is less significant, which could be due to an overall higher reaction enthalpy, possibly due to the stronger interaction of the more acidic surface silanol groups (compared to H-TBOS) and/or the Van-Der-Waals interaction between the chemisorbed species and the silica surface. Nevertheless, when evacuating and heating 1-Ir/SiO2-700 at 10 -5 mbar and 180 °C for 2 h, a partial degrafting can be observed as evidenced by the sublimation of 1-Ir onto the walls (out of the heating zone) of the reaction vessel (confirmed by 1 H NMR spectroscopy). At this temperature, however, decomposition of the grafted species is also evidenced with the formation of a dark material. Comparing the IR spectra before and after evacuation at 180 °C, the band at 1600 cm -1 (assigned to C=N) almost completely vanishes (the intensity drops by ca. 95% in intensity) along with the N-H stretching bands indicating complete removal of amidine fragments (Figure ). Meanwhile, the OH stretching band of isolated silanol groups re-emerges, regaining ca. 50% of the intensity of SiO2-700, corroborating the (partial) reprotonation and reversible grafting. Overall, ca. half of 1-Ir is readily and reversibly removed, which could indicate the presence of different environments around metals sites (some with or without siloxane bridges): Notably, this observation, presence of ca. 1:1 mixture of similar yet different surface species on SiO2-700, is in line with earlier proposals that the environment around isolated surface silanols on amorphous silica supports is quite heterogenous.