Reactivity of dicoordinated stannylones (Sn⁰) versus stannylenes (Sn^II): An investigation using DFT-based reactivity indices

The reactivity of dicoordinated Sn⁰ compounds, stannylones, is probed using density functional theory (DFT)-based reactivity indices and compared with the reactivity of dicoordinated Sn^II compounds, stannylenes. For the former compounds, the influence of different types of electron-donating ligands, such as cyclic and acyclic carbenes, stannylenes and phosphines, on the reactivity of the central Sn atom is analyzed in detail. Sn⁰ compounds are found to be relatively soft systems with a high nucleophilicity, and the plots of the Fukui function f^- for an electrophilic attack consistently predict the highest reactivity on the Sn atom. Next, complexes of dicoordinated Sn compounds with different Lewis acids of variable hardness are computed. In a first part, the double-base character of stannylones is demonstrated in interactions with the hardest Lewis acid H ⁺. Both the first and second proton affinities (PAs) are high and are well correlated with the atomic charge on the Sn atom, probing its local hardness. These observations are also in line with electrostatic potential plots that demonstrate that the tin atom in Sn⁰ compounds bears a higher negative charge in comparison to Sn^II compounds. Stannylones and stannylenes can be distinguished from each other by the partial charges at Sn and by various reactivity indices. It also becomes clear that there is a smooth transition between the two classes of compounds. We furthermore demonstrate both from DFT-based reactivity indices and from energy decomposition analysis, combined with natural orbitals for chemical valence (EDA-NOCV), that the monocomplexed stannylones are still nucleophilic and as reactive towards a second Lewis acid as towards the first one. The dominating interaction is a strong σ-type interaction from the Sn atom towards the Lewis acid. The interaction energy is higher for complexes with the cation Ag⁺ than with the non-charged electrophiles BH₃, BF₃, and AlCl ₃. The difference of two electrons: The reactivity of dicoordinated Sn⁰ compounds, stannylones, is probed using density functional theory (DFT)-based reactivity indices and compared with the reactivity of dicoordinated Sn^II compounds, stannylenes.