Mono-silicon isoelectronic replacement in CAl4: van't hoff/le bel carbon or not?

In cluster studies, the isoelectronic replacement strategy has been successfully used to introduce new elements into a known structure while maintaining the desired topology. The well-known penta-atomic 18 valence electron (ve) species CAl42- and its Al-/Si or Al/Si+ isoelectronically replaced clusters CAl3Si-, CAl2Si2, CAlSi3-, and CSi42+, all possess the same anti-van't Hoff/Le Bel skeletons, that is, nontraditional planar tetracoordinate carbon (ptC) structure. In this article, however, we found that such isoelectronic replacement between Si and Al does not work for the 16ve-CAl4 with the traditional van't Hoff/Le Bel tetrahedral carbon (thC) and its isoelectronic derivatives CAl3X (X = Ga/In/Tl). At the level of CCSD(T)/def2-QZVP//B3LYP/def2-QZVP, none of the global minima of the 16ve mono-Si-containing clusters CAl2SiX+ (X = Al/Ga/In/Tl) maintains thC as the parent CAl4 does. Instead, X = Al/Ga globally favors an unusual ptC structure that has one long CX distance yet with significant bond index value, and X = In/Tl prefers the planar tricoordinate carbon. The frustrated formation of thC in these clusters is ascribed to the C(sic)Si bonding that prefers a planar fashion. Inclusion of chloride ion would further stabilize the ptC of CAl2SiAl+ and CAl2SiGa+. The unexpectedly disclosed CAl2SiAl+ and CAl2SiGa+ represent the first type of 16ve-cationic ptCs with multiple bonds. (c) 2019 Wiley Periodicals, Inc.