pi Back-Donation from a Beryllium Dibromide Fragment at the Expense of Its sigma Strength

It is common knowledge that metal-to-ligand pi back-donation requires filled atomic orbitals at the metal center. However, we show through a combined experimental and theoretical approach that Be(II) -> N-heterocyclic carbene (NHC) pi back-donation is present in the two carbene adducts [(iPr)BeBr2] (1) and [(iPr)(2)BeBr2] (2) (iPr = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene). These complexes were characterized with NMR, IR, and Raman spectroscopy as well as with single-crystal X-ray diffractometry. The unusual bonding situation is understood from the results of energy decomposition analysis in combination with natural orbital for chemical valence and quantum theory of atoms-in-molecules analysis. The obtained findings shed light on the unusually high Be-C bond strength in carbene adducts to beryllium compounds and rationalize their geometry and reactivity.