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

Abstract Bis(carbazolide) complexes M[3,6‐tBu 2 ‐1,8‐(RC≡C) 2 Carb] 2 (THF) n (R=SiMe 3 , n=0, M=Ca, Yb; R=Ph, n=1, M=Ca, Yb; n=0, M=Yb) were synthesized through transamination reaction of M[N(SiMe 3 ) 2 ] 2 (THF) 2 with two molar equivalents of carbazoles. The complexes feature M(η 2 ‐C≡C) 4 structural motif composed of M(II) ions encapsulated by four acetylene fragments due to atypical for alkaline‐ and rare‐earth metals η 2 ‐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.