Bonding and reactivity study of chalcogen adducts of N-heterocyclic Silylene and Germylene: Fisher-carbene like bond polarity in heavier main group adducts

The chalcogen-stabilized N-heterocyclic carbenes, Imidazolin-2-chalcogenones (NHC=X, X  = O, S, Se), represent an important class of stable compounds with wide applications in catalysis. However, the chalcogen-stabilized N-heterocyclic Silylenes and Germylenes (NHE=X, E = Si, Ge; X  = O, S, Se), which could potentially lead to a new class of low valent compounds, are not yet isolated, while their Lewis-base-supported adducts are. Therefore, the nature of the Si/Ge-chalcogen (_NHEE = X) bond is investigated and compared to the _NHCC=X bond using quantum mechanical methods to elucidate the differences in the stability and reactivity. The NBO, AIM, and MO analyses suggest that the _NHEE=X bond in the heavier analogues exhibits a Fischer-carbene like bond polarity for the main group adduct, and the polarity decreases going down the group. The EDA-NOCV (Energy Decomposition Analysis combined with Natural Orbitals for Chemical Valence) analysis confirms that the bonding in NHE=X except for NHGe=Se can be best represented by considering one electron sharing and one donor–acceptor interaction between positively charged NHE fragment (_NHEE⁺) and negatively charged chalcogen, which confirms the polar nature of the E─X bond. On the other hand, the preference for the singlet electronic state is translated in double donor–acceptor interactions between two neutral fragments (NHGe and Se) in NHGe=Se. In addition, an in-plane hyperconjugative _NHEE ← X (π_\parallel) interaction is also observed. Our analysis further confirms the preference for ammonia addition to the Silylene/Germylene centre, thus corroborating the experimentally reported reactivity of Lewis base stabilized NHE=X.