Ca(II) and Yb(II) complexes featuring M(CC)₄ structural motif: enforced proximity or genuine ²-bonding?

Bis(carbazolide) complexes M[3,6-tBu(2)-1,8-(RC equivalent to C)(2)Carb](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(eta(2)-C equivalent to C)(4) structural motif composed of M(II) ions encapsulated by four acetylene fragments due to atypical for alkaline- and rare-earth metals eta(2)-interactions with triple C equivalent to 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.