Infrared Photodissociation Spectroscopy of Be(N ₂ ) _n ⁺ and Be(CO)

Beryllium dinitrogen and carbonyl cation complexes, Be(N ₂ ) _n ⁺ and Be(CO) _n ⁺ ( n  = 3–4), were produced in the gas phase by a laser vaporization‐supersonic expansion ion source and investigated by infrared photodissociation spectroscopy in conjunction with quantum chemical calculations. Be(N ₂ ) ₃ ⁺ is assigned to a planar D _3h ‐symmetric structure with three equivalent end‐on coordinated molecular N ₂ ligands, whereas Be(N ₂ ) ₄ ⁺ is characterized as a weakly bound N ₂ adduct of Be(N ₂ ) ₃ ⁺ . The corresponding carbonyl complexes exhibit analogous structural motifs. Bonding analyses reveal that the L → Be (L = NN/CO) interactions in these end‐on complexes are governed by L → Be σ donation, accompanied by weaker L ← Be π back‐donation. In contrast to our previous study on Be(N ₂ ) ₃ ( Angew. Chem. Int. Ed . 2020, 59, 10603), where both end‐on and side‐on coordination motifs coexist, only end‐on structures are observed for the cationic species. The absence of side‐on coordination is attributed to the positive charge, which significantly weakens π back‐donation while only modestly enhancing σ donation, thereby destabilizing side‐on binding motifs.