Ca(II) and Yb(II) complexes featuring M(C≡C)4 structural motif: enforced proximity or genuine η2‐bonding?

Bis(carbazolide) complexes M[3,6‐tBu2‐1,8‐(RC≡C)2Carb]2(THF)n (R = SiMe3, n = 0, M = Ca, Yb; R = Ph, n = 1, M = Ca, Yb; n = 0, M = Yb) were synthesized through transamination reaction of M[N(SiMe3)2]2(THF)2 with two molar equivalents of carbazoles. The complexes feature M(η2‐C≡C)4 structural motif composed of M(II) ions encapsulated by four acetylene fragments due to atypical for alkaline‐ and rare‐earth metals η2‐interactions with triple C≡C bond. This interaction is evidenced experimentally by X‐ray diffraction, Raman spectroscopy in the solid state and by NMR‐spectroscopy in the solution. According to QTAIM analysis there are 4 bond critical points (3;−1) between the metal atom and each of the triple bonds, which are connected by a strongly curved, almost T‐shaped bond pathway.