LAUSR

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 C─X 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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. © 2019 Wiley Periodicals, Inc.